PHYSICS 9 - CURRENT ELECTRICITY AND CIRCUITS

What are some examples of natural electrical circuits?

- The nerves in your body form an electrical circuit that carries messages from your brain to your muscles and other parts of the body. - The tail of an electric eel makes a circuit when it stuns its prey with a jolt of electricity. - An electric circuit with a large amount of energy is formed when lightning carries electric current between clouds and the ground.

Fuse Symbol

-A fuse is a fast-acting switch that opens a circuit if it detects too much current. -Some fuses burn out, and must be replaced. -Others, called "circuit breakers", trip, break the circuit, and have to be reset, but do not generally have to be replaced. -To avoid blowing a fuse, be sure there is a resistor in the circuit when you measure current, and make the meter part of the circuit.

Resistor (Symbol)

-A resistor is an electrical device that uses or controls the energy carried by electric current. -In many circuit diagrams, any electrical device that uses energy is shown with a resistor symbol. -A light bulb, heating element, speaker, or motor can be drawn with a resistor symbol. -In the textbook, the light bulb symbol is used to represent the light bulb in a circuit and the resistor symbol is used to represent a fixed resistor.

What does voltage mean?

-A voltage difference of 1 volt allows 1 amp of current to do 1 joule of work in 1 second. Therefore, voltage is the power per amp of current that flows. -Every amp of current flowing out of a 1.5 V battery carries 1.5 watts of power. -The higher the voltage, the more power is carried by each amp of electric current.

How do you measure voltage?

-A voltmeter measures voltage. -A more useful meter is a multimeter, which can measure voltage or current, and sometimes resistance. -To measure voltage, the meter's probes are touched to two places in a circuit or to a battery's poles. The meter shows the voltage difference between the poles. -The meter reads zero volts if both probes are connected at the same end.

Battery (Symbol)

-All electric circuits must have a source of energy. -Circuits in flashlights, cell phones, and cameras get their energy from the Sun or other sources of energy.

How is electric current analogous to water current?

-Electric current can carry energy and do work, and so can a current of water. -For example, a waterwheel turns when a current of water exerts a force on it. -Before electricity was available, waterwheels were used to supply energy to many machines. -Today, the same tasks are done using energy from electric current.

How is resistance analogous to water flow out of a bottle?

-Higher resistance results in low water flow. -Lower resistance results in high water flow. -If there is a high resistance in a circuit, very little electric current will flow. -If there is a low resistance in a circuit, a large electric current can flow.

How do you apply Ohm's Law?

-If the resistance in a circuit is low, then a given voltage will result in a large amount of current. -Devices that need a large amount of current typically have lower resistance, which allows the device to get the current it needs.

How do you set up the meter?

-Multimeters can measure two types of electric current, called alternating current (AC) and direct current (DC). -For circuits with light bulbs and batteries, make sure to read the direct current, or DC.

Switch (Symbol)

-Switches are used to turn electricity on and off. -Flipping a switch to the "off" position creates an open circuit by making a break in the wire. The break stops the current from flowing in the circuit. -Flipping a switch to the "on" position closes the break. The current flows again to supply energy to the light bulb, camera, or other device.

What is a battery/how does it work?

A battery is a device that transforms chemical energy into electrical energy. A battery uses chemical energy to create a voltage difference between its two terminals. When current leaves a battery, it carries energy. The current gives up its energy as it passes through a resistor, such as a light bulb. The bulb transforms the current's electrical energy into light and heat energy. The current returns to the battery, where it gets more energy. A 1.5 volt battery allows the particles to carry 1.5 joules out of the battery every second, which produces a power of 1.5 watts.

How are conductors and insulators used as materials in technology?

A wire has one or more conductors on the inside and an insulator on the outside and sometimes between the conductors. A computer cable might have 20 or more conductors, each separated from the others by an insulator. The insulation prevents the conductors from being exposed to the current and voltage carried by the other conductors.

What is an electric circuit? Provide an example.

An electric circuit is a complete path through which electricity travels. A good example of a circuit is the one in an electric toaster. Bread is toasted by a heater that converts electrical energy to heat. The toaster circuit has a switch that turns on when the toaster's handle is pushed down. With the switch on, electric current enters through one prong of the plug from the wall socket and goes through the toaster heater and out the other prong.

What is a circuit diagram? How is it drawn?

Circuits consist of wires and electrical parts such as batteries, light bulbs, and switches. Electrical drawings are called circuit diagrams. In a circuit diagram, electrical symbols are used to represent each part of the circuit. Electrical symbols are easier to draw than realistic illustrations of circuit parts.

