Unit Two: Energy

Earth's magnetic field

A compass can help determine direction because the needle's north pole points to a location near Earth's geographic north pole. Earth's geographic north pole is the northernmost point on Earth. Earth acts like a giant bar magnet with a magnetic field that extends into space. A compass needle will align with Earth's magnetic field lines, as shown in Figure 6. Earth's magnetic field is not very strong in comparison to common magnets. The magnetic field of a refrigerator magnet is about 100 times stronger than Earth's magnetic field. Earth's magnetic poles The north pole of a magnet is defined as the end of the magnet that points toward Earth's geographic north pole. Sometimes the north pole and south pole of magnets are called the north-seeking pole and the south-seeking pole. Opposite magnetic poles attract, so the north pole of a compass needle is attracted to a south magnetic pole. Therefore, Earth's south magnetic pole is near its geographic north pole, as seen in Figure 6. However, Earth's magnetic poles move slowly with time and sometimes switch places. Measurements of magnetism in rocks show that Earth's magnetic poles have changed places over 150 times in the past 70 million years. The source of Earth's magnetic field No one is certain what produces Earth's magnetic field, but many scientists think that it is generated by Earth's core. Earth's core is thought to be made of iron and nickel. The solid inner core is surrounded by a liquid outer core. The circulation of the molten iron and nickel in Earth's outer core is believed to produce Earth's magnetic field.

Heat engines

A heat engine is a device that converts some thermal energy into mechanical energy. A car's engine is an example of a heat engine. A car's engine converts the chemical energy in gasoline into thermal energy. The engine then transforms some of this thermal energy into mechanical energy and rotates the car's wheels, as shown in Figure 18. However, only about 25 percent of the thermal energy released by the burning gasoline is converted into mechanical energy.

Heat pumps

A heat pump is a two-way air conditioner. In warm weather, a heat pump operates as an ordinary air conditioner. It does work to transfer thermal energy from the cooler building to the warmer outdoors. This cools the building while warming the outdoors very slightly. In cold weather, a heat pump operates like an air conditioner in reverse. It does work to transfer thermal energy from the cooler outdoors to the warmer building. This warms the building while cooling the building's surroundings very slightly. Energy transformations and

Machines

A machine is a device that changes the force or increases the motion from work. Imagine trying to lift a grand piano. Without help, this would be impossible. However, you could probably push the piano up a ramp or lift the piano with a pulley system. In both cases, you use a machine to change the force on the piano. However, you might be more interested in increasing speed than increasing force. Using only the power of your muscles, how fast could you move? You could travel much faster on a machine called a bicycle than on foot. Types of machines When you cut your food with a knife, use a screwdriver, or chew your food, you are using a simple machine. A simple machine is a machine that does work with only one movement of the machine. There are six types of simple machines: lever, pulley, wheel and axle, inclined plane, screw, and wedge. The pulley and the wheel and axle are modified levers, and the screw and the wedge are modified inclined planes. A common example of each type of simple machine is shown in Figure 4. A compound machine is a combination of two or more simple machines. For example, a pair of scissors is a compound machine. It combines two wedges and two levers. A bicycle is also a compound machine.

Magnetic Fields

A magnet can exert a force on a distant object because of its magnetic field. A magnetic field is a region of space that surrounds a magnet and exerts a force on other magnets and objects made of magnetic materials. The magnetic field and the magnetic force are related. A stronger magnetic field means that a magnetic object placed in the field will experience a stronger magnetic force. In other words, stronger magnets have stronger magnetic fields. The strength of the magnetic field affects the use of the magnet. For example, the magnetic field used for a type of medical test called Magnetic Resonance Imaging (MRI) is about 200 times stronger than the magnetic field of a refrigerator magnet. This is why you must remove all magnetic metal objects before entering the MRI room. Magnetic field lines The magnetic field can be represented by lines of force called magnetic field lines. Figure 2 shows iron filings lined up along the magnetic field lines surrounding a bar magnet. The closer together the magnetic field lines are, the stronger the magnetic field is at that point. Field lines are closer together near the magnet, as shown in Figure 2. This agrees with our observation that the magnetic force and the magnetic field are strongest close to the magnet. Figure 3 shows the magnetic fields around a horseshoe magnet and a disk magnet.

Nuclear Reactors

A nuclear reactor uses energy from controlled nuclear reactions to generate electricity. Although nuclear reactors vary in design, all reactors share some similarities. They contain fuel that can undergo fission and control rods that are used to control the nuclear reactions. They have a cooling system that keeps the reactor from being damaged by the enormous amount of heat produced. The actual fission of the radioactive fuel occurs in a relatively small part of the reactor known as the core, shown in Figure 11. Nuclear fuel Only certain elements have nuclei that can undergo fission. Naturally occurring uranium contains the isotope U-235 with nuclei that can be split apart. Naturally occurring uranium typically contains 0.72 percent of the isotope U-235. The uranium used in a reactor is enriched so it contains 3-5 percent U-235. The fuel that is used in a nuclear reactor is usually uranium dioxide. Fuel rods The reactor core contains uranium dioxide fuel in the form of tiny pellets like the ones shown in Figure 11. The pellets are about the size of a pencil eraser and are placed endto-end in a fuel rod. The fuel rods are then bundled and covered with a metal alloy. The core of a typical reactor, shown in Figure 12, has about 100,000 kg of uranium contained in fuel rods. For every kilogram of uranium that undergoes fission in the core, 1 g of matter is converted into energy. You would have to burn more than 3 million kg of coal to generate an equivalent energy output.

Measuring Specific Heat

A scientist can calculate the specific heat of a material from the measurements that he or she takes from a calorimeter, such as the one shown in Figure 5. To determine the specific heat of a material using a calorimeter, a scientist measures the mass of a sample of the material and the mass and initial temperature of the water in the calorimeter. The scientist then heats the sample, measures the sample's temperature, and places the sample in the water in the inner chamber of the calorimeter. The sample cools as thermal energy is transferred to thewater, and the temperature of the water increases. The transferof thermal energy continues until the sample and the water are at the same temperature. Then the initial and final temperatures of the water are known, and the amount of heat gained by the water can be calculated from the water's mass, temperature change, and specific heat. At this point, the scientist knows the mass of the substance, the thermal energy change of the substance, and the temperature change of the substance. From there, the specific heat of the unknown substance can be calculated using the thermal energy equation. With this information, the scientist might be able to accurately identify the substance.

Coolant

A substance that can absorb a great amount of thermal energy with little change in temeprature. Water is a coolant, because it can absorb thermal energy without a change in temeprature. Water is a good coolant because the thermal energy has to overcome its strong intermolecular forces before it can move faster, meanwhile metals can move their electrons freely, so no strong attractions need to be overcome.

Isolated and non-isolated systems

A system is isolated if there are no energy transfers between that system and its surroundings. The total energy of an isolated system cannot change. However, recall that energy can be converted between forms. According to the first law of thermodynamics, the thermal energy of an isolated system can still change, as long as the total energy of that system does not change. A system is non-isolated if energy is transferred between the system and its surroundings. For example, a pan on a hot stove is a non-isolated system. This means that the total energy of a nonisolated system can change. However, remember that energy cannot be created or destroyed. Energy can only be transferred from one system to another or converted from one form to another

Open circuits

A typical graphing calculator requires four AAA batteries to operate, as shown in Figure 18. If you remove one the batteries, the calculator will not turn on. Why does removing this battery prevent the calculator from turning on? You might think that the other three batteries do not provide enough electrical energy to power the calculator. However, there is another reason. The batteries in a typical graphing calculator are connected in series. The arrow in Figure 18 shows the path of electric current through the batteries. Because the parts of a series circuit are wired one after another, the amount of current is the same through every part. When any part of a series circuit is disconnected, there is no current through the circuit. This is called an open circuit. Removing a battery from a typical graphing calculator results in an open circuit. Parallel circuits What would happen if your home were wired in a series circuit and you turned off one light? All the other lights and appliances in your home would go out too . This is why houses are wired with parallel circuits. Parallel circuits contain two or more branches for current. Devices on each branch can be turned on or off separately. Look at the parallel circuit in Figure 19. There can be a current through both or either of the branches. Because all branches connect the same two points of the circuit, the voltage difference is the same in each branch. Then, according to Ohm's law, the current is greater through the branches that have lower resistance. When one branch of a parallel circuit is opened, such as when you turn off a light, the current continues through the other branches. Homes use parallel wiring so that individual devices can be turned off without affecting the entire circuit.

Electrical outlets

A voltage difference is also provided at electrical outlets, such as a wall socket. This voltage difference is usually higher than the voltage difference provided by batteries. Most types of household devices are designed to use the voltage difference supplied by a wall socket. In the United States, the average voltage difference across the two holes in a wall socket is usually 120 V. Certain devices, such as clothes dryers, plug into special 240-V electrical outlets. In other areas of the world, the average voltage difference across the two holes of an electric outlet may be 110 V, 240 V, or another value. Resistance If you look inside a computer, a radio, or a telephone, you might find striped objects like those in Figure 15. These objects are called resistors, and they are designed to resist the flow of electrons. Electrical engineers use resistors to reduce the current through all or part of a circuit. Resistors help protect more delicate electronic components. These components can melt or break if too much current is sent through them. Current and resistance When electrons move through a resistor, some energy is transferred to that resistor. Resistance, which is measured in ohms (Ω), is the tendency for a material to resist the flow of electrons and to convert electrical energy into other forms of energy, such as thermal energy. This is why cell phones can get hot during a long telephone call. Almost all materials have some resistance. For example, even copper wires have some resistance. The resistances of long copper wires can noticeably affect the currents through a circuit.

Electric circuits

A water circuit is shown in Figure 13. Water flows out of the tank and falls on a water wheel, causing it to rotate. A pump then lifts the water back up into the tank. The constant flow of water would stop if the pump stopped working. The flow of water would also stop if one of the pipes broke. Then water could no longer flow in a closed loop, and the water wheel would stop rotating. Figure 13 also shows an electric circuit. Just as the water current stops if there is no longer a closed loop, the electric current stops if there is no longer a closed path to follow. An electric circuit is a closed path that electric current follows. If the battery, the lightbulb, or one of the wires is removed from the path in Figure 13, the electric circuit is broken and there will be no current. Current and electron flow For historical reasons, we think of current as being in the direction that positive charge flows. However, in almost all circuits, positive charge does not flow. Instead, it is the negatively charged electrons that actually move through the circuit. Because current is in the direction of positive charge flow, the flow of electrons and the current are in opposite directions. The direction of the electric current is always from higher voltage to lower voltage, but the electrons in a circuit actually flow from lower voltage to higher voltage.

The Disposal of Nuclear Waste

After about three years of use, the amount of U-235 in the fuel pellets in the reactor core is too small for the chain reaction to continue. The fuel pellets left are now referred to as spent fuel. The spent fuel includes radioactive fission products in addition to some leftover U-235. Spent fuel is a form of nuclear waste. Nuclear waste is any radioactive material that results when radioactive materials are used. Reading Check Describe the formation of spent fuel. While many people support the idea of using nuclear energy as an alternative to fossil fuels, they do not necessarily support the idea of nuclear waste disposal in their state. Many people refer to this antinuclear attitude as the "Not in My Backyard" syndrome. Nuclear waste disposal has been a controversial subject and continues to fuel debate about nuclear energy use. Low-level waste Low-level nuclear waste usually contains a small amount of radioactive material. Additionally, low-level waste usually contains radioactive materials with short halflives. Low-level waste is a by-product of electricity generation, medical research and treatments, the pharmaceutical industry, and food preparation. Low-level wastes also include used water and air filters from nuclear power plants and discarded smoke detectors. Low-level waste is kept isolated from people and the environment. It is treated as hazardous material and is stored in spill-safe containers underground. High-level waste High-level nuclear waste is generated in nuclear power plants and by nuclear weapons programs. After spent fuel is removed from a reactor, it is stored in steel-lined concrete pools filled with water, as shown in Figure 16, or in airtight steel or concrete and steel canisters. Many of the radioactive materials in high-level nuclear waste become nonradioactive after a relatively short amount of time. However, the spent fuel also contains materials that will remain radioactive for tens of thousands of years. For this reason, high-level waste must be disposed of in extremely durable, safe, and stable containers. Reading Check Describe What are the differences between low-level and high-level nuclear wastes? One method proposed for the disposal of highlevel waste is to seal the waste in ceramic glass, which is placed in protective metal containers. The containers then are buried hundreds of meters belowground in stable rock formations or in salt deposits.

Nonrenewable Resources

All fossil fuels are nonrenewable resources. Nonrenewable resources are resources that cannot be replaced by natural processes as quickly as they are used. Reading Check Identify three examples of nonrenewable resources. Because they are nonrenewable resources, fossil fuels are decreasing in supply. As supplies of fossil fuels run out, fossil fuels will become more difficult to obtain. This will cause fuel prices to become more costly than they are today. Even as fossil fuel supplies decrease, the demand for energy continues to increase. One way to meet these energy demands is to search for energy alternatives. Scientists have discovered numerous oil shale reserves in the United States, as shown in Figure 9. When oil shale is heated to extremely high temperatures, it releases an organic chemical compound called kerogen. Kerogen is a petroleum-like substance that has potential for meeting increasing energy demands as fossil fuel resources are consumed.

Internal combustion engines

Almost all cars are powered by internal combustion engines. A heat engine that burns fuel inside a set of cylinders is an internal combustion engine. Each cylinder contains a piston that moves up and down. Each up or down movement of the piston is called a stroke. Automobile and diesel engines have four strokes per cycle. Figure 19 shows the four-stroke cycle in an automobile engine.

Thermal Expansion

Almost all substances expand when they are heated and contract wen cooled. (Except water)

Heating Systems

Almost everywhere in the United States, air temperatures at some time become cold enough that people seek out sources of warmth in addition to the Sun. As a result, most modern homes and public buildings contain some type of heating system. The best heating system for any building depends on the local climate and how the building is constructed. The simplest and earliest heating system was probably a campfire. Later, people began burning wood and coal in stoves and furnaces to keep warm. Burning a material transforms chemical energy into thermal energy. Conduction, convection, and radiation then transferred this thermal energy from the stove to the surrounding air. One disadvantage of this system is that energy transfers from the room in which the stove is located to other rooms in the building can be slow. Forced-air systems- The most common type of heating system that people in the United States use today is the forced air system, as shown in Figure 14. In this system, a furnace burns fuel and heats a volume of air. A fan then blows the warm air through a series of large pipes called ducts. The ducts lead to vents in each room. Cool air returns to the furnace through additional vents, and the furnace reheats it. Radiator systems- Before forced-air systems were widely used, radiators heated many homes and buildings. A radiator is a closed metal container that contains hot water or steam. In radiator heating systems, a central furnace heats a tank of water. A system of pipes carries the hot water or steam to radiators in other rooms. Contrary to the name, radiators do not only transfer energy through radiation. Conduction transfers the thermal energy in the hot water or steam to the metal of the radiator and then to the surrounding air. Convection helps spread this energy throughout the room. Electric heating systems- An electric heating system has no central furnace. Instead, electric heating coils transform electrical energy into thermal energy. Portable space heaters contain such coils. Conduction transfers thermal energy from the heating coils to the surrounding air, and convection distributes thermal energy throughout the room.

