Physics Final Exam

When you touch a cold surface, does the cold travel from the surface to your hand or does energy travel from your hand to the cold surface? Explain.

Heat (energy transfer) always occurs naturally from the higher temperature object (in this case the hand) to the lower temperature object (cold surface).

How does the second law of thermodynamics relate to the direction of heat flow?

Heat always flows from hot objects to colder objects.

What is evaporation, and why is it a cooling process? Exactly what is it that cools?

During bumping in a liquid some water molecules gain kinetic energy while other lose kinetic energy. Molecules at the surface that gain enough kinetic energy can break free from the liquid. The molecules with more kinetic energy leave the liquid, leaving behind molecules with less kinetic energy. The average kinetic energy of the liquid decreases and thus the temperature decreases. When we sweat and our sweat evaporates our bodies cool down.

What is condensation, and why is it a warming process? Exactly what is it that warms? Warming Process

Less energetic molecules of water in the air hit the surface. Some bounce off, but less energetic molecules are attracted and stay in the liquid. As a result the liquid gains more kinetic energy because these molecules are moving faster than those in the liquid and the average kinetic energy increases, thus the temperature increases. The gas loses less energetic molecules and therefore the average kinetic energy of the gas increases.

Distinguish between a calorie and a Calorie.

1 Calorie = 1000 calories (Calories are food Calories).

Distinguish between a calorie and a joule.

1 calorie = 4,186 Joules

Why is a steam burn more damaging than a burn from boiling water of the same temperature?

1 gram of steam that comes into contact with skin that is at a lower temperature will burn the skin because it contains more internal energy. The steam will go through a phase change and release a great deal of energy when place in contact with cooler skin. 1 gram of water at 100 °C does not release as much energy as steam when it is placed in contact with cool skin.

Why does a bimetallic strip bend with changes in temperature?

A bimetallic strip is made up of two different metals that are bound together. Since materials expand at different rates, when a bimetallic strip is heated (or cooled) the strip will bend. If it is heater and its temperature increases it bends so that the outside of the arc is made of a material that has a larger coefficient of expansion and if it is cooled the strip will bend so that the inside of the arc is made of the material that has the larger coefficient of expansion. A material with a large coefficient of expansion both expands more when heated and contracts more when cooled as compared to a material with a smaller coefficient of expansion.

Does a gas give off energy or absorb energy when it turns into a liquid? How about a solid when it turns to a liquid?

A gas gives off energy when it turns into a liquid. A solid absorbs energy when it turns into a liquid.

Does a liquid give off energy or absorb energy when it turns into a gas? When it turns into a solid?

A liquid must absorb energy when it turns into a gas. A liquid must give off energy when it turns into a solid.

Does a substance that heats up quickly have a high or low specific heat capacity?

A substance that heats up quickly has a low specific heat capacity.

What is meant by the statement "a thermometer measures its own temperature"?

A thermometer when placed in contact with an object at a given temperature reaches thermal equilibrium with object and thus the thermometer is measuring both the temperature of the object and its own temperature.

Do all the molecules in a liquid have about the same speed, or do they have a wide variety of speeds?

A variety of speeds

Because all objects emit energy to their surroundings, why doesn't the temperature of all objects continuously decrease?

All object both emit energy to their surroundings and absorb energy from their surroundings. When the amount of absorbed is equal to that emitted the temperature of the object no longer changes.

Which is greater, an increase in temperature of 1 °C or an increase in 1 °F?

An increase in 1 °C is greater than an increase in 1 °F. A change in temperature of 100 °C = 212 °F.

Distinguish between heat and thermal energy

An object can contain thermal/internal energy due to its temperature. When an object has thermal energy it has the ability to give away or take on more thermal energy. Heat is the process of energy transfer. When an object loses "heat" it is actually losing internal energy and the process of transferring the energy is heat. Heat is energy in transit (movement) while internal energy is something that a substance has.

How does buoyancy relate to convection?

As a fluid is heated, it expands. As the fluid expands, its volume increase and density decreases. Less dense substances float. So when part of a fluid becomes warmer (and less dense) it will float and be replaced by a cooler part of the fluid. The process then continues to repeat itself and you can get convection currents.