How are voltage and current different?

Current is what actually flows and does work. Voltage differences provide the energy that causes current to flow.

Compare electric current to water current.

Current is what carries energy in a circuit. Like water current, electric current only flows when there is a difference in energy between the two locations that are connected. Water flows downhill from higher gravitational potential energy to lower energy. Electric current flows from higher electrical potential energy to lower electrical potential energy.

What are electrical conductors?

Current passes easily through some materials, such as silver, gold, copper, aluminum, tungsten, and iron--which are called conductors. A conductor can conduct, or carry electric current. The electrical resistance of wires made from conductors is low. Most metals are good conductors.

How do you measure electric current?

Electric current can be measured with a multimeter, but the meter is not used in the same way as if you were measuring voltage. To measure current, the current must pass through the meter. -The meter has been inserted into the circuit to measure current. -The current comes out of the positive end of the battery, and passes through the bulb and the meter and back. Some electrical meters, called ammeters, are designed to specifically measure only current.

Which way does current flow from a battery?

Electric current from a battery flows out of the positive end and returns back to the negative end. In most electric circuits, negative charge flows, so you would think the current arrows would show a negative to positive direction. It is practical and conventional, however, to describe current as flowing from positive to negative, or from high voltage to low voltage. The amount of electric current coming out of the positive end of the battery must always be the same as the amount of current flowing into the negative end.

How is electric current measured?

Electric current is measured in units called amperes (A), or amps for short. The unit is named in honor of Andre-Marie Ampere, a French physicist who studied electricity and magnetism. A small battery-powered flashlight bulb uses about half and amp of electric current.

How is resistance measured?

Electrical resistance is measured in units called ohms. This unit is abbreviated with the Greek letter omega. For a given voltage, the greater the resistance, the lower the current. If a circuit has a resistance of 1 ohm, then a voltage of 1 volt causes a current of 1 amp to flow.

What is electrical voltage?

Electrical voltage is a measure of the difference in electrical potential energy between two places in a circuit. Differences in voltage cause electric currents to flow.

How is electricity dangerous?

Electricity can carry a great deal of power. Touching an electric wire through which current is flowing can result in serious injury.

What is electricity? What is electric current?

Electricity is made up of charged particles that carry energy from one place to another. The flow of electric charge is called electric current. Electric current comes from the motion of electrons or other charged particles.

What happens when you add or subtract resistance from a circuit?

Every device that uses electrical energy adds resistance to a circuit. The more resistance the circuit has, the lower the current. For example, if you string several light bulbs together, the resistance in the circuit increases and the current decreases. This makes each bulb dimmer than a single bulb would be. -One bulb: single resistance/full current -Two bulbs: twice the resistance/half the current -Three bulbs: three times the resistance/one third the current

What other way(s) are there to create breaks in circuits?

Incandescent light bulbs, burn out when the thin, glowing wire in the bulb breaks. This creates an open circuit and explains why a burned-out bulb cannot light.

What is the difference between open and closed circuits?

It is necessary to turn light bulbs, radios, and other devices on and off. One way to turn off a device is to stop the current by "breaking" the circuit. Electric current can only flow when there is a complete and unbroken path throughout the circuit. -A CLOSED circuit is a circuit with no breaks. (A light bulb will only light if it is a part of a closed circuit.) -An OPEN circuit describes a circuit with a break in it. (Opening a switch or disconnecting a wire "breaks" the circuit.)

Does a perfect conductor or insulator exist?

No, no material is a perfect conductor or insulator. Some amount of current will always flow in any material if a voltage is applied to it. Even a good conductor such as copper has some resistance.

What are electrical insulators?

Other materials, such as wood, air, paper, ice, glass, rubber, and most plastics, do not allow current to pass easily through them. These materials are called insulators. An insulator is a material that does not conduct electric current well, because it opposes or slows the flow of current.

For a given voltage, the amount of current that flows depends on the ________________ of the circuit.

Resistance

What is resistance?

Resistance is the measure of how strongly an object "resists" current flowing through it. A copper wire has low resistance and can easily carry a large current. A material with a high resistance, such as a rubber band, carries a very small current that is difficult to measure.

What is Ohm's Law?

The current in a circuit depends on the battery's voltage and the circuit's resistance. Voltage and current are directly related. Doubling the voltage doubles the current. Resistance and current are inversely related. Doubling resistance halves the current. These two relationships form Ohm's Law. Ohm's Law states that current is directly related to voltage and inversely related to resistance. Current (amps, A) I = Voltage (volts, V) V / Resistance (ohms) R I = V/R

What are semiconductors?