Air conditioners

An air conditioner operates like a refrigerator, except that warm air from the room is forced to pass over tubes containing the coolant. Thermal energy is transferred from the warm air to the coolant. The thermal energy that is absorbed by the coolant is then transferred to the air outdoors.

Properties of electromagnets

An electromagnet behaves like any other magnet when there is current in the solenoid. One end of the electromagnet is a north pole, and the other end is a south pole. If placed in a magnetic field, an electromagnet will align itself along the magnetic field lines. An electromagnet will also exert a magnetic force magnetic materials and other magnets. Electromagnets are useful because their magnetic properties can be controlled by the user. The strength of the magnetic field can be increased by adding more turns of wire to the solenoid or by increasing the current in the wire. An electromagnet can even be turned off by shutting off the current to the wire. Reading Check Compare and contrast permanent magnets and electromagnets. Making an electromagnet rotate The forces exerted on an electromagnet by another magnet can be used to make the electromagnet rotate. Figure 11 shows an electromagnet suspended between the poles of a permanent magnet. The poles of the electromagnet are repelled by the like poles and attracted by the unlike poles of the permanent magnet. When the electromagnet is in the position shown on the left side of Figure 11, there is a downward force on the left side and an upward force on the right side of the electromagnet. These forces cause the electromagnet to rotate as shown. The electromagnet continues to rotate until its poles are next to the opposite poles of the permanent magnet, as shown on the right of Figure 11. Like a compass needle, the electromagnet will come to rest so that it is aligned in the magnetic field..

Electroscopes

An electroscope is a device that can detect electric charge. One kind of electroscope is shown in Figure 10. This electroscope is made of two thin, metal leaves attached to a metal rod with a metal knob at the top. The leaves are allowed to hang freely from the metal rod. When the device is not charged, the leaves hang straight down, as shown in Figure 10. When the device is charged, electric forces push the leaves apart so they are separated. Charging an electroscope Suppose a negatively charged object touches the knob. Because the metal is a good conductor, electrons travel down the rod into the leaves. Both leaves become negatively charged as they gain electrons, as shown in Figure 11 on the left. Because the leaves have similar charges, they repel each other. When a positively charged object touches the knob of an uncharged electroscope, electrons flow out of the metal leaves and onto the rod. Each leaf then becomes positively charged, as shown in Figure 11 on the right. Once again, the leaves repel each other.

The Wedge

An inclined plane with one or two sloping sides

Deforestation

Approximately 25 percent of Earth's total land area is covered by forests. Whether you are writing on paper with a pencil, sitting in a wooden chair, or wiping your face with a napkin, you are using products derived from wood. This wood comes from forests worldwide. Deforestation is the clearing of forest land for agriculture, grazing, urban development, or logging. It is estimated that the amount of forested land decreases by 94,000 km2 each year. Many of these forests are home to diverse populations of plants and animals. Cutting down trees could lead to the extinction of some of these organisms. In addition, plants remove carbon dioxide from the atmosphere. Deforestation increases the concentration of carbon dioxide in the atmosphere. Scientists believe that an increase in carbon dioxide has contributed to an increase in atmospheric temperatures worldwide. Urban development With a growing population, the percentage of land area devoted to urban development has increased. Highways, office buildings, stores, housing developments, and parking lots are under construction every day. This development can lead to negative impacts on land. For example, paving land prevents water from soaking into the soil. Instead, water runs off into sewers or streams, increasing stream discharge and the threat of flooding. Because water is unable to seep through pavement, this also decreases the amount of water that seeps into the ground. Some communities, businesses, and private organizations preserve areas rather than pave them. As population grows, more urban areas have been set aside for recreation and preservation for future generations to enjoy. Some urban areas have been designated as historic sites, parks, and monuments by the U.S., state, and local governments, such as Central Park in New York City shown in Figure 28. Waste Whether or not you realize it, you impact land when you throw garbage into your trash can. About 55 percent of our garbage is disposed of in sanitary landfills. The rest is recycled or burned. Some of the wastes release substances, such as lead from batteries, that are harmful to humans and animals. Wastes that are poisonous, that cause cancer, or that can catch fire are classified as hazardous wastes

Efficiency of heat engines

Approximately three-fourths of the chemical energy that is transformed into thermal energy in a car engine is never converted into mechanical energy. The inefficiency of heat engines is not due only to friction. Even if friction could be completely eliminated, a typical heat engine still would not be 100 percent efficient. Instead, the efficiency of a heat engine depends on the difference in the temperature of the burning gases in the cylinder and the temperature of the air outside the engine. Increasing the temperature of the burning gases and decreasing the temperature of the surroundings make the engine more efficient.

Lever

Bar that is free to pivot around fixed point (fulcrum) 1st class Lever: Fulcrum is between the input and output force 2nd Class Lever: Output force is between fulcrum and input force 3rd Class Lever: Input force is between fulcrum and output force

Direct and Alternating Currents

Because power outages can occur, some electric devices, such as alarm clocks, use batteries as a backup source of electrical energy. However, the current produced by a battery is different from the current produced by an electric generator. A battery produces a direct current. Direct current (DC) is electric current that is always in one direction through a wire. When you plug an appliance into a wall outlet, it is receiving an alternating current. Alternating current (AC) is electric current that reverses the direction in a regular pattern. In North America, the alternating current in a wall socket has a frequency of 60 Hz and an average voltage of 120 V. Electronic devices that use backup batteries usually require direct current to operate. The device's electronic components reduce the voltage of the alternating current and convert it to direct current.

Population and Carrying

Capacity A population includes all the individuals of one species living in a particular area. You can see in Figure 25 that it took thousands of years for the human population to reach 1 billion people. In the mid-1800s, human population began to increase at a rapid rate because of advances in modern medicine and the availability of clean water and better nutrition. People began to live longer. In addition, the number of births increased because more people survived to a child-bearing age. Carrying capacity Every person alive today uses and is dependent on Earth's natural resources. However, Earth has a carrying capacity. Carrying capacity is the largest number of individuals of a particular species that the environment can support, given the natural resources available. If natural resources are consumed too quickly or the environment becomes threatened, then populations suffer. Unless Earth's natural resources are treated with care, the human population could reach its carrying capacity.

How are machines useful?

Change the way work is done. They can increase speed, change the direction of force, or increase force. Increase Speed: Bicycles increase speed. To increase speed one needs force. If you wanted to go up a hill you can get there faster by riding a bike, but you need to apply more force. Change direction of force: The wedge shape of a blade of an axe. You exert a downward force on an axe to split wood. The blade changes downward force into outward for that splits the wood. Increase Force: A car jack. The upward force exerted on the car is greated than the downard force on the handle. You move the car jack handle faster than you lift the car.

Gravitational potential energy

Consider the blue vase in Figure 9. Together, the blue vase and Earth have potential energy. Gravitational potential energy is energy that is due to the gravitational forces between objects. Gravitational potential energy is often shortened to GPE. Any system that has objects that are attracted to each other through gravity has gravitational potential energy. An apple and Earth have gravitational potential energy. The solar system also has gravitational potential energy. The gravitational potential energy of a system containing just Earth and another object depends on the object's mass, Earth's gravity, and the object's height. Recall that near Earth's surface, g is equal to 9.8 N/kg. Gravitational Potential Energy Equation gravitational potential energy (J) = mass (kg) × gravity (N/kg) × height (m) GPE = mgh Height and gravitational potential energy Look at the bookcase in Figure 9. Imagine that this bookcase is on the second floor of a building and that this building is at the top of a large hill. How should you measure the heights of the objects on the shelves? You could measure the heights from the floor. You could also measure the heights from the ceiling, the ground outside, the bottom of the hill, or Earth's center. To calculate gravitational potential energy, height is measured from a reference level. This means that gravitational potential energy varies depending on the chosen reference level. Relative to the floor, the GPE of a system containing just the blue vase and Earth is about 90 J. Relative to the ceiling, the GPE of this same system might be about -40 J. Relative to Earth's center, this system's GPE is about 300 million J. All these statements are correct. In addition, the GPE of the blue vase-Earth system is greater than the GPE of the green vase-Earth system for every reference level. However, statements such as, "The gravitational potential energy is 100 J," are meaningless, unless a reference level is given.

Other energy transformations

Consider the swing again. Think about what happens when you continue to swing without getting a push or pumping. The swing slows down and eventually comes to a stop. The mechanical energy of the swing-Earth system decreases. At first, it might appear that energy is being destroyed. However, recall that there are other forms of energy aside from mechanical energy. Energy transformations often involve these other forms. The effect of friction If the mechanical energy of the swing Earth system decreases, then some other forms of energy must increase by an equal amount to keep the total amount of energy the same. What could these other forms of energy be? Think about friction and air resistance. With every movement, the swing's ropes or chains rub on their hooks and air pushes on the rider, as illustrated in Figure 14. Friction and air resistance convert some of the mechanical energy into thermal energy. Thermal energy is the energy of heat and hot objects. With every pass of the swing, the temperature of the hooks and the air increases slightly. Mechanical energy is not destroyed. Instead, friction and air resistance transform mechanical energy into thermal energy. This thermal energy is soon transferred to the surrounding air. Reading Check Infer why the wheels of a car get hot when the car is driven. To keep the swing going, you must constantly put energy into the swing-Earth system. You can do this by pumping the swing, transforming the chemical potential energy from the food that you eat into additional mechanical energy

Wheel and Axle

Consists of an axle attached to center of a larger wheel sp that wheel and axle rotate

Household Circuits

Count how many different things in your home require electrical energy. You do not see the wires because most of them are hidden behind the walls, ceilings, and floors. Figure 20 shows a few rooms of a home without the walls, ceilings, or floors so that you can see these wires. This wiring is mostly a combination of parallel circuits. In the United States, the voltage difference in most of the branches is 120 V. In some branches that are used for electric stoves or electric clothes dryers, the voltage difference is 240 V. The main switch and circuit breaker or fuse box serve as an electrical headquarters for your home. Parallel circuits branch out from the breaker or fuse box to wall sockets, major appliances, and lights. Safety devices In a home, many appliances draw current from the same circuit. If more appliances are connected, there will be more current through the wires. As the amount of current increases, so does the amount of heat produced in the wires. If the wires get too hot, the insulation can melt and the bare wires can cause a fire. To protect against overheating of the wires, all household circuits should contain either a fuse or a circuit breaker Fuses An electrical fuse is shown on the left in Figure 21. An electrical fuse contains a small piece of metal that melts if the current becomes too high. When this piece of metal melts, it causes a break in the circuit, stopping the current. To enable charge to flow again in the circuit, the fuse must be replaced. Circuit breakers A circuit breaker is another device that prevents a circuit from overheating and causing a fire. In a circuit breaker, a switch is automatically flipped when the current becomes too great. Flipping the switch opens the circuit and stops the current. Circuit breakers usually can be reset by pushing the switch back to its "on" position. Many residences in the United States contain a box of circuit breakers similar to the one shown on the right in Figure 21.

Energy from inside

Earth Unstable radioactive elements in Earth's core transform nuclear energy into thermal energy. As these unstable elements decay, thermal energy is transferred from the core into Earth's mantle and crust. This is called geothermal heat. Geothermal heat can cause the rock beneath Earth's crust to melt. Molten rock beneath Earth's surface is called magma. The thermal energy that is contained in and around magma is called geothermal energy. Reading Check Identify the process that transforms energy inside Earth into thermal energy. In some areas, Earth's crust has cracks or areas of weakness that allow magma to rise toward the surface. Active volcanoes, for example, permit hot gases and magma from deep within Earth to escape. Perhaps you have seen a geyser, such as Old Faithful in Yellowstone National Park, spewing hot groundwater and steam. The groundwater erupting from the geyser is heated by magma close to Earth's surface. In some areas, hot groundwater is pumped directly into homes to provide warmth. Geothermal power plants Geothermal energy can be converted into electrical energy, as shown in Figure 23. Where magma is close to the surface, the surrounding rocks are hot. Water is pumped into the ground through a well, where it comes into contact with hot rock and changes into steam. The steam then returns to the surface, where it spins a turbine that powers an electric generator. The efficiency of geothermal power plants is about 16 percent. Although geothermal power plants can produce sulfur-based compounds, pumping the water that condenses from steam back into the ground reduces this pollution. This makes geothermal power plants a source of clean energy. However, one disadvantage is that the use of geothermal energy is limited to volcanically active areas where magma is close to the surface. Five states in the United States have geothermal power plants—California, Nevada, Hawaii, Montana, and Utah

Electric fields

Electric charges do not need to be touching to exert forces on each other. An electric field surrounds every electric charge and exerts the force that causes other electric charges to be attracted or repelled. Any charge that is placed in an electric field will be pushed or pulled by the field. Electric fields are usually represented by arrows that indicate how the electric field would make a positive charge move, as shown in Figure 4. Electric field arrows point away from positive charges and toward negative charges. Comparing electric forces and gravitational forces The force of gravity between you and Earth seems to be strong. Yet, compared with electric forces, the force of gravity is very weak. For example, the electric force between a proton and an electron in a hydrogen atom is about a thousand trillion trillion trillion times larger, which is about 1 0 39 times larger, than the gravitational force between these two particles. All atoms are held together by electric forces between protons and electrons that are tremendously larger than the gravitational forces between the same particles. The chemical bonds that form between atoms in molecules are also due to the electric forces between the atoms. These electric forces are much larger than the gravitational forces between the atoms. Reading Check Compare the strength of electric forces and gravitational forces between protons and electrons. However, the electric forces between the objects around you are much less than the gravitational forces between them. Most objects that you see are nearly electrically neutral and have almost no net electric charge. There is usually no noticeable electric force between these objects. But even if a small amount of charge is transferred between objects, the electric force between those objects can be noticeable

Current and Voltage Difference

Electric devices, such as stereos, lights, and toasters, work when there is an electric current through them. Electric current is the net movement of electric charges in a single direction. Electric current is measured in amperes (A). One ampere is equal to one coulomb of electric charge flowing past a point every second. In a metal wire without an electric current, electrons are in constant motion in all directions. As a result, there is no net movement of electrons in one direction. However, when there is an electric current in the wire, electrons continue their random movements but they also drift in the direction of the current. The movement of an electron in an electric current is similar to a ball bouncing down a flight of stairs. Even though the ball changes direction when it strikes a stair, the net motion of the ball is downward. Voltage difference In some ways, the electric force that causes charges to flow is similar to the force acting on the water in a pipe. Water flows from higher pressure to lower pressure, as shown in Figure 12. In a similar way, electric current is from higher voltage to lower voltage. A voltage difference is related to the force that causes electric charges to flow. Voltage difference is measured in volts (V)

Transforming electrical energy

Electric stoves and toasters transform electrical energy into thermal energy. Television transforms electrical energy into radiant and sound energy. The electric motor in a washing machine transforms electrical energy into mechanical energy. Light bulbs transform electrical energy into radiant energy.