State whether water at the following temperatures expands or contracts when warmed a little: 0 °C; 4 °C; 6 °C.

At 0°C it will contract when warmed a little; at 4°C it will expand, and at 6°C it will expand.

Distinguish between evaporation and boiling.

Boiling is a phase change between liquid and gas at a specific temperature depending on the substance as heat is added. Evaporation is a phase change between a liquid and a gas that can occur at various temperatures.

What are the three common ways in which heat is transferred?

Conduction, Convection, and Radiation

An old remedy for a pair of nested drinking glasses that stick together is to run water at different temperatures into the inner glass and over the surface of the outer glass. Which water should be hot, and which cold?

Cool the inner glass (so that it contracts) and heat the out glass (so that it expands). If the reverse is done, the glasses will become more stuck together.

What are the temperatures for freezing water on the Celsius and Fahrenheit scales? For boiling water

Freezing: 0 °C and 32 °F Boiling: 100 °C and 212 °F

Why does decreasing the temperature of a liquid make it freeze?

Decreasing the temperature of a liquid takes away kinetic energy of the molecules causing them to slow down. As they slow down, bonds between molecules can form and the liquid freezes.

Desert sand is very hot in the day and very cool at night. What does this tell you about its specific heat?

Desert sand has a very low specific heat capacity. It is "easy" to change the temperature of the sand.

Why does adding the same amount of heat to two different objects not necessarily produce the same increase in temperature?

Different materials have different capacities for "holding" heat. An object with a high specific heat capacity will not change temperature as much as an object with a low specific heat capacity.

How does the law of conservation of energy relate to the first law of thermodynamics?

Energy can neither be created nor destroyed.

Which has a greater amount of thermal energy, an iceberg or a cup of hot coffee? Explain.

Even though an iceberg is at a much lower temperature than a cup of hot coffee, the iceberg has more thermal energy due to the fact that it has much more mass than the cup of hot coffee. Internal energy depends on both the temperature of the substance but also the amount of the substance (mass) that is present.

By what means is heat transferred by the convection?

Heat is transferred by the bulk movement of a fluid.

Distinguish between heat of fusion and heat of vaporization.

Heat of fusion is the amount of energy per unit mass required to change the phase of a given substance between solid - liquid. Heat of vaporization is the amount of energy per unit mass required to change the phase of a given substance between liquid - gas. Both of these must occur at a specific temperature depending on the type of phase change and the type of substance.

If all of the molecules in a liquid had the same speed, and some were able to evaporate, would the remaining liquid be cooled?

If all the molecules in a liquid had the same speed then they would also all have the same kinetic energy. Thus, the average kinetic energy is the same even if some of the molecules are removed. Since temperature is directly proportional to the average kinetic energy of the molecules the temperature would not change.

Why is it important that your finger be wet if you intend to touch it briefly to a hot clothes iron to test its temperature.

If your finger is wet, some of the heat transmitted to your finger will be given to the water which has a high specific heat capacity and also a larger latent heat of vaporization. The heat will go into warming the water and change the phase rather than directly to your finger.

Which generally expands more for an equal increase in temperature- solids or liquids?

In general, solids tend to expand more than liquids.

How does the specific heat capacity of water compare with the specific heat capacities of other common materials?

In general, the specific heat capacity of water is large compared to other materials and thus it is more difficult to change the temperature of water as compared to other substances.

What determines the direction of heat flow?

In natural situation, heat flows (energy flows) from the higher temperature object to the lower temperature object. The only way the reverse is possible is if it is forced to move in the opposite direction.

A piece of metal and an equal mass of wood are both removed from a hot oven at equal temperatures and dropped onto blocks of ice. The metal has a lower specific heat capacity than the wood. Which melts more ice before cooling to 0 °C?

In order to answer this question, we must consider which substance will lose more heat as it cooled. Both will change temperature by the same amount, ∆T, and both have the same mass, m. Since the amount of heat added or removed depends on Q = m c ∆T, the substance that has the higher specific heat capacity, c, will give up more heat Q. This means that the wood which has a higher specific heat capacity would melt more ice than the metal. The wood has more heat, Q, to give to the ice in order to melt it. The metal would change temperature very quickly when place in contact with the ice.

Which warms up faster when heat is applied- iron or silver?

