Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | ssm Ignore friction and assume that air behaves as an ideal gas. The plunger of a bicycle tire pump is pushed down rapidly with the end of the pump sealed so that no air escapes and there is little time for heat to flow through the cylinder wall. Explain why the cylinder of the pump becomes warm to the touch.

| 2 | One hundred joules of heat is added to a gas, and the gas expands at constant pressure. Is it possible that the internal energy increases by 200 J? Account for your answer with the aid of the first law of thermodynamics.

| 3 | A gas is compressed isothermally, and its internal energy increases. Is the gas an ideal gas? Justify your answer.

| 4 | A material undergoes an isochoric process that is also adiabatic. Is the internal energy of the material at the end of the process greater than, less than, or the same as it was at the start? Justify your answer.

| 5 | (a) Is it possible for the temperature of a substance to rise without heat flowing into it? (b) Does the temperature of a substance necessarily have to change because heat flows into or out of it? In each case, give your reasoning and use the example of an ideal gas.

| 6 | The drawing shows a pressure-volume graph in which a gas expands at constant pressure from A to B, and then goes from B to C at constant volume. Complete the table below by deciding whether each of the remaining four entries should be positive (+), negative (−), or zero (0). Give a reason for each answer you choose.

|  | Q | W
A→B | + |  |
A→C |  | + |

| 7 | The drawing shows an arrangement for an adiabatic free expansion or “throttling” process. The process is adiabatic because the entire arrangement is contained within perfectly insulating walls. The gas in chamber A rushes suddenly into chamber B through a hole in the partition. Chamber B is initially evacuated, so the gas expands there under zero external pressure and the work is zero. Assume that the gas is an ideal gas and explain how the final temperature of the gas after expansion compares to its initial temperature.

| 8 | ssm Suppose a material contracts when it is heated. Follow the same line of reasoning used in the text to reach Equations 15.7 and 15.8 and deduce which specific heat capacity for the material is larger, CP or CV.

| 9 | When a solid melts at constant pressure, the volume of the resulting liquid does not differ much from the volume of the solid. Using what you know about the first law of thermodynamics and the latent heat of fusion, describe how the internal energy of the liquid compares to the internal energy of the solid.

| 10 | Suppose you want to heat a gas so that its temperature will be as high as possible. Would you heat it under conditions of constant pressure or constant volume? Why?

| 11 | Consider a hypothetical device that takes 10 000 J of heat from a hot reservoir and 5000 J of heat from a cold reservoir and produces 15 000 J of work. (a) Does this device violate the first law of thermodynamics? Explain. (b) Does the device violate the second law of thermodynamics? Explain.

| 12 | If you saw an advertisement for an automobile that claimed the same gas mileage with and without the air conditioner operating, would you be suspicious? Explain, using what the second law of thermodynamics has to say about work and the direction of heat flow in an air conditioner.

| 13 | The second law of thermodynamics, in the form of Carnot’s principle, indicates that the most efficient heat engine operating between two temperatures is a reversible one. Does this mean that a reversible engine operating between the temperatures of 600 and 400 K must be more efficient than an irreversible engine operating between 700 and 300 K? Provide a reason for your answer.

| 14 | Concept Simulation 15.1 at allows you to explore the concepts that relate to this question. Three reversible engines, A, B, and C, use the