Transferring charge

Electrons are bound more tightly to some atoms and molecules. For example, compared to the electrons in the atoms that make up a carpet, electrons are bound more tightly to the atoms that make up the soles of your shoes. Figure 1 shows that when you walk on the carpet, electrons transfer from the carpet to the soles of your shoes. As a result, the soles of your shoes gain an excess of electrons and become negatively charged. The carpet has lost electrons and has an excess of positive charge. Static electricity is the accumulation of excess electric charge on an object.

Potential energy

Energy does not always involve motion. Even motionless objects can have energy. Potential energy is energy that is stored due to the interactions between objects. One example is the energy stored between an apple hanging on a tree and Earth. Energy is stored between the apple and Earth because of the gravitational force between the apple and Earth. Another example is the energy stored between objects that are connected by a compressed spring or a stretched rubber band. Elastic potential energy If you stretch a rubber band and let it go, it sails across the room. As it flies through the air, it has kinetic energy due to its motion. Where did this kinetic energy come from? Just as there is potential energy due to gravitational forces, there is also potential energy due to the elastic forces between the particles that make up a stretched rubber band. The energy of a stretched rubber band or a compressed spring is called elastic potential energy. Elastic potential energy is energy that is stored by compressing or stretching an object. Chemical potential energy The food that you eat and the gasoline in cars also have stored energy. This stored energy is due to the chemical bonds between atoms. Chemical potential energy is energy that is due to chemical bonds. You might notice chemical potential energy when you burn a substance. When an object is burned, chemical potential energy becomes thermal energy and radiant energy. Figure 8 shows the process for burning methane.

The second law of thermodynamics

Energy spontaneously transfers from the warm radiator to the cat in Figure 17. Could the reverse ever happen? Could energy spontaneously transfer from the cat to the warmer radiator? Energy could never spontaneously transfer from the cat to the radiator. This is due to the second law of thermodynamics. The second law of thermodynamics states that energy spontaneously spreads from regions of higher concentration to regions of lower concentration. It is possible to transfer energy from regions of lower concentration to regions of higher concentration, but this process does not happen spontaneously.

Projectile motion

Energy transformations also occur during projectile motion when an object moves in a curved path. Look at Figure 12 and consider the ball-Earth system. When the ball leaves the bat, the ball is moving fast, so the system's kinetic energy is relatively large. The ball's speed decreases as it rises, so the system's kinetic energy decreases. However, the system's gravitational potential energy increases as the ball goes higher. At the top of the ball's path, the system's GPE is larger and kinetic energy is smaller. Then, as the baseball falls, the system's GPE decreases as its kinetic energy increases. However, the mechanical energy of the ball-Earth system remains constant as the ball rises and falls. Swings The mechanical energy transformations for a swing, like the one shown in Figure 13, are similar to the mechanical energy transformations for a roller coaster. The ride starts with a push, which transfers kinetic energy to the rider. As the swing rises, the rider loses speed but gains height. In energy terms, kinetic energy changes to GPE. At the top of the rider's path, GPE is at its greatest. Then, as the swing moves back downward, gravitational potential energy changes back to kinetic energy. At the bottom of each swing, the kinetic energy is at its maximum and the GPE is at its minimum. As the rider swings back and forth, energy is continually transformed between kinetic energy and GPE. However, the rider swings less and less on each cycle unless he or she pumps the swing or gets someone to provide a push. What is happening to the rider's mechanical energy?

Energy transformations with Projectile motion

Energy transformations also occur during projectile motion when an object moves in a curved path. When the ball leaves the bat, the ball is moving fast, so the system's kinetic energy is relatively large. The ball's speed decreases as it rises so the system's kinetic energy decreases. However, the system's gravitational potential energy increases as the ball goes higher. At the top of the ball's path, the system's GPE is larger and kinetic energy is smaller. As the base ball falls the GPE decreases as its kinetic energy increases. However, the mechanical energy of the ball-Earth system remains constant as the ball rises and falls.

Transforming electrical energy

Energy transformations can also involve electrical energy. Think about all the electric devices that you use every day. Electric stoves and toasters transform electrical energy into thermal energy. Televisions transform electrical energy into radiant energy and sound energy. The electric motor in a washing machine transforms electrical energy into mechanical energy. Lightbulbs transform electrical energy into radiant energy. Figure 15 illustrates the energy change that occurs in a lightbulb. What other devices have you used today that make use of electrical energy? You might have been awakened by an alarm clock, styled your hair, made toast, listened to music, or played a video game. What form or forms of energy is electrical energy converted to in each of these examples? Transforming chemical potential energy Fuel stores energy in the form of chemical potential energy. For example, most cars run on gasoline, which has chemical potential energy. A car engine transforms this chemical potential energy into thermal energy and then into mechanical energy for the car's motion. A car engine also gets very hot when it is used. This is evidence that much of the thermal energy is never converted to mechanical energy. Some energy transformations are less obvious because they do not result in visible motion, sound, heat, or light. Every green plant converts radiant energy into chemical potential energy. If you eat an ear of corn, the chemical potential energy from the corn is transferred to your body. Your body then extracts this energy for functions such as breathing, pumping blood, moving, speaking, and thinking.

Charging by Contact

Every example of charging that has been discussed so far is an example of charging by contact. Charging by contact is the process of transferring charge by touching or rubbing. Rubbing two materials together can result in a transfer of electrons. Then one material is left with a positive charge and the other with an equal amount of negative charge. Charging by Induction Because electric forces act at a distance, charged objects brought near a neutral object will cause electrons to rearrange their positions on the neutral object. Suppose you charge a balloon by rubbing it with a cloth. If you bring the negatively charged balloon near your sleeve, the extra electrons on the balloon repel the electrons in the sleeve. The electrons near the sleeve's surface move away from the balloon, leaving a positively charged area on the surface of the sleeve, as shown in Figure 7. The rearrangement of electrons on a neutral object caused by a nearby charged object is called charging by induction. Reading Check Contrast charging by contact with charging by induction. Lightning How is getting shocked when you touch a metal doorknob similar to lightning? Both are static discharges. A static discharge is a transfer of charge between two objects because of a buildup of static electricity. A thundercloud is a mighty static electricity generator. As air masses move and swirl in the cloud, areas of positive and negative charge build up. Eventually, enough charge builds up to cause a static discharge between the cloud and the ground. As the electric charges move through air, they collide with atoms and molecules. These collisions cause the atoms and molecules in the air to emit light. You see this light as a flash of lightning, as shown in Figure 8.

Explain whether the following statement is true: If the thermal energy of an object increases, the temperature of the object must also increase.

False. The increase in the object's thermal energy could be a potential energy increase. Heat energy is actually made up partly of kinetic energy and partly of potential energy. In a solid, for example, it's the kinetic energy and potential energies of the atoms as they wiggle around. When something is really hot, it's because all of the atoms inside of it (the tiny bits of stuff that make it up) are wiggling around a lot. And the colder it is, the less the atoms wiggle. When the atoms wiggle, they have kinetic energy because they are moving. They also have potential energy because the spacing between the atoms is changing as they wiggle; as you stretch or squeeze the distance, you store potential energy just like when you stretch or squeeze a spring.

Electricity

Figure 6 shows that almost 70 percent of the electrical energy used in the United States is produced by burning fossil fuels, such as coal. How is the chemical potential energy stored in fossil fuels converted to electrical energy in a power plant? The process of energy conversion is shown in Figure 7. Fuel burned in a boiler or combustion chamber converts chemical potential energy into thermal energy, which heats water and produces pressurized steam. This steam strikes the blades of a turbine, causing it to spin, converting thermal energy into mechanical energy. The shaft of the turbine connects to an electric generator, which converts mechanical energy into electrical energy. The electrical energy is then transmitted to homes, schools, and businesses through power lines. Power plant efficiency In the power plant, not all of the chemical potential energy stored in fuel is converted into electrical energy. Some energy is converted into thermal energy. As a result, no stage of the process of electricity production is 100 percent efficient. The overall efficiency of a fossil fuel-burning power plant is roughly 35 percent. This means that only 35 percent of the energy stored in fossil fuels is transported to homes, schools, and businesses as electrical energy. The remaining 65 percent is converted into thermal energy. Often, this heat is released into the environment.

Transforming Chemical Potential Energy

Fuel stores energy in the form of chemical potential energy. Most cars run on gasoline, which has chemical potential energy. A car engine transforms this chemical potential energy into thermal energy and then into mechanical energy for the car's motion. A car engine gets very hot when it is used, which proves most of the thermal energy is never converted into mechanical energy. Green plants converts radiant energy into chemical potential energy. When you eat food, chemical potential energy is transferred to your body which extracts it for bodily functions.

The Cost of Fossil

Fuels Although fossil fuels are common energy resources, their uses have undesirable effects. Burning fossil fuels releases small particulates into the atmosphere that can cause breathing problems. Fossil fuels also release carbon dioxide (CO2) when they are burned. Figure 8 shows how the CO2 concentration in the atmosphere has increased from 1958 to 2010. Many scientists think this increase in atmospheric CO2 concentrations has contributed to global warming.

Pulley

Grooved wheel with a rope or chain or cable running along the cable Fixed pulley: Attached to something that does not. Moveable Pulley: One end of the rope is fixed and wheel is free to move. Block and table- System of pulleys consisting of fixed and moveable pulley

Specific Heat of a material is

Have you ever been to the beach during the summer? The ocean was probably cool, but the sand was probably hot. Energy from the Sun falls on the water and sand at nearly the same rate. However, the Sun's energy changes the sand's temperature more quickly than the water's temperature. A substance's temperature changes when that substance absorbs thermal energy. This temperature change depends on the amount of thermal energy that the substance absorbs and the mass of the substance. This temperature change also depends on the nature of the substance. The specific heat of a material is the amount of heat needed to raise the temperature of 1 kg of that material by 1°C. Scientists measure specific heat in joules per kilogram degree Celsius [J/(kg · °C)]. Table 1 compares the specific heats of some familiar materials. Compare water with iron in Table 1. Water has a very high specific heat. Metals, such as iron, have low specific heats. To raise equal masses of water and iron 1°C, water must absorb almost 10 times more thermal energy than iron. Figure 4 explains why this is so. Thermal Energy Equation: Change in thermal energy (J)= mass (kg) x temperature (Celsius) x specific heat (J/Kg xc) (Q= m(Tf - Ti)C) m= mass C= specific heat

Alternating Current and Direct Current

Have you ever seen an AC/DC adapter like the one in Figure 16? This is a special piece of equipment that allows you to plug a battery-powered device into a wall socket. Most chargers for portable music players, laptops, and cell phones are AC/DC adapters. Alternating current The AC in AC/DC stands for alternating current. This is the kind of current from household electrical outlets. In the United States, electric power grids are built so that alternating current changes direction 120 times each second. Household appliances, such as toasters and hair dryers, are built to use alternating current. Direct current The DC in AC/DC stands for direct current. Battery-powered devices, such as flashlights, use direct current. Unlike alternating current, direct current never changes direction. An AC/DC adapter is a device that converts the alternating current of an electrical outlet into direct current. With AC/DC adapter, you can charge a cell phone battery with the current from an electrical outlet.

Series and Parallel Circuits

Have you ever wondered how a hair dryer works? If you were to look inside one, you would see the components of an electric circuit. You would see wires, an electric motor, a heating element, and other components. You can see the parts of a circuit if you look inside any electrical device, whether that device is a video-disc player, a flashlight, or a computer. As a safety precaution, you should always check the instruction manual before investigating a device's circuitry. In addition, you should always unplug a device and remove its batteries before investigating that device's circuitry. Most real circuits are not simple circuits with only one power source, one device, and one path for the current. Multiple power sources, multiple devices, and multiple paths for the current are very common. The two main types of more complex circuits are series circuits and parallel circuits. Series circuits One kind of circuit is called a series circuit. A series circuit is an electric circuit with only one branch, as shown in Figure 17. Series circuits are used in flashlights. Because the parts of a series circuit are wired one after another, the amount of current is the same through every part. When any part of a series circuit is disconnected, there is no current through the circuit.

The nuclear chain reaction

How does a fission reaction proceed in the reactor core? As U-235 nuclei undergo fission, neutrons are released and are absorbed by other U-235 nuclei. When a U-235 nucleus absorbs a neutron, it splits into two smaller nuclei and two or three free neutrons, as shown in Figure 13. These neutrons strike other U-235 nuclei, triggering the release of more neutrons, and fission continues. Because every uranium atom that splits apart releases free neutrons that cause other uranium atoms to split apart, this process is called a nuclear chain reaction. In the chain reaction, the number of nuclei that are split can more than double at each stage of the process. As a result, an enormous number of nuclei can be split after only a small number of stages. For example, if you start with one uranium nucleus and the number of nuclei involved doubles at each stage, after only 50 stages, more than a quadrillion nuclei might be split. Nuclear chain reactions take place in a matter of milliseconds. If the process is not controlled, the chain reaction could release a tremendous amount of energy in the form of an explosion. A constant rate To control the chain reaction, some of the neutrons that are released when U-235 splits apart must be prevented from colliding with other U-235 nuclei. These neutrons are absorbed by control rods containing boron or cadmium inserted into the reactor core, as shown in Figure 11. Moving these control rods deeper into the reactor causes them to absorb more neutrons and to slow down the chain reaction. Eventually, only one of the neutrons released in the fission of each of the U-235 nuclei strikes another U-235 nucleus, so energy is released at a constant rate.