In order to answer this question, we must know the specific heat capacity of iron and silver. Material Specific heat Capacity [J/(kg °C)] Iron 448 Silver 234 When equal amounts of heat are added to the same amount of iron and silver, the silver will change temperature by a greater amount (or quicker if you think about putting both items over a flame) since it has a lower specific heat capacity. The lower the specific heat capacity, the easier it is to change the temperature of the substance.

Many tongues have been injured by licking a piece of metal on a very cold day. Why would no harm result if a piece of wood were licked on the same day?

In touching the tongue to very cold metal, enough heat can be quickly conducted away from the tongue to bring the saliva to sub-zero temperature where it freezes, locking the tongue to the metal. In the case of relatively non-conducting wood, much less heat is conducted from the tongue and freezing does not take place fast enough for sudden sticking to occur.

Why does the presence of large bodies of water tend to moderate the climate of nearby land - makes it warmer in the cold weather and cooler in hot weather?

In winter months when the water is warmer than the air, the air is warmed by the water to produce a seacoast climate warmer than inland. In summer months when the air is warmer than the water, the air is cooled by the water to produce a seacoast climate cooler than inland. This is why seacoast communities and especially islands do not experience the high and low temperature extremes that characterize inland locations.

Why does increasing the temperature of a solid make it melt?

Increasing the temperature of a solid adds energy to the molecules that make up the solid and eventually breaks the bonds between the molecules causing the object to go into its liquid phase.

Why is it important to protect water pipes so that they don't freeze?

It is important to keep water in pipes from freezing because when the temperature drops below freezing, the water expands as it freezes, whereas the pipe (if metal) will fracture if water in them freezes as the pipes contract as they get colder.

Does a good insulator prevent heat from getting through it, or does it simply slow its passage?

It slows down the passage of heat. Insulators still allow energy transfer but it happens more slowly.

State the third law of thermodynamics.

No system can reach absolute zero.

After a machinist very quickly slips a hot, snugly fitting iron ring over a very cold brass cylinder, there is no way that the two can be separated intact. Can you explain why this is so?

Once the iron ring and the cold brass reach an equilibrium temperature, the cold brass cylinder has expanded as it heats up to room temperature and the iron ring has contracted as it cooled down to room temperature. They are now fitted together snugly. It turns out that brass expands and contracts more than iron for the same changes in temperature. Since they are both good conductors and are in contact with each other, one cannot be heated or cooled without also heating or cooling the other. If the iron ring is heated, it expands—but the brass cylinder expands even more. Cooling the two will not result in separation either, for even at the lowest temperatures the shrinkage of brass over iron would not produce separation since the iron had been at a high temperature when it was originally placed on the cold brass cylinder.

What is sublimation?

Phase change between solid - gas directly.

In the old days, on a cold winter night, it was common to bring a hot object to bed with you. Which would be better to keep warm through the cold night- a 10-kilogram iron brick or a 10-kilogram jug of hot water at the same temperature? Explain

Since water has a higher specific heat capacity as compared to iron, it would be better to have a 10-kg jug of hot water. It is "more difficult" to change the temperature of the water as compared to iron because it has a higher specific heat capacity than iron. The hot water would be able to "give up" more internal energy without changing temperature as much.

What are the four common phases of matter?

Solid, liquid, gas, plasma

Is the temperature of an object a measure of the total kinetic energy of molecules in the object or a measure of the average kinetic energy per molecule in the object?

Temperature is a measure of the average kinetic energy per molecule in the object. Internal energy or Thermal energy is a measure of the total kinetic energy of the molecules of the object. The Internal/Thermal energy is directly related to the temperature of the substance. If an object is at a higher temperature is has more internal energy than an object at a lower temperature.

Distinguish between temperature and heat

Temperature is related to how hot or cold something is and more specifically the average kinetic energy of the molecules. Heat is a measure of energy transfer. When energy is transferred to or from an object its temperature can change.

What is the role of "loose" electrons in heat conductors?

The "loose" electrons are able to transfer energy to their neighbors more easily than bound electrons.

What are the temperatures for freezing water and boiling water on the Kelvin temperature scale?

The Kelvin scale is directly proportional to the Celsius scale but shifted by 273 K, thus the freezing point of water in Kelvin is 273 K and the boiling point is 373 K.