The electrical conductivities of some materials is between that of a conductor and an insulator. These materials are called semiconductors because their ability to carry current is higher than an insulator but lower than a conductor. Computer chips, televisions, portable radios, are among the many devices that use semiconductors. Other materials that are considered to be semiconductors are carbon, germanium, and silicon (usually used in making computer chips).

What do meters measure?

The meter reads positive voltage if the red positive probe is at a higher voltage than the black probe. The meter reads negative when the black probe is at the higher voltage. The meter reads voltage DIFFERENCES between its probes.

Explain positive and negative battery terminals.

The positive end on a AA, C, or D battery has a raised bump, and the negative end is flat. In a circuit diagram, a battery's electrical symbol uses a long line to show the positive end and a short line to show the negative end.

Why does resistance match operating voltage?

The resistance of electrical devices can vary from as little as 0.001 ohms to over a million ohms. Every electrical device is designed with a resistance that causes the right amount of current to flow when the device is connected to the proper voltage. A 100-watt light bulb has a resistance of 145 ohms. When connected to a wall socket's 120 volts, the current is 0.83 amps and the bulb lights. If you connect the same light bulb to a 1.5 volt battery, it won't light. According to Ohm's Law, the current would be 0.01 amps when 1.5 volts is applied to a resistance of 145 ohms. This amount of current will not light the bulb. All electrical devices are designed to use the "right" amount of current only when connected to a specific voltage value.

What does it mean to be non-ohmic?

The resistance of many electrical devices varies with temperature. For example, the amount of resistance a light bulb contributes to a circuit increases as its temperature increases, due to the current running through it. Devices that have a variable resistance like this are referred to as non-ohmic, because you cannot use Ohm's law to predict the current when there is a changing resistance.

Why are electrical outlets so dangerous?

The typical resistance of dry skin is 100,000 ohms or more. According to Ohm's Law, 9 V / 100,000 ohms is only 0.00009 amps. This is not enough current to be harmful. On average, nerves in the skin can feel current of about 0.0005 amps. You can get a dangerous shock from 120 volts from a wall socket because that is enough voltage to force 0.0012 amps through your skin, and you can certainly feel that.

Explain the voltage of a battery.

The voltage of a battery depends on how the battery is made. -Household zinc-carbon batteries are 1.5 volts each. -Lead-acid batteries, like those used in cars, are usually 12 volts. -Different voltages can also be made by combining multiple batteries.

Do wires have resistance?

The wires used to connect circuits are made of metals that have low resistance, such as copper or aluminum. The resistance of wires is usually low compared to other devices in a circuit. You can ignore wire resistance when calculating the total resistance in a circuit. However, if the current is large, the resistance of the wires may be important.

Why are batteries like pumps?

Two water-filled reservoirs connected by a pump is a good analogy for a battery in a circuit. The pump pulls the water to a higher level, giving it potential energy. The water current can flow back down and turn the waterwheel. In a battery, chemical reactions provide a voltage difference to give energy to the current. The current then flows through the circuit, carrying the energy to light the bulb. The current gets a "refill" of energy each time it passes through the battery, for as long as the battery's energy lasts.

What is voltage?

Voltage is a measure of electric potential energy, just as height is a measure of gravitational potential energy. Voltage is measured in volts (V). Like other forms of potential energy, a voltage difference means there is energy that can be used to do work.

Why is it even more dangerous to touch an electrical outlet with wet hands?

Wet skin has much lower resistance than dry skin. Because of the lower resistance, the same voltage will cause even more current to pass through when your skin is wet. The combination of water and 120 volts from a wall socket can give you a fatal, dangerous shock. The high voltage and lower resistance make it possible for large amounts of current to flow.

Why are wires like pipes for electricity?

Wires in electric circuits are similar in some ways to water pipes in a house. Wires act like pipes for electric current. Electric current enters the house on a supply wire and leaves on a return wire. The big difference between wires and water pipes is that you cannot get electricity to leave a wire the way water can leave a pipe. If you cut a water pipe, the water flows out. If you cut a wire, the electric current stops immediately.

How do you measure resistance?

You can use a multimeter to measure the resistance of wires, light bulbs, and other devices. Remove the device from the circuit. Then, set the dial on the multimeter to the resistance setting and touch the probes to each end of the device. The meter will display the resistance in ohms, kilo-ohms, or mega-ohms.