Energy Resources

How many different ways have you relied on energy resources today? You can see energy being used in many ways throughout the day, such as those shown in Figure 1. Furnaces and stoves use thermal energy to heat buildings and to cook food, respectively. Air conditioners use electrical energy to cool homes. Cars and other vehicles use mechanical energy to transport people and materials from one area to another. Energy transformation According to the law of conservation of energy, energy cannot be created or destroyed. Energy can only be transformed from one form to another. To use energy means to transform energy from one form into another. For example, you use energy when the chemical potential energy from coal, oil, or natural gas is transformed into thermal energy that is used to heat your home. Sometimes, energy is transformed into a form that is not useful. For example, when an electric current travels through power lines, about 10 percent of the electrical energy is converted to thermal energy. This reduces the amount of useful electrical energy that is delivered to homes, schools, and businesses Energy use in the United States In 2009, more energy was used annually in the United States than in any other country in the world. Figure 2 shows energy use in the United States in 2008. About 22 percent of the energy was used in homes for heating and cooling, to run appliances, to provide lighting, and for other household needs. About 28 percent was used for transportation, powering vehicles such as cars and planes. Another 19 percent was used by businesses to heat, cool, and light shops and buildings. About 31 percent of this energy was used by industry and agriculture for manufacturing and food production. As shown in Figure 2, about 85 percent of the energy used in the United States in 2008 came from burning fossil fuels. Nuclear power plants provided 8 percent, and alternative energies supplied 7 percent.

The effect of friction

If the mechanical energy of the swing-earth system decreases, then some other forms of energy must increase by an equal amount to keep the total amount of energy the same. With every movement the swing's ropes or chains rub on their hooks and air pushes on the rider. Friction and air resistance convert some of the mechanical energy into thermal energy. Thermal energy is the heat of energy and hot objects. With every pass of the swing, the temperature of the hooks and air increases slightly. Mechanical energy is transformed into thermal energy that is released into the air.

Converting Thermal Energy into Mechanical Energy

If you push a book sitting on a table, the book will slide andcome to a stop. Friction between the book and the table converted the book's mechanical energy into thermal energy. In the example above, mechanical energy was converted completely into thermal energy. Is it possible to do the reverse and convert thermal energy completely into mechanical energy? Remember that a system's mechanical energy is related to the motion of the objects in that system as well as the interactions between the objects of that system. A system's thermal energy is related to the motions and interactions of all the particles in that system. Because there are far more particles than objects in a system, thermal energy is spread among more particles than objects than mechanical energy. As a result, mechanical energy tends to transform into thermal energy. Therefore, it is not possible to completely convert thermal energy into mechanical energy.

Conductors and Insulators

If you reach for a metal doorknob after walking across a carpet, you might see a spark. The spark is caused by electrons moving from your hand to the doorknob, as shown in Figure 5. Recall that electrons were transferred from the carpet to your shoes. How did these electrons move from your shoes to your hand? Conductors A conductor is a material through which electrons move easily. Electrons on your shoes repel each other, and some are pushed onto your skin. Because your skin is an effective conductor, the electrons spread over your skin, including your hand. The best electrical conductors are metals. The atoms that make up metals have electrons that are able to move easily through the material. In Figure 5, the electrons move from your hand to the doorknob because the doorknob is made of metal. What would happen if the doorknob were made of glass? Insulators Glass is an insulator. An insulator is a material in which electrons are not able to move easily. Electrons are held tightly to the atoms that make up insulators. Most plastics are insulators. The plastic coating around electric wires, such as the one shown in Figure 6, prevents dangerous electric shocks. If the doorknob in Figure 5 were made of glass, you would not experience an electric shock from moving charges

Electric Current and Magnetism

In 1820, Danish physics teacher Hans Christian Oersted found that electricity and magnetism are related. While doing a demonstration involving electric current, he happened to have a compass near an electric circuit. He noticed that the electric current affected the direction of the compass needle. Oersted hypothesized that the electric current must produce a magnetic field around the wire and that the direction of the field depends on the direction of the current. Oersted's hypothesis that an electric current creates a magnetic field was correct. It is now known that all moving charges, like those in an electric current, produce magnetic fields. Around a current-carrying wire, the magnetic field lines form circles, as shown in Figure 9. The direction of the magnetic field around the wire reverses when the direction of the current in the wire reverses. The strength of the magnetic field can be increased by increasing the current in the wire. Also, as you move farther away from the wire, the strength of the magnetic field decreases.

Magnetic domains—a model for magnetism

In magnetic materials such as iron, the magnetic fi eld created by each atom exerts a force on other nearby atoms. Th ese forces cause large groups of neighboring atoms to align. Th is means that almost all north magnetic poles in the group point in the same direction. These groups of atoms with aligned magnetic poles are called magnetic domains. Because the magnetic poles of the individual atoms are aligned, the domain itself behaves like a larger magnet with a north pole and a south pole. Random arrangement of domains An iron nail contains an enormous number of these magnetic domains. So, why does the nail not behave like a magnet? Even though each domain behaves like a magnet, the poles of the domains are arranged randomly and point in different directions, as shown on the left in Figure 7. As a result, the magnetic fields from the domains cancel each other and the nail is not magnetic. Lining up domains If you place a nail in a magnetic field, many of the domains align in the direction of the magnetic field, as shown on the right in Figure 7. The like poles of many of the domains point in the same direction and no longer cancel each other out. The nail itself now acts as a magnet. Permanent magnets When domains align, their magnetic fields add together and create a magnetic field inside the material. In a permanent magnet, this field is strong enough to prevent the constant motion of the atoms from bumping the domains out of alignment. A permanent magnet is any magnet whose domains remain aligned without an external field.

Fossil Fuel Formation

In one hour of driving, a car might use two or three gallons of gasoline. It might be hard to believe that it took millions of years to make the fuels that are used to power your car, to produce electricity, and to heat your home. Coal, natural gas, and petroleum, also called crude oil, are fossil fuels because they form from the remains of ancient plants and animals that were buried and altered over millions of years. Combustion reactions When fossil fuels are burned, a combustion reaction occurs. During this reaction, carbon and hydrogen atoms combine with oxygen in the air to form carbon dioxide and water. This process converts the chemical potential energy that is stored in the bonds between atoms into thermal energy and light. Compared to wood, the energy stored in fossil fuels is much more concentrated. In fact, burning 1 kg of coal releases two to three times more energy than burning 1 kg of wood. Figure 3 shows the energy content of different fuels

Batteries

In order to keep water flowing continually in a water circuit, a pump is used to provide a pressure difference. In a similar way, to keep electric charges continually flowing in an electric circuit, a voltage difference needs to be maintained in the circuit. Power supplies, such as the batteries shown in Figure 14, can provide this voltage difference. Dry-cell batteries You are probably most familiar with dry-cell batteries. The cylindrical batteries that are placed in flashlights are dry-cell batteries. A cell consists of two electrodes surrounded by a material called an electrolyte. The electrolyte enables charges to move from one electrode to the other. Look at the dry cell shown in Figure 14. One electrode is the carbon rod, and the other is the zinc container. The electrolyte is a moist paste containing several chemicals. When the two terminals of a dry-cell battery are connected in a circuit, a chemical reaction occurs. Electrons are transferred between some of the compounds in this chemical reaction. As a result, the carbon rod becomes positive, forming the positive terminal. Electrons accumulate on the zinc, making it the negative terminal. The voltage difference between these two terminals causes a current through a closed circuit. Wet-cell batteries A wet-cell battery is another common type of battery. A wet cell, like the one shown in Figure 14, contains two connected plates made of different metals or metallic compounds in an electrolyte. Unlike for the dry cell, the electrolyte in a wet cell is a conducting liquid solution. A wet-cell battery contains several wet cells connected together. The most common type of wet-cell battery in use today is a car battery Lead-acid batteries Most car batteries are lead-acid batteries, like the wet-cell battery shown in Figure 14. A lead-acid battery contains a series of six wet cells composed of lead and lead dioxide plates in a sulfuric acid solution. The chemical reaction in each cell provides a voltage difference of about 2 V, giving a total voltage difference of 12 V.

The Screw

Inclined plane wrapped around in a spiral around cylindrical post.

What is output and input force?

Input: Force that a person or a device such as a motor applies to the machine. Output: Force that the machines applies to another object.

A system?

Is anything around which you can imagine a boundary. Can be a single object such as a tennis ball, or a group of objects such as the solar system. When one system does work on a second system then energy is transferred from one system to the next.

Doing Work to Transfer Thermal Energy

Is it possible to transfer thermal energy from a cooler area to a warmer area? You may think that the answer is no due to the second law of thermodynamics. However, thermal energy can be transferred from a cooler area to a warmer area if work is done in the process. Refrigerators and air conditioners function based on this principle.

Doing Work to Transfer Thermal Energy

Is it possible to transfer thermal energy from a cooler area to a warmer area? You may think that the answer is no due to the second law of thermodynamics. However, thermal energy can be transferred from a cooler area to a warmer area if work is done in the process. Refrigerators and air conditioners function based on this principle. Refrigerators A refrigerator does work as it transfers thermal energy from inside the cool refrigerator to the warmer room. The energy to do the work comes from the electrical energy the refrigerator obtains from an electrical outlet. A refrigerator makes the room that it is in warmer. Figure 20 shows how a refrigerator operates. Liquid coolant is pumped through an expansion valve and changes into a gas. When the coolant expands as it changes into a gas, it pushes outward on its surroundings. This means that the coolant does work on its surroundings. As a result, the coolant transfers energy to its surroundings and the coolant cools. The cold gas is pumped through pipes inside the refrigerator, where it absorbs thermal energy from the area where you keep your food. When this happens, the inside of the refrigerator cools. The gas then is pumped to a compressor that does work on the gas by compressing it. This makes the gas warmer than the temperature of the room. The warm gas is pumped through the condenser coils. Because the gas is warmer than the room, thermal energy spreads from the gas to the room. As the gas heats the room, it cools and changes back to a liquid and enters the expansion valve again. The cycle is then repeated. Air conditioners An air conditioner operates like a refrigerator, except that warm air from the room is forced to pass over tubes containing the coolant. Thermal energy is transferred from the warm air to the coolant. The thermal energy that is absorbed by the coolant is then transferred to the air outdoors. Heat pumps A heat pump is a two-way air conditioner. In warm weather, a heat pump operates as an ordinary air conditioner. It does work to transfer thermal energy from the cooler building to the warmer outdoors. This cools the building while warming the outdoors very slightly. In cold weather, a heat pump operates like an air conditioner in reverse. It does work to transfer thermal energy from the cooler outdoors to the warmer building. This warms the building while cooling the building's surroundings very slightly. Energy transformations and thermal energy Many energy transformations occur around you every day that convert one form of energy into a more useful form. However, when these energy transformations occur, some of the energy is usually converted into thermal energy. For example, friction converts mechanical energy into thermal energy when the electric generators in Figure 21 rotate. A laptop computer converts electrical energy into thermal energy. The thermal energy from these energy transformations is no longer in a useful form and is transferred to the surroundings by conduction and convection.

Energy from water

Just as the expansion of steam can spin a turbine and power an electric generator, rapidly moving water can do the same. The gravitational potential energy of water is great when the water is retained by a dam. This energy is released when the water flows through tunnels near the base of the dam. Figure 19 shows how the rushing water spins a turbine, converting gravitational potential energy to mechanical energy and then to electrical energy. Dams built for this purpose are called hydroelectric dams. Hydroelectricity Electric current produced from the energy of moving water is called hydroelectricity. About nine percent of the electrical energy used in the United States comes from hydroelectric power plants. Hydroelectric power plants convert mechanical energy into electrical energy with almost no pollution. They are almost twice as efficient as fossil fuel-burning or nuclear power plants. In addition to efficiency, another advantage of hydroelectric power is that the bodies of water held back by dams can form lakes that can provide water for drinking and crop irrigation. These lakes can also be used for boating and swimming. After the initial cost of dam construction, hydroelectric power plants are more cost effective than other energy resources. However, dams and hydroelectric power plants can disturb the balance of natural ecosystems. Some species of fish that live in the ocean migrate back to the rivers in which they were hatched to breed. This migration can be blocked by dams, causing a decline in the fish population. Fish ladders, such as those shown in Figure 20, have been designed to enable fish to migrate upstream past some dams. Also, operation of a hydroelectric power plant can change the temperature of the water, which affects plant and animal habitats. Finally, river sediments can build up behind the dam and affect life downstream.

What are the six simple machines

Lever, pulley, wheel and axle, inclined plane, screw and wedge. The pulley and the wheel and axle are modified levers, and the screw and wedge are modified inclined planes.

Impact on Air

Like water, air is essential for all life on Earth. Air pollution can affect human health and threaten plants and animals. Air pollution comes from natural and manufactured sources. For example, cars, buses, and trucks burn fuel for energy and release exhaust into the atmosphere. Factories and power plants emit pollutants during production, as shown in Figure 31. Dust from farms and construction sites also contributes to air pollution. Natural sources of pollution include particles and gases emitted into air from erupting volcanoes and forest fires. Types of air pollution Have you ever observed a thick, brown haze on the horizon? The brown haze that you see forms from vehicle exhaust and factory and power plant pollution. This haze is often referred to as photochemical smog. Photochemical smog is a term used to describe the pollution that results from the reaction between sunlight and vehicle or factory exhaust. Smog Major sources of photochemical smog include cars, factories, and power plants. Pollutants are released into the air when fossil fuels, such as gasoline are burned, as shown in Figure 32, emitting sulfur-, nitrogen-, and carbon-based compounds. These compounds react with oxygen in the presence of sunlight. One of the products of this reaction is ozone (O3). Ozone that forms high in the atmosphere protects you from ultraviolet (UV) radiation from the Sun. Ozone near Earth's surface, however, can cause breathing problems. CFCs The protective ozone high in the atmosphere is concentrated in a layer roughly 20 km above Earth's surface. This layer is called the ozone layer, and it is at risk of being destroyed. Chlorofluorocarbons (CFCs) are compounds that leak from old air conditioners and refrigerators and react with ozone. This reaction destroys ozone molecules. Even though the use of CFCs has been declining due to environmental laws, these compounds can remain in the atmosphere for decades. Acid precipitation When sulfur-, nitrogen-, and carbon-based compounds from vehicles and factories react with moisture in the air, they form acids. When acidic moisture falls from the sky as precipitation, it is called acid precipitation. Acid precipitation can corrode metals and cause harm to plants and animals.

Efficiency

Machines can increase force or increase speed. You might think this means that you get more work out of a machine than you put into a machine because work is related to force and motion. However, no machine can increase both force and speed at the same time. In fact, you always put more work into a machine than you get out of that machine. This is a fundamental scientific law that cannot be broken by building better machines. Efficiency is the ratio of output work to input work. Efficiency is often measured in percent. Efficiency Equation efficiency (%) = ___ output work (in joules) input work (in joules) × 100 e = _ Wout Win × 100 Machines can be made more efficient by reducing friction. This is usually done by adding a lubricant, such as oil or grease, to surfaces that rub together. However, all machines are less than one hundred percent efficient.