On which temperature scale does the average kinetic energy of molecules double when the temperature doubles?

The Kelvin scale is the scientific scale. If the temperature in Kelvin doubles, the average kinetic energy of the molecules also doubles.

When you boil potatoes, is your cooking time reduced if the water is vigorously boiling instead of gently boiling?

The cooking time is the same regardless of whether the boil is gentle or vigorous since both are boiling at the same temperature and it is the temperature that cooks the food not the boiling.

State the first law of thermodynamics

The first law of thermodynamics states that the heat added to a system transforms to an equal amount of some other form of energy (energy conservation). Energy can neither be created nor destroyed.

Suppose you cut a small gap in a metal ring. If you heat the ring, does the gap become wider or narrower?

The gap would become wider.

Why are such materials as wood, fur, feathers, and even snow good insulators?

The have very few "loose" electrons.

When a 1-kg metal pan containing 1 kg of cold water is removed from the refrigerator and set on a table, which absorbs more heat from the room - the pan or the water? Defend your answer.

The higher specific heat of the water results in water absorbing more heat than the metal pan.

Is it the boiling of the water or the higher temperature of the water that cooks food faster in a pressure cooker?

The higher temperature of the water allows the food to cook faster. Under pressure, water boils at a higher temperature.

State the second law of thermodynamics.

The second law of thermodynamics states that heat never spontaneously flows from a cold substance to a hot substance but only from hot to cold. Heat can flow from cold to hot only when work is done on the system or by adding energy from another source

Would a bimetallic strip function if the two different metals happened to have the same rates of expansion? Is it important that they expand at different rates? Defend your answer.

The strip would not function if the two different materials had the same rate of expansion. The strip bends because one side expands more than the other due to the difference in the coefficients of expansion.

In the power plant of a nuclear submarine, the temperature of the water in the reactor is above 100 °C. How is this possible?

The water is under higher than atmospheric pressure. Under higher pressures, water boils at a temperature greater than 100 °C.

Why can't you establish whether you are running a high temperature by touching your own forehead?

The way that we check temperature is by having energy transferred to our hand (something we are touching is warm) or taken away from our hand (something we touch is cold). Since our entire body is at the same temperature, if we use our own hand to touch our forehead we don't feel warm or cold because our hand is at the same temperature as our forehead. Heat is not transferred (no temperature difference) and thus our brain does not interpret a difference in temperature.

What exactly is radiant energy?

This is energy that is transmitted via electromagnetic waves.

State an exception to the claim that all substance expand when heated?

Water is an exception. Below 4 degrees Celsius, it expands when cooled.

How much energy can be taken from a system at 0 K?

When a system is at 0 K no more energy can be taken. This is absolute zero and implies that all the molecules are completely at rest (no kinetic energy) and no more energy can be taken. The temperature can not drop below 0 K.

Why does the boiling temperature of water decrease when the water is under reduced pressure, such as at higher altitudes?

When atmospheric pressure is reduces this is transmitted through the liquid. With less pressure, gas molecules are able to make it to the surface of the liquid under the reduce pressure at temperatures that are less than the standard boiling temperature.

A metal ball is just able to pass through a metal ring. When the ball is heated, however, it will not pass through the ring. What would happen if the ring, rather than the ball, was heated? Does the size of the ball increase, stay the same, or decrease?

When the ball is heated and its temperature rises, the size of the ball increases and it will not pass through the ring. If the ring rather than the ball was heated, the ball would still pass through the ring. A hole in a substance expands when heated at the same rate as the material that surrounds the hole. The ball size would stay the same if it was not heated.

Why does water not boil at 100 °C when it is under greater than normal atmospheric pressure?

When water is under higher pressure there is a greater force/pressure at the surface of the liquid which is transmitted throughout the liquid. The water molecules that go into the gas state do not have enough pressure to make it to the surface when the temperature is 100 °C. They must reach a higher temperature in order to have enough pressure to make it to the surface.

Room-temperature water will boil spontaneously in a vacuum - on the Moon, for example. Could you cook an egg in this boiling water? Explain.

You could not cook an egg in this boiling water. It is the temperature of the water that cooks the food. Even the water is boiling, it is at room temperature and cannot cook the egg.