Petroleum

Millions of liters of petroleum, a fossil fuel, are pumped from wells within Earth's crust every day. Petroleum is a flammable liquid formed from the decay of ancient organisms, such as microscopic plankton and algae. It is a mixture of thousands of chemical compounds. Most of these compounds are hydrocarbons, which means that their molecules are made of different arrangements of carbon and hydrogen atoms. Fractional distillation Hydrocarbon compounds found in petroleum differ based on the number and arrangement of carbon and hydrogen atoms. The composition and structure of a hydrocarbon determines its chemical and physical properties. The many different hydrocarbon compounds found in petroleum can be separated in a process called fractional distillation. This separation occurs in distillation towers at oil refineries. First, petroleum is pumped into the bottom of the tower and heated. The chemical compounds in the petroleum boil at different temperatures. Materials with the lowest boiling points rise to the top of the tower as vapor and are collected from the tower. Hydrocarbons with high boiling points, such as asphalt and waxes, remain liquid and are drained from the bottom of the tower. Petroleum use Petroleum supplies nearly 38 percent of all energy generated in the United States each year. However, about 15 percent of petroleum-based materials in the United States are not used for fuel. Look at the materials in your home or classroom. Do you see any plastics? In addition to fuel, plastics, synthetic fabrics, cosmetics, and medicines, such as those shown in Figure 4, are made from petroleum. Also, lubricants such as grease and motor oil, as well as wax-based products and asphalt, are made from petroleum

Magnets

More than 2,000 years ago, the Greeks discovered deposits of a mineral that was a natural magnet. They noticed that chunks of this mineral could attract pieces of iron. This mineral was found in a region of Turkey that was known as Magnesia, so the Greeks called the mineral magnetic stone. This mineral is now called magnetite. Since the discovery of magnetite, many devices have been developed that rely on magnets. In the twelfth century, Chinese sailors used magnetite to make compasses that improved navigation. Figure 1 shows common magnets and familiar devices that use magnets today. In science, magnetism refers to the properties and interactions of magnets. Magnetic force You probably have played with magnets and might have noticed that two magnets exert forces on each other. Depending on which ends of the magnets are close together, the magnets either repel or attract each other. You might have noticed that the interaction between two magnets can be felt even before the magnets touch. The strength of the force between two magnets increases as magnets move closer together and decreases as the magnets move farther apart. Magnetic strength You might also have noticed that some magnets are stronger than others. For example, a magnet from your refrigerator can be used to pick up paper clips, but a much stronger magnet would be needed to lift a car.

Generators

Most of the electrical energy that you use every day is provided by generators. A generator uses electromagnetic induction to transform mechanical energy into electrical energy. Figure 18 shows a hand-operated generator being used to power a flashlight. The mechanical energy is provided by turning the handle on the generator. The generator transforms the mechanical energy into electrical energy, which lights the bulb. Simple generators A diagram of a simple generator is shown in Figure 19. In this type of generator, a current is produced in the coil as the coil rotates between the poles of a permanent magnet. As the generator's wire coil rotates through the magnetic field of the permanent magnet, a current is induced in the coil. After the wire coil makes one-half of a revolution, the ends of the coil are moving past the opposite poles of the permanent magnet. This causes the current to change direction. As the coil keeps rotating, the current that is produced continues to change direction. The direction of the current in the coil changes twice with each revolution. The frequency with which the current changes direction can be controlled by regulating the rotation rate of the generator. In the United States, electrical energy for residential use is produced by generators that rotate 60 times a second (60 Hz). Therefore, the current changes direction 120 times per second. Using electric generators Generators similar to the one in Figure 19 are used in cars, where they are called alternators. The alternator provides electrical energy to operate lights and other accessories. Spark plugs in the car's engine also use this electrical energy to ignite the fuel in the cylinders of the engine. Once the engine is running, it provides the mechanical energy that is used to turn the coil in the alternator

Electromagnetism

Moving electric charges are surrounded by magnetic fields. As a result, there is a force between the moving electric charge and magnets. These interactions led scientists to the realization that the electric force and the magnetic force are parts of the same force, called the electromagnetic force. The electromagnetic force is the attractive or repulsive force between electric charges and magnets. Like gravity, the electromagnetic force is one of the fundamental forces. The interaction between electric charges and magnets is called electromagnetism. Electromagnetism is what makes magnets so useful. Many devices, including MP3 players, operate because of these interactions. Electromagnetism is also essential in producing, transmitting, and using electricity. Electromagnets The magnetic field around a current-carrying wire can be made stronger by changing the shape of the wire. When there is a current in a wire loop, such as the one shown on the left in Figure 10, the magnetic field inside the loop is stronger than the field around a straight wire. A single wire wrapped into a cylindrical wire coil is called a solenoid. The magnetic field inside a solenoid is stronger than the field in a single loop. The magnetic field around each loop in the solenoid adds together to form the field shown in Figure 10 . An electromagnet is a temporary magnet created when there is a current in a wire coil. Often, the current-carrying coil is wrapped around an iron core, as shown on the right in Figure 10. The coil's magnetic field temporarily magnetizes the iron core. As a result, the electromagnet's field can be more than 1,000 times greater than the field of the solenoid.

National and state parks

National and state parks are areas of land, like those shown in Figure 29, that are preserved and protected by the U.S. government. These forests, wetlands, grasslands, and parks in the United States are safe from urban development, waste disposal, and extensive deforestation. Parks are home to plants, animals, and waterways. Millions of people visit parks, such as Grand Canyon National Park each year. Many countries throughout the world also set aside land for protection and preservation. As the world population grows, the impact on land may worsen. Preserving this land in its natural state will benefit generations to come. Impact on Water Life on Earth would not be possible without water. Plants need water to convert radiant energy into food energy. Some animals, such as fish, frogs, and whales, make bodies of water their homes. Approximately 60 percent of the human body is composed of water. How are living things affected when water becomes polluted? Sources of water pollution Many streams and lakes in the United States are polluted. Polluted water contains harmful chemicals and sometimes organisms that cause disease. Water can also be polluted with sediments, such as silt and clay. Sediment from runoff makes water cloudy and can limit the sunlight and oxygen supply, which then affects fish and wildlife. Industry Mining can release metals into water. Metals such as mercury, lead, nickel, and cadmium are poisonous. However, environmental laws limit the amount of these harmful chemicals that can be released into the environment, and they protect natural resources and the people that depend upon them. Oil and gas Oil and gas can run off roads and parking lots into lakes and rivers when it rains. It can also leak from oil tankers or pipelines associated with offshore drilling, as shown in Figure 30. Oil and gas are pollutants, which can cause cancer. Today, environmental laws require that all new gasoline storage tanks have a double layer of steel or fiberglass to prevent spills. These laws help protect soil and water from oil spills. Human waste When you flush a toilet or take a shower, you create wastewater. Wastewater, also called sewage, contains human waste, household detergents, and soaps. Sewage contains harmful organisms that can make people ill. In most cities in the United States, underground pipes route water from homes, schools, and businesses to sewage treatment plants. Sewage treatment plants remove pollutants in a series of steps. These steps purify the water by removing solid materials from the sewage, killing harmful microorganisms, and reducing the amount of nitrogen and phosphorous in the water. The water is then recycled back into the environment.

Thunder

Not only does lightning produce a brilliant flash of light, it also generates powerful sound waves. The electrical energy in a lightning bolt rips electrons off atoms in the atmosphere and produces great amounts of heat. The surrounding air temperature can rise to about 30,000°C, several times hotter than the Sun's surface. The heat causes air in the lightning bolt's path to expand rapidly, producing sound waves that you hear as thunder. Grounding Lightning strikes can cause power outages, injury, fires, and loss of life. The sensitive electronics in a computer can be harmed by large static discharges. A discharge can occur any time that charge builds up in one area. Providing a path for charge to reach Earth prevents any charge from building up. Earth is a large, neutral object that is also a conductor of charge. Any object connected to Earth by a good conductor will transfer any excess electric charge to Earth. Connecting an object to Earth with a conductor is called grounding. Reading Check Explain the purpose of grounding. Buildings often have a metal lightning rod that provides a conducting path from the highest point on the building to the ground to prevent damage by lightning, as shown in Figure 9. Plumbing fixtures, such as metal faucets, sinks, and pipes, often provide a convenient ground connection. Look around. Do you see anything that might act as a path to the ground?

The release of radioactivity

Nuclear power plants around the world operate safely every day. However, one of the serious risks of nuclear power is the release of harmful radiation from power plants. The fuel rods contain radioactive elements. Some of these radioactive elements could harm living organisms if they are released from the reactor core of a nuclear power plant. To prevent accidents, nuclear reactors have elaborate systems of safeguards, strict safety precautions, and highly trained workers. In spite of this, accidents have still occurred. For example, an accident occurred when a reactor core at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine, overheated during a routine safety test on April 26, 1986. Materials in the core caught fire and caused a chemical explosion that blew a hole in the reactor, as shown in Figure 15. This resulted in the release of radioactive materials that were carried by winds and deposited over a large area. As a result of the accident, 50 people died from acute radiation sickness and about 4,000 cancer-related cases have been attributed to the release of radioactivity from the explosion. The World Health Organization estimates that approximately 600,000 people were exposed to levels of radiation that continue to pose a risk to their health. Newer nuclear power plants are designed to prevent accidents like the one that occurred at Chernobyl. But there is always a possibility that an accident might occur.

Nuclear Power Plants

Nuclear power plants produce an electric current in a way that is similar to fossil fuel-burning power plants. As shown in Figure 14, the thermal energy released in fission is used to heat water and produce pressurized steam. To transfer thermal energy from the reactor core, the core contains a fluid coolant. The hot coolant is pumped into a heat exchanger. In the heat exchanger, thermal energy is transferred from the hot coolant to water, causing the water to boil and produce pressurized steam that spins a turbine. When the steam leaves the turbine, it enters a chamber where it is condensed back into liquid water. Cool water absorbs the thermal energy released during condensation. The thermal energy is then carried to the cooling tower where it is released into the environment. The overall efficiency of nuclear power plants is about 35 percent, which is similar to that of fossil fuel-burning power plants. Benefits and Risks of Nuclear Power Extracting energy from atomic nuclei has its advantages. Nuclear power plants do not produce the air pollutants that fossil fuel-burning power plants release into the atmosphere. Also, nuclear power plants do not release carbon dioxide into the atmosphere. Nuclear power plants also have their disadvantages. For example, nuclear power plants are very expensive to build, and the building process can take 10 or more years to complete. Nuclear power plants also produce radioactive waste that can be harmful to living organisms and the environment.

Making permanent magnets

Permanent magnets can be made by stroking a magnetic material, as you did with the sewing needle in the MiniLab. They can also be made by heating magnetic material and letting it cool in a magnetic field. How long does a magnet last? The domains in permanent magnets do not have to remain aligned forever. The magnetized needle that you made in the MiniLab, for example, will not remain magnetized for very long. However, it is still considered a permanent magnet because it remained magnetized when there was no external magnetic field. Why did the sewing needle become unmagnetized? When the external magnetic field is removed, the constant motion and vibration of the atoms bump the magnetic domains out of alignment and the domains return to a random arrangement. How quickly this happens depends on the magnet's material and its environment. For example, heating a permanent magnet causes its atoms to move faster. If the magnet is heated enough, its atoms move fast enough to bump the domains out of alignment. Can a pole be isolated? If a magnet is broken in two, is one piece a north pole and one piece a south pole? Look at the domain model of the broken magnet in Figure 8. Recall that even individual atoms of magnetic materials act as tiny magnets. Because every magnet is made of many aligned smaller magnets, even the smallest pieces have both a north pole and a south pole.

Reducing Pollution

Pollution is often difficult to contain. Airborne pollutants travel wherever the wind carries them. Even if one state or country reduces air pollution, pollutants from another state or country can blow across borders. For example, burning coal in the midwestern United States might cause acid precipitation in Canada. Water pollution can enter a river or stream and travel several kilometers downstream, into groundwater supplies, and across state borders. How can you help? The United States uses more natural resources per person than most countries in the world. There are ways that you can help conserve resources. You can reduce the amount of consumable materials you use. You can compost some yard and kitchen waste rather than throwing it in the trash. You can also reuse and recycle many different materials, as shown in Figure 33. Energy-efficient appliances can help your family reduce its energy dependence. Low-flush toilets, leak-free faucets, and dishwashers and washing machines that run on less water will help you reduce your water use. Driving fuel-efficient vehicles or using alternate modes of transportation, such as a bicycle or a bus, will help lessen your impact on air.

Radiation

Radiation Energy from the Sun reaches Earth, but how does that energy travel through space? Almost no matter exists in the space between Earth and the Sun, so neither conduction nor convection could warm Earth. Instead, radiation transfers energy from the Sun to Earth. Radiation is the transfer of energy by electromagnetic waves, such as light and microwaves. These waves travel through space even when matter is not present. Energy that is transferred by radiation is often called radiant energy. When you stand near a fire, radiation transfers energy from the fire and increases the thermal energy of your body. Radiation and matter When radiation strikes a material, that material absorbs, reflects, and transmits some of the energy. Figure 10 shows what happens to radiation from the Sun as it reaches Earth. The amount of energy that a material absorbs, reflects, and transmits depends on the type of material. The thermal energy of a material increases when that material absorbs radiant energy. Radiation in solids, liquids, and gases In a solid, liquid, or gas, radiation travels through the space between particles. Particles can absorb and re-emit this radiation. This energy then travels through the space between particles, and other particles then absorb and re-emit the energy. Radiation usually passes more easily through gases than through solids or liquids because particles are much farther apart in gases than in solids or liquids. Thus, radiation transfers energy more rapidly and efficiently through gases than through liquids or solids.

Refrigerators

Refrigerators A refrigerator does work as it transfers thermal energy from inside the cool refrigerator to the warmer room. The energy to do the work comes from the electrical energy the refrigerator obtains from an electrical outlet. A refrigerator makes the room that it is in warmer. Figure 20 shows how a refrigerator operates. Liquid coolant is pumped through an expansion valve and changes into a gas. When the coolant expands as it changes into a gas, it pushes outward on its surroundings. This means that the coolant does work on its surroundings. As a result, the coolant transfers energy to its surroundings and the coolant cools. The cold gas is pumped through pipes inside the refrigerator, where it absorbs thermal energy from the area where you keep your food. When this happens, the inside of the refrigerator cools. The gas then is pumped to a compressor that does work on the gas by compressing it. This makes the gas warmer than the temperature of the room. The warm gas is pumped through the condenser coils. Because the gas is warmer than the room, thermal energy spreads from the gas to the room. As the gas heats the room, it cools and changes back to a liquid and enters the expansion valve again. The cycle is then repeated.

Compound and Simple Machine

Simple: does work with only one movement (Cut your food with a knife, chew your food) Compound: combination of two or more simple machines (Scissors: two wedges/two levers and bicycle)

Inclined Plane

Sloping surface, such as a ramp that reduces the amount of force required to do work.

An ___ is a device that transforms the Suns radiant energy into thermal energy.

Solar collectors

Rotating the permanent magnet

Suppose that the coil in a generator were fixed and the permanent magnet rotated. The current generated would be the same as when the coil rotates and the magnet does not move. The huge generators used in electrical power plants are made this way. Mechanical energy is used to rotate the magnet, and the current is induced in the stationary coil. Generating electricity for your home Unless you have a generator in your home, the electrical energy needed to watch television or to wash your clothes comes from a power plant. Each power plant must have an energy source. For example, some power plants burn fossil fuels or use nuclear reactions to produce thermal energy. This thermal energy is used to heat water and produce steam. Thermal energy is then converted into mechanical energy as the steam pushes the turbine blades. A turbine (TUR bine) is a large wheel that rotates when pushed by water, wind, or steam. In some areas, fields of windmills, like those shown in Figure 20, can be used to capture the mechanical energy in wind. Other power plants use the mechanical energy in falling water to drive the turbine. The turbine is connected to the rotating magnet in the generator. Power plant generators, like those shown in Figure 20, are very large. The electromagnets in these generators have many coils of wire wrapped around huge iron cores. The generator changes the mechanical energy of the rotating turbine into the electrical energy that you use. Generators and motors Both generators and electric motors use electromagnets to convert electrical and mechanical energy. Figure 21 summarizes the differences and similarities between electric motors and generators.

The Law of Conservation of Energy

Suppose you are riding on a roller coaster like the one in Figure 10. As your height above the ground changes, gravitational potential energy changes. As your speed changes, kinetic energy changes. Think about the motion of the roller-coaster cars. When the cars are high above the ground, GPE is large and kinetic energy is small. When the cars are low, GPE is small and kinetic energy is large. Energy is changing back and forth between GPE and kinetic energy. In addition, some kinetic energy is slowly converted into other forms of energy during a roller-coaster ride. However, the total energy remains constant. The law of conservation of energy states that energy cannot be created or destroyed. Energy can only be converted from one form to another or transferred from one place to another. Reading Check State the law of conservation of energy. Conserving resources You might have heard about energy conservation or have been asked to conserve energy. These ideas are related to using energy resources, such as coal and oil, wisely. The law of conservation of energy, on the other hand, is a universal principle that states that total energy remains constant.

Heat

Suppose you place a pot of water on a hot burner, as shown in Figure 3. When you do this, you transfer thermal energy from the warmer stove to the cooler pot and water. Heat is energy that is transferred between objects due to a temperature difference between those objects. Warmer objects always heat cooler objects, but the reverse never occurs. For example, a hot stove will heat a cold pot of water. However, a cold pot of water can never heat a hot stove.

Thermal Energy depends on

Temperature and thermal energy Thermal energy depends on temperature. The average kinetic energy of the particles that make up an object increases when the temperature of that object increases. Thermal energy is the total kinetic and potential energy of all of the particles that make up an object. The thermal energy of the object increases when the average kinetic energy of the particles that make up that object increases. Therefore, the thermal energy of an object increases as its temperature increases. Mass and thermal energy Thermal energy also depends on mass. If the mass of the object increases, the thermal energy of that object also increases. Suppose you have a glass of water and a beaker of water, both at the same temperature. However, the beaker contains twice as much water as the glass. The average kinetic energy of the water molecules is the same in both containers. However, there are twice as many water molecules in the beaker. Therefore, the water in the beaker has twice as much thermal energy as the water in the glass

Solar heating

The Sun emits an enormous amount of radiant energy that strikes Earth every day. This energy can be used to help heat homes and other buildings through both passive solar heating and active solar heating. Passive solar heating In passive solar heating systems, materials inside a building absorb radiant energy from the Sun and heat up during the day. At night when the building begins to cool, thermal energy absorbed by these materials helps keep the room warm. Active solar heating In active solar heating, a solar collector is used. A solar collector is a device that transforms radiant energy from the Sun into thermal energy. Radiation from the Sun heats air or water in the solar collector. A pump circulates the hot fluid to radiators in rooms of the house. Both passive solar heating and active solar heating are shown in Figure 15.

Fusion

The Sun is a giant nuclear reactor in the sky. It transforms energy through a process called fusion. Fusion occurs when atomic nuclei combine at very high temperatures. In this process, a small amount of mass is transformed into a tremendous amount of thermal energy. Fusion-based power plants are not practical. One problem with fusion is that it occurs at millions of degrees Celsius. Under these conditions, reactors use a great deal of energy. Another problem is containment—what kind of chamber can hold a reaction under these extreme conditions? Fission Energy is released when the nucleus of an atom splits apart in a process called fission. During fission, a small amount of mass is converted into a tremendous amount of thermal energy. Unlike fusion, fission-based power plants are practical. Sixtyfive power plants in the United States, including the one shown in Figure 10, transform energy by fission reactions. These plants convert nuclear energy into electrical energy and produce 8 percent of the energy used in the United States.

Natural Gas

The chemical processes that produced petroleum when ancient organisms decayed and were buried on the seafloor also formed natural gas. Due to differences in density, lightweight natural gas compounds are found trapped on top of petroleum deposits. Natural gas is a fossil fuel composed mostly of methane, but it also contains other gaseous hydrocarbon compounds, such as propane and butane. Natural gas contains more chemical potential energy per kilogram than petroleum or coal does. Additionally, natural gas burns cleaner than other fossil fuels, produces fewer pollutants, and leaves no ash residue. Natural gas is burned to provide energy for cooking, heating, and manufacturing. About one-fourth of the energy used in the United States comes from the combustion of natural gas. There is a good chance that your home has a stove, a furnace, a hot-water heater, or a clothes dryer that is powered by natural gas. Some cars and buses also are powered by natural gas.

Another type of heat engine is an external combustion engine.

The cylinders in external combustion engines are heated by burning fuel outside the cylinders. Old-fashioned steam engines are external combustion engines.

Energy Options

The demand for energy is increasing every day as Earth's population increases. As demand increases, our supply of nonrenewable energy resources decreases. Use of nuclear energy produces high-level waste that has to be disposed of safely. As a result, alternative energy sources are being developed to meet increasing energy demands. Some alternative energy sources are renewable resources. A renewable resource is an energy source that is replaced by natural processes faster than humans can consume the resource. Energy from the Sun The average amount of solar energy that shines down on the United States in one year is 1,000 times more energy than the total energy used in one year. Because the Sun is expected to continue producing energy for billions of years, solar energy is inexhaustible in our lifetime. Solar energy is a renewable resource. Despite being renewable, only 1 percent of the energy in the United States is produced using solar power. There are several ways to produce solar power. One way is to use a photovoltaic cell, as shown in Figure 17. A photovoltaic cell converts radiant energy directly into electrical energy. Photovoltaic cells are also called solar cells. How solar cells work Solar cells are made of two layers of semiconducting material sandwiched between two layers of conducting metal, as shown in Figure 18. One layer of the semiconducting material is rich in electrons, and the other layer is electron poor. When sunlight strikes the surface of the solar cell, electrons flow through an electric circuit from the electron-rich material to the electron-poor material. This process of converting radiant energy from the Sun directly to electrical energy is only about 7-11 percent efficient. The transformation of radiant energy into electrical energy using solar cells is more expensive than the transformation of thermal energy into electrical energy by combustion. However, in remote areas where power lines are not available, solar cells are a practical energy source. Parabolic troughs Other promising solar technologies concentrate solar power into a receiver. One such system is called the parabolic trough. The trough focuses the sunlight on a tube that contains a heat-absorbing fluid, such as synthetic oil or liquid salt. Sunlight heats the fluid, which circulates through a boiler, where it turns water to steam that spins a turbine to generate an electric current. One of the world's largest concentrating solar power plants is located in the Mojave Desert in California. This facility consists of nine units that generate more than 350 megawatts of power. These nine units can generate enough electricity to meet the demands of approximately 500,000 people. The units also use natural gas as a backup power source for generating an electric current at night and on cloudy days when solar energy is unavailable.

The first law of thermodynamics

The first law of thermodynamics states that if the mechanical energy of a system is constant, the increase in thermal energy of that system equals the sum of the thermal energy transfers into that system and the work done on that system. This means that there are two ways to increase the temperature of a system. One way is to heat that system. Another way is to do work on that system. For example, you could increase the temperature of your hands both by warming them near a fire and by rubbing them together

Energy from the oceans

The gravitational pull of the Moon and the Sun on Earth's oceans causes tides. Hydroelectric power can be produced by these tides. As the tide rises, water spins a turbine, which transforms mechanical energy into electrical energy. The water is then trapped behind a dam. At low tide, the water behind the dam is released to flow back out to sea, converting even more energy to electricity. Hydroelectric power can also be produced by waves. There are several new technologies that harness wave energy. One type focuses wave energy into a channel. As waves enter a channel, they spin turbines, converting mechanical energy into electrical energy. Plans are also in place to harness mechanical energy from ocean currents, as shown in Figure 21. Energy from the ocean is nearly pollution free, and the efficiency of tidal and wave power plants is similar to that of hydroelectric power plant. However, only a few places on Earth have large enough differences between high and low tide for oceans to be a useful energy source.

Energy transformations with swings

The mechanical energy transformation for a swing. The ride starts with a push, which transfers kinetic energy to the rider. As the swing rises, the rider loses speed but gains height. In energy terms, kinetic energy is changed to GPE. At the top of the path the GPE is the greatest. As the swing moves back downward, gravitational potential energy changes back to kinetic energy. At the bottom of the swing, kinetic energy is at its max while GPE is at its minimum. GPE and Kinetic energy change back and forth as the swing moves.

Thermal Conductor

The rate at which conduction transfers thermal energy depends on the material. Conduction is faster in solids and liquids than it is in gases. In gases, particles are farther apart. Therefore, collisions among particles occur less frequently in gases. The best conductors of thermal energy are metals. This is one reason why manufacturers often make cooking pots, like those in Figure 7, out of metal. In a piece of metal, some electrons are not bound to individual atoms. These electrons can move easily through the metal. Collisions between these electrons and other particles in the metal enable more rapid thermal energy transfers than in other materials. Silver, copper, and aluminum are among the best conductors of thermal energy

Electrical Power and Energy

The reason that electricity is so useful is that electrical energy is converted easily to other types of energy. For example, electrical energy is converted to mechanical energy as the blades of a fan rotate to cool you. An electric heater converts electrical energy into thermal energy. Electrical power is the rate at which electrical energy is converted to another form of energy. The electrical power used by appliances varies. In general, heating appliances, such as electric stoves, use much more electrical power than electronic devices, such as computers. Appliances, like the microwave oven in Figure 22, often are labeled with a power rating that describes how much power the appliance uses. Calculate electrical power A device's electrical power depends on the voltage difference across that device and the current through that device. Electrical power can be calculated from the following equation. Electrical Power Equation electrical power (in watts) = current (in amperes) × voltage difference (in volts) P = IV If current is measured in amperes and if voltage difference is measured in volts, then electrical power is calculated in watts (W). Because the watt is a small unit of power, electrical power is often expressed in kilowatts (kW). One kilowatt equals 1,000 watt Calculate electrical energy Electric companies provide the electrical energy that you can then transform into other forms of energy, such as mechanical energy or thermal energy. The electrical energy that electric companies provide can be calculated from this equation. Electrical Energy electrical energy (in kWh) = electrical power (in kW) × time (in h) E = Pt Electrical energy is usually measured in units of kilowatt hours (kWh). One kilowatt hour is equal to 3.6 million joules and is enough energy to lift an average-sized car more than 200 meters. The cost of electrical energy The electrical energy that power companies provide costs money. The cost of using an appliance can be computed by multiplying the electrical energy used by the amount that the power company charges for each kWh. For example, if a 100-W lightbulb is left on for 5 h, the amount of electrical energy provided by the power company is E = Pt = (0.1 kW) (5 h) = 0.5 kWh If the power company charges $0.15 per kWh, the cost of using the bulb for 5 h is cost = (kWh provided) (cost per kWh) = (0.5 kWh) ($0.15/kWh) = $0.08 The cost of using some sample household appliances is given in Table 1, where the cost per kWh is assumed to be $0.15/kWh.

Mechanical Energy

The sum of the kinetic and potential energy of objects in a system. If Kinetic and potential don't equal mechanical then energy must have been lost in other forms. Includes: kinetic energy, elastic potential energy, and gravitational potential energy. Doesn't include: thermal, nuclear, chemical potential energy.

Alternative Fuels

The use of fossil fuels would be greatly reduced if cars could run on alternative energy resources alone. For example, cars have been developed that use electrical energy supplied by batteries as their primary power source. Hybrid cars use electric motors and gasoline engines. Hydrogen Hydrogen fuel cells are another possible alternative energy resource. A fuel cell behaves like a battery. It combines hydrogen with oxygen in air to generate electrical energy, water, and heat. There are several problems with using hydrogen fuel as an alternative energy resource, however. First, obtaining hydrogen requires more energy than the energy that is released by the fuel-cell reaction. Second, hydrogen fuel cells are built from expensive platinum parts. And third, there is a lack of hydrogen fueling stations, as storing hydrogen is considered to be dangerous and difficult. Biomass Are there any other materials that can be used to heat water and produce electricity other than fossil fuels, nuclear fission, or hydrogen? Biomass is one of the oldest energy sources. Biomass is renewable organic matter, such as wood, soy, corn, sugarcane fibers, rice hulls, and animal manure. It can be burned in the presence of oxygen, which converts the stored chemical potential energy to thermal energy. Figure 24 shows a bus powered by recycled cooking oil derived from biomass

Ohm's Law

The voltage difference, current, and resistance in a circuit are related. The relationship between voltage difference, current, and resistance in a circuit is known as Ohm's law. If I stands for electric current, Ohm's law can be written as the following equation. Ohm's Law, current (amperes) = ___ voltage difference (volts)/resistance (ohms) I = V/R According to Ohm's law, the current in a circuit equals the voltage difference divided by the resistance. If voltage is measured in volts (V) and resistance is measured in ohms (Ω), then current is measured in amperes (A)

When a force is perpendicular to motion what is the work?

The work from the force is zero. (When carrying books there is a 90 degree angle between this force on books and the motion of them.)

Thermal Energy

Thermal Energy If you let cold butter sit at room temperature for awhile, it warms and becomes softer. The particles that make up the room temperature air have more kinetic energy than the particles that make up the cold butter. Collisions between the air and the butter transfer energy from the faster-moving air particles to the slower-moving butter particles. The butter particles then move faster, and the temperature of the butter increases. Recall that Earth and a ball exert gravitational forces on each other. Earth and the ball have potential energy. Similarly, the particles that make up matter exert attractive electromagnetic forces on each other and have potential energy. When you move a ball away from Earth, gravitational potential energy increases. When you move the particles that make up matter farther apart, the potential energy of that matter increases. Figure 2 shows the kinetic and potential energies of particles. The thermal energy of an object is the sum of the kinetic energy and the potential energy of all of the particles that make up that object. As the butter warmed and the kinetic energy of its particles increased, the butter's thermal energy increased.

Thermal Insulator

Thermal insulators A thermal insulator is a material through which thermal energy moves slowly. Wood, fiberglass, and air are all good thermal insulators. Metals and other good conductors are poor thermal insulators. In conductors, thermal energy moves more rapidly from one place to another. Insulated clothing Gases, such as air, are usually much better thermal insulators than solids or liquids. Some thermal insulators contain many pockets of trapped air. These air pockets conduct thermal energy poorly and also keep convection currents from forming. Fleece jackets, like the one worn by the cyclist in Figure 12, work in this way. When you put on a jacket like this one, the fibers in fleece trap the air and hold it next to you. The air slows the transfer of your body's thermal energy into its surroundings. Under the jacket, a blanket of warm air covers you. Reading Check Explain how trapped air makes a material, such as fleece, a good thermal insulator. Insulated buildings Insulation, or materials that are thermal insulators, helps prevent thermal energy transfers out of buildings in cold weather and thermal energy transfers into buildings in warm weather. Manufacturers usually make building insulation from a fluffy material, such as fiberglass, that contains pockets of trapped air. Builders pack insulation into a structure's outer walls and attic, where it reduces thermal energy transfers between the structure and the surrounding air. Insulation helps furnaces and air conditioners work more efficiently. In the United States, about 50 percent of the energy used in homes is for climate control. Repairing and improving insulation can dramatically reduce heating and cooling costs.

Power—how fast energy changes

Think again about the energy that your body extracts from food every day. You probably get enough energy from food in one day to jump nearly 10 km into the air. If this is true, then why can't you do this? You might have enough energy, but you don't have enough power. Power is the rate at which energy is converted. Power can be found using the following equation: Power Equation Power (in watts) = __ Energy (in joules) time (in seconds) P = _ E t Power is measured in watts; 1 watt equals 1 joule per second. A 13-W lightbulb transforms 13 J of electrical energy into radiant energy each second. A typical person can develop a power of only about 500 W for a jump. This results in a jump that is less than 1 m high for a person with average mass

Definition of Work

To many people, the word work means something that people do to earn money. In that sense, work can be anything from fixing cars to designing Web sites. The word work might also mean exerting a force with muscles. However, in science, the word work is used in a different way. Motion and work Press your hand against the surface of your desk as hard as you can. Have you done any work on the desk? The answer is no, no matter how tired you get from the effort. In science, work is force applied through a distance. If you push against the desk and it does not move, then you have not done any work on the desk because the desk has not moved. Force and direction of motion Imagine that you are pushing a lawn mower, as shown in Figure 1. You could push on this mower in many different directions. You could push it horizontally. You could also push down on the mower or push on it at an angle. Think about how the mower's motion would be different each time. The direction of the force that you apply to the lawn mower affects how much work you do on it. Force parallel to motion Imagine that you push on the lawn mower in Figure 1 with a force of 25 N and through a distance of 4 m. In what direction would you push to do the maximum amount of work on the mower? You do the maximum amount of work when you push the lawn mower in the same direction as it is moving. When force and motion are parallel, which means they are in the same direction, work is equal to force multiplied by distance. Work Equation work (in joules) = applied force (in newtons) × distance (in meters) W = Fd If force is measured in newtons (N) and distance is measured in meters (m), then work is measured in joules (J). You do about 1 J of work on a cell phone when you pick it up off the floor.

Transformers

To reduce the voltage without changing the amount of electrical energy, many devices use transformers. A transformer is a device that increases or decreases the voltage of an alternating current. A transformer is made of a primary coil and a secondary coil wrapped around the same iron core. A transformer that increases the voltage is called a step-up transformer, and a transformer that decreases the voltage is called a step-down transformer. Both types of transformers are shown in Figure 22. How a transformer works As an alternating current passes through the primary coil, the coil's magnetic field magnetizes the iron core. The primary coil's magnetic field changes direction at the same frequency as the current in the primary coil. The magnetic field in the iron core also changes direction at that frequency. The changing magnetic field in the iron core induces an alternating current in the secondary coil. The input current and the output current alternate at the same frequency. Calculate output voltage The output voltage of a transformer depends on the input voltage and the number of turns in the primary and secondary coils. This relationship can be expressed in an equation. Transformer Voltage Equation ___ output voltage (in volts) input voltage (in volts) = ___ turns in secondary coil turns in primary coil _ V out V in = _ N 2 N 1 In this equation, N stands for the number of turns in the coil. Because N is just a number, it has no units. Consider the step-up transformer in Figure 22. The secondary coil has twice as many turns as the primary coil has. So, the ratio of the output voltage to the input voltage is two to one. For this transformer, the output voltage is twice as large as the input voltage.

Conduction

Transfer of thermal energy by collisions between particles that make up matter. Occurs because particles are constantly in motion. you leave a metal spoon in a pot of soup while it cooks on the stove, the spoon might get too hot to touch. As one end of the spoon heats up, the kinetic energy of the particles that make up that part of the spoon increases. These particles collide with neighboring particles. Conduction transfers thermal energy to the other end of the spoon as particles with more kinetic energy transfer kinetic energy to particles with less kinetic energy. Conduction transfers thermal energy without transferring matter. Conduction spreads thermal energy from warmer areas to cooler areas, as shown in Figure 6. Faster in solids and liquids than gases. Best conductors of thermal energy are metals. This is why pots are made of metal, they can conduct thermal energy easier.

Different Forms of Energy

Turn on an electric light, and a dark room becomes bright. Turn on a portable music player, and sound comes through your headphones. In both situations, a change occurs. These changes differ from each other and the tennis racket hitting the tennis ball in Figure 6. This is because energy has many different forms. These forms include mechanical energy, electrical energy, chemical energy, and radiant energy. Figure 7 shows some everyday situations in which you might notice energy. Automobiles make use of the chemical energy of gasoline. Many household appliances require electrical energy to function. Radiant energy from the Sun warms Earth. In short, energy plays a role in every activity that you do.

Convection

Unlike solids, liquids and gases are fluids that flow. In fluids, convection can transfer thermal energy. Convection is the transfer of thermal energy in a fluid by the movements of warmer and cooler fluid. When conduction occurs, more energetic particles collide with less energetic particles and transfer thermal energy. When convection occurs, more energetic particles move from one place to another. Most substances expand as their temperatures increase. That is, as the particles move faster, they tend to be farther apart. Recall that density is the mass of a material divided by its volume. When a fluid expands, its volume increases, but its mass does not change. Therefore, a fluid's density decreases when that fluid is heated. Because fluids decrease in density as they are heated, a fluid that absorbs thermal energy also decreases in density. The density of a warmer sample of a fluid is less than the density of a cooler sample of that same fluid. The same is true for the parts of a fluid. The warmer parts of a fluid are less dense than the cooler parts of a fluid. These differences in density within a fluid drive convection. The warmer portions of the fluid rise to the top of the fluid, and the cooler portions sink to the bottom. If a fluid is heated from below, convection currents form

Specific heat of common materials

Water: 4,184 Wood: 1,760 Carbon: 710 Glass: 664 Iron: 450

Using Electromagnets

When an electromagnet rotates, electrical energy is converted into mechanical energy to do work. Electromagnets do work in various devices, such as stereo speakers and electric motors. Speakers How does musical information stored on an MP3 player become sound that you can hear? The sound is produced by a loudspeaker, which is an electromagnet connected to a flexible speaker cone. The electromagnet changes electrical energy into the mechanical energy that vibrates the speaker cone to produce the sounds that you hear. Figure 12 shows the parts of the loudspeaker. When you listen to music, the MP3 player produces a voltage that changes according to the musical information in the music file. This varying voltage causes a varying electric current in the electromagnet. Both the size and the direction of the electric current change, depending on the information in the music file. The varying electric current causes both the strength and the direction of the magnetic field in the electromagnet to change. This changing magnetic field direction causes the electromagnet to be attracted to or repelled by the surrounding permanent magnet. The permanent magnet is fixed, so the changing magnetic force makes the electromagnet move back and forth. This causes the speaker cone to vibrate and makes the surrounding air vibrate to create sound waves.

Change Requires Energy

When something is able to change its surroundings or itself, it has energy. Energy is the ability to cause change. Without energy, nothing would ever change. The moving tennis racket in Figure 6 has energy. That racket causes change when it deforms the tennis ball and changes the tennis ball's motion. Work transfers energy The tennis racket in Figure 6 also does work on the tennis ball, applying a force to that ball through a distance. When this happens, the racket transfers energy to the ball. Therefore, energy can also be described as the ability to do work. Because energy can be described as the ability to do work, energy can be measured with the same units as work. Energy, like work, can be measured in joules. Imagine that the tennis racket in Figure 6 does 250 J of work on the tennis ball. Then, 250 J of energy are transferred from the racket to the ball. Systems The tennis racket and the tennis ball in Figure 6 are systems. A system is anything around which you can imagine a boundary. A system can be a single object, such as a tennis ball, or a group of objects, such as the solar system. When one system does work on a second system, energy is transferred from the first system to the second system.

Convection: Think about a lava lamp

When the oil is cool, its density is greater than the acohol, so it sits at the bottom of the lamp. When the light heats the two liquids in the lamp, the oil becomes less dense than the alcohol. The oil rises to the top of the lamp because it is less dense than the acohol. As the oil rises, conduction transfers thermal energy from the warmer oil to the cooler acohol. As a result the oil cools Convection currents How does convection occur? Look at the lamp shown in Figure 8. Some of these lamps contain oil and alcohol. When the oil is cool, its density is greater than the alcohol so it sits at the bottom of the lamp. When the light heats the two liquids in the lamp, the oil becomes less dense than the alcohol. The oil rises to the top of the lamp because it is less dense than the alcohol. As the oil rises, conduction transfers thermal energy from the warmer oil to the cooler alcohol. As a result, the oil cools as it rises. By the time the oil reaches the top of the lamp, it cools and again becomes denser than the alcohol. As a result, the oil sinks. This rising-and-sinking action illustrates a convection current. Convection currents transfer thermal energy from warmer to cooler parts of the fluid. In a convection current, both conduction and convection transfer thermal energy. Reading Check Contrast conduction with convection.

Force perpendicular to motion

When you carry books while walking at a constant velocity, you might think that your arms are doing work on those books. After all, you are exerting a force on the books to hold them, and the books are moving with you. Your arms might even feel tired. However, in this case, the force exerted by your arms does zero work on the books. This is because there is a 90° angle between this force on the books and the motion of the books. When a force is perpendicular to motion, the work from that force is zero. Reading Check Describe the work done on an object when the force on that object and the motion of that object are perpendicular. Other directions If a force on an object and that object's motion are parallel, then the work from that force equals that force's magnitude multiplied by distance. If the force on an object and that object's motion are perpendicular, then the work from that force equals zero. How much work is done when the angle between force and motion is not parallel or perpendicular? For these other directions, the work done is less than the force multiplied by the distance but more than zero. Figure 2 shows a graph of how direction affects work. When is work done? Suppose you give a book a push; it leaves your hand and slides along a table for a distance of 1 m before it comes to a stop. The distance that you used to calculate the work you did on the book is how far the object moves while you apply a force. Even though the book moved a total of 1 m, you do work on the book only while you touch it. You can only do work on an object while you are applying a force to that object. In Figure 3, the girl only did work on the softball while her hand was in contact with the softball.

Kinetic energy

When you think of energy, you might think of objects in motion. Objects in motion can collide with other objects and cause change. Therefore, objects in motion have energy. Kinetic energy is energy due to motion. A car moving along a highway and a ballet dancer leaping through the air have kinetic energy. The kinetic energy from an object's motion depends on that object's mass and speed. Kinetic Energy Equation kinetic energy (in joules) = _1 2 mass (in kg) × [speed (in m/s)]2 KE = _ 1 2 mv2 If mass is measured in kg and speed is measured in m/s, then kinetic energy is measured in joules. If you drop a softball from just above your knee, the kinetic energy from that ball's falling motion is about 1 J, just before the ball reaches the floor.

Wind energy

Windmills can convert wind energy into electrical energy. As the wind blows, it spins a propeller that is connected to an electric generator. The greater the wind speed and the longer the wind blows, the greater the amount of wind energy that is converted into electrical energy. Windmill farms, like the one shown in Figure 22, can contain several hundred windmills. One disadvantage of wind energy is that only a few places on Earth have enough wind to meet our energy needs. Even then, wind energy cannot be stored without the use of batteries. Windmills can be noisy and change the appearance of a landscape. They also can disrupt the migration patterns of some birds. The advantages of using wind energy are that wind generators do not consume nonrenewable resources and they do not pollute air or water. Research is underway to improve the design of wind generators and to increase their efficiency. By 2030, the U.S. Department of Energy wants to increase our use of wind energy so it provides 20 percent of our total electrical power

Electromagnetic Induction

Working independently in 1831, Michael Faraday in Britain and Joseph Henry in the United States both found that moving a loop of wire through a magnetic field caused an electric current in the wire. They also found that moving a magnet through a loop of wire produces a current, as shown in Figure 17. In both cases, the mechanical energy associated with the motion of the wire loop or the magnet is converted into electrical energy associated with the current in the wire. When the magnet and wire loop are moving relative to each other, the magnetic field inside the loop changes with time and induces, which means generates, an electric current in the wire coil. The generation of an electric current by a changing magnetic field is electromagnetic induction. Reading Check Define the term electromagnetic induction. Electromagnetic induction can occur in other ways, too. For example, if the current in a wire changes with time, then the magnetic field around the wire is also changing. This changing magnetic field can induce a current in a nearby coil. Both types of electromagnetic induction are important to generating and transmitting the electrical energy that you use

Temperature

You can use the words hot and cold to describe temperature. Something is hot when its temperature is high. When you heat water on a stove, its temperature increases. How are temperature and heat related? Matter in motion The matter around you is made of tiny particles—atoms, ions, and molecules. In all materials, these particles are in constant, random motion. They move in all directions at different speeds. These particles have kinetic energy because they are moving. The greater their speeds, the greater their kinetic energy. If the particles that make up an object have more kinetic energy, then that object feels hotter. For example, in Figure 1, the particles that make up the electric stove burner on the left have more kinetic energy than the particles that make up the burner on the right. As a result, the burner on the left feels hotter than the burner on the right. Is there a more exact relationship between temperature and kinetic energy? Kinetic energy and temperature The temperature of an object is the measure of the average kinetic energy of the particles that make up that object. The temperature of hot tea is higher than the temperature of iced tea. Therefore, the particles that make up the hot tea have more kinetic energy on average. In SI units, scientists measure temperature in kelvins (K). However, people around the world use the Celsius scale much more often. One kelvin is the same size as one degree Celsius

Heat and work increase thermal energy

You can warm your hands by placing them near a fire because the fire heats your hands by radiation. If you rub your hands and hold them near a fire, the increase in your hands' thermal energy is even greater. Both the work you do and the heat from the fire increase your hands' thermal energy. In the example above, your hands can be considered as a system. Recall that a system is anything around which you can draw a boundary. A system can be a group of objects, such as a galaxy, a car's engine, or something as simple as a ball. An example of a system is shown in Figure 16. A system can interact with its surroundings in many ways. You increase the energy of a system whenever you do work on that system or heat that system. The work done on a system is the work done by something outside the system's boundary on something inside the system's boundary. A system can also heat its surroundings or do work on its surroundings. When a system does work on its surroundings or heats its surroundings, the total energy of the system decreases. The total energy of the surroundings increases by the same amount.

People and the Environment

You have an impact on the environment every day. The electrical energy that you use most likely comes from burning fossil fuels. The cars and buses you use for transportation burn fossil fuel. Fossil fuels are mined from Earth and have an impact on the air that you breathe. The water that you use must be treated, as shown in Figure 26, to remove as many pollutants as possible before it is recycled back into waterways. Pollutants include any substance that contaminates the environment. You also use plastics and paper every day. Plastics are petroleum-based products. When petroleum is refined, it produces pollutants. In the process of harvesting trees to make paper, trees are cut down. They are transported using fossil fuels, and water and air can be polluted in the paper-making process. Impact on Land Land is affected when resources such as fossil fuels, water, soil, or trees, are extracted from Earth. You might not think of land as a natural resource, but it is as important as fossil fuels, clean water, and clean air. We use land for agriculture, forests, urban development, and even waste management. These uses impact the land and the natural resources it provides. Agriculture The pears and apples that you purchase at the grocery store were grown on farms, which cover 16 million km2 of Earth's total land area. To feed the world's growing population, some farmers are planting higher-yielding seeds and using stronger nitrate- and phosphate-based fertilizers. Herbicides and pesticides are also used for weed and pest control. These methods increase the amount of food grown, but if they not managed properly, they can have a negative impact by possibly polluting soil and water and endangering animals Organic farms Organic farming methods, as shown in Figure 27, use natural fertilizers, crop rotation, and biological pest controls. These methods help reduce pollution and other negative impacts on land. However, organic farming methods cannot currently produce the food that is necessary to feed the world's growing population.

Galvanometers

You have probably noticed the gauges in the dashboard of a car. One gauge shows the amount of gasoline in the tank. How does a change in the amount of gasoline in a tank make a needle move in a gauge on the dashboard? The fuel gauge is a galvanometer. A galvanometer is a device that uses an electromagnet to measure electric current. A diagram of a galvanometer is shown in Figure 13. In a galvanometer, the electromagnet is connected to a small spring. When there is a current in the electromagnet, it will rotate until the force exerted by the spring is balanced by the magnetic forces on the electromagnet. Changing the amount of current in the electromagnet changes the strength of the force between the electromagnet and the permanent magnet. Therefore, the amount that the needle rotates is related to the amount of current in the electromagnet. If the galvanometer is marked with a calibrated scale, it can be used to measure the current in a circuit. In a car, a float in the fuel tank is attached to a sensor. The sensor sends a current to the fuel gauge galvanometer. As the level of the float in the tank changes, the amount of current sent by the sensor changes and rotation of the needle changes. The gauge is calibrated so that the current sent when the tank is full causes the needle to rotate to the full mark on the scale.

Magnetic poles

You might have also noticed in Figure 2 that the magnetic field lines are closest together at the ends of the bar magnet. This means the magnetic field is strongest at the ends. The regions of a magnet that exert the strongest force are called magnetic poles. All magnets have a north pole and a south pole. For a bar magnet, the north and south poles are at the opposite ends. How magnetic poles interact Whether two magnets attract or repel each other depends on which poles are brought close together. Two north poles or two south poles repel each other. However, a north pole and a south pole always attract each other. Like magnetic poles repel each other, and unlike poles attract each other. Figure 4 illustrates these interactions. When two magnets are brought close to each other, their magnetic fields combine to produce a new magnetic field. If you look again at Figure 4, you will see iron filings illustrating the magnetic fields that result when like poles and unlike poles of bar magnets are brought close to each other. Magnetic field direction A magnetic field also has a direction. The magnetic field always points away from north magnetic poles and toward south magnetic poles. The direction of the magnetic field around a bar magnet is shown by the arrows on the magnetic field lines in Figure 5. When a compass is brought near a bar magnet, the compass needle rotates. The compass needle is a small bar magnet with a north pole and a south pole. The force exerted on the compass needle by the magnetic field causes the needle to rotate until it lines up with the magnetic field lines, as shown in Figure 5. The north pole of a compass points in the direction of the magnetic field. Notice that in Figure 5 the north compass needles point along the field lines toward the south magnetic pole of the magnet.

Magnetic Materials

You might have noticed that a magnet will not attract all metal objects. For example, a magnet will not attract pieces of aluminum foil. Only a few metals, such as iron, cobalt, and nickel, are attracted by magnets or can be made into permanent magnets. What makes these elements magnetic? Remember that every atom contains electrons. Electrons have magnetic properties. In the atoms of most elements, the magnetic properties of the electrons cancel out. But in the atoms of magnetic elements, these magnetic properties do not cancel out and each atom behaves like a small magnet with its own magnetic field and north and south poles. Reading Check Explain why the atoms of magnetic materials behave like small magnets. Even though these atoms have their own magnetic fields, objects made from these metals are not always magnets. For example, if you hold an iron nail close to a refrigerator door and let go, the nail falls to the floor. However, you can make the nail behave like a magnet by placing it in a magnetic field. A magnetized nail would stick to the refrigerator door.

Thermoses

You might have used a thermos bottle, like the one shown in Figure 13, to carry hot soup or iced tea. A thermos bottle reduces energy transfers between the bottle's contents and its surroundings. As a result, the temperature of the contents changes very little, even over many hours. In order to minimize thermal energy transfers from conduction and convection, a thermos bottle has two glass walls with very little air between those walls. Scientists call any region with very low gas density a vacuum. Glass is a good thermal insulator, and a vacuum is an extremely good thermal insulator. To reduce energy transfers from radiation, manufacturers often coat the thermos bottle's inside and outside glass surfaces with aluminum. This makes each surface highly reflective. The inner reflective surface prevents radiation from transferring energy out of the liquid. The outer reflective surface prevents radiation from heating the liquid. A thermos bottle keeps the liquid inside it warm or cool. Think about the things that you do to stay warm or cool. Sitting in the shade reduces energy transfers from radiation. Opening or closing windows affects thermal energy transfers from convection. Putting on a jacket reduces thermal energy transfers from conduction. In what other ways do you control heat?

Controlling Heat

You might not realize it, but you probably do a number of things every day to control thermal energy transfers between your body and your body's surroundings. For example, when it is cold outside, you put on a coat or a jacket before you leave your home. When you reach into an oven to pull out a hot dish, you might put a thick, cloth mitten over your hand. In both cases, you have used various materials to help control transfers of thermal energy. Your jacket decreased how much thermal energy your body transferred to the surrounding air, keeping you from getting cold. The oven mitten decreased how much thermal energy the hot dish transferred to your hand, preventing that hot dish from burning your hand. Animals and heat The animals shown in Figure 11 have special features that help them control thermal energy transfers between their bodies and their bodies' surroundings. For example, the Antarctic fur seal's thick coat and the emperor penguin's thick layer of fat help to keep them from transferring thermal energy to their surroundings. This helps them survive in a climate where the temperature is often below freezing. In the desert, however, the scaly skin of the desert spiny lizard has just the opposite effect. It reflects the Sun's rays and keeps the animal from becoming too hot. An animal's color also can play a role in keeping it warm or cool. The black feathers on the penguin's back, for example, allow it to absorb radiant energy.

Energy Transformations

You might not think that a vase on a table has any relationship with energy—until it falls. You probably associate energy more with race cars roaring by you or the Sun warming your skin on a summer day. All these situations involve energy transformations. Mechanical energy transformations Bicycles, roller coasters, and swings can often be described in terms of mechanical energy. Mechanical energy is the sum of the kinetic energy and potential energy of the objects in a system. Mechanical energy includes the kinetic energy of objects, elastic potential energy, and gravitational potential energy. It does not include nuclear energy, thermal energy, or chemical potential energy. Mechanical energy and total energy are not the same, because there are types of energy that are not mechanical energy. As a result, mechanical energy is not necessarily conserved. However, the mechanical energy of a system often remains constant or nearly constant. When this is the case, energy is only transformed between different kinds of mechanical energy. Falling objects Look at the apple tree in Figure 11. An apple-Earth system, which is a system that includes an apple and Earth, has gravitational potential energy. The apple-Earth system does not have kinetic energy while the apple is hanging from the tree because the apple is not moving. However, when the apple falls, it gets closer to Earth, so the GPE of the apple-Earth system decreases. This potential energy is transformed into kinetic energy as the apple's speed increases. If potential energy is being converted into kinetic energy, then the mechanical energy of the apple-Earth system does not change as the apple falls. The potential energy that the apple-Earth system loses is gained back as kinetic energy. The form of mechanical energy changes, but the total amount of mechanical energy remains the same.

Energy conversions in your body

You transfer energy from your surroundings to your body when you eat. The chemical potential energy of food supplies the cells in your body with the energy that they need to function. Energy from food is often measured in Calories (C). You have probably seen descriptions of Calories per serving on food packages, such as the sides of cereal boxes or milk cartons. One Calorie is equal to about 4,000 J. Every gram of fat in a food supplies a person with about 10 C (40,000 J) of energy. Carbohydrates and proteins each supply about 5 C (20,000 J) of energy per gram. Everything that your body does requires energy. The number of Calories that you need for different activities depends on your weight, your body type, and your degree of physical activity. Table 1 shows the amount of energy needed to do various activities.

What is energy that is transferred between objects due to a temperature difference between those objects?

heat

An ___ is a device that converts thermal energy into mechanical energy.

heat engine

What device is fuel burned inside chambers called cylinders?

internal combustion engine

Positive and Negative Charges Matter

is composed of atoms. These atoms are composed of protons, neutrons, and electrons. Protons have positive electric charge and electrons have negative electric charge. Neutrons have no electric charge. The amount of positive charge on a proton equals the amount of negative charge on an electron. An atom has equal numbers of protons and electrons, so the positive and negative charges cancel out and an atom has no net electric charge. Objects with no net charge are said to be electrically neutral. The amount of electric charge is measured in coulombs (C). There are 6,250 million billion protons in 1 C of electric charge and 6,250 million billion electrons in -1 C of electric charge.

What device transfers thermal energy from a cooler region to a warmer region

refriderator

How does an active solar heating system work?

solar energy collected to heat water->hot water to house->heat exchanger(heated air circulates through home) -> Cold water supply that returns to solar collector site

Deserts and rainforests Earth's atmosphere is a fluid

that is made of various gases. The atmosphere is warmer at the equator than it is at the North Pole and the South Pole. Also, the atmosphere is warmer at Earth's surface than it is at higher altitudes. These temperature differences produce convection currents that carry thermal energy away from the equator. Moist, warm air near the equator rises. As this air rises, it cools and loses moisture. Rain then falls over the equator. The cooler, drier air sinks down toward the ground north and south of the equator. Desert zones form as a result. The temperature of this air increases as the air sinks. Figure 9 shows how these convection currents result in rain forests and desert

If a force on an object and that object's motion are parallel then

the work from that force equals that force's magnitude multiplied by distance.

Charges Exert Forces Have

you noticed how clothes sometimes cling together when removed from the dryer? These clothes cling together because of the forces that electric charges exert on each other. Figure 2 shows that unlike charges attract each other, and like charges repel each other. Charged particles do not attract or repel particles with no charge, such as neutrons, through the electric force. The force between electric charges depends on the amount of charge as well as on the distance between charges. The force increases as the amount of charge increases and as the charges get closer together. Just as with two electric charges, the force between any two objects that are electrically charged decreases as the objects get farther apart. This force also depends on the amount of charge on each object. As the amount of charge on either object increases, the electrical force also increases. As clothes tumble in a dryer, the atoms that make up some clothes gain electrons and become negatively charged. The atoms that make up other clothes lose electrons and become positively charged. Electrons are transferred from some objects onto other objects. Clothes that are oppositely charged attract each other and stick together, as shown in Figure 3

what term indicates the number of times a machine multiplies the input force?

mechanical advantage

Conservation of charge

When your shoe soles become charged, they illustrate the law of conservation of charge. The law of conservation of charge states that charge can be transferred from object to object, but it cannot be created or destroyed. In Figure 1, electrons are transferred from the carpet to the shoes. However, the total charge does not change. Usually, it is the electrons that transfer from one object to another and not the protons.

Order the events that occur in the removal of thermal energy from an object by a refrigerator. Draw the complete cycle, from the placing of a warm object in the refrigerator to the change in the coolant.

Thermal energy is transferred from the warm object to the coolant in the refrigerator; coolant is compressed and the coolant's temperature rises; thermal energy is transferred from the warm coolant to the outside air; the coolant expands as it passes through the expansion valve and cools. Diagram would resemble Figure 20.

Predict- suppose a beaker of water is heated from the top. Predict which is more likely to occur in the water: thermal energy transfer by conduction or convection.

Thermal energy transfer by conduction is more likely to occur. The warm water would remain at the top instead of circulating for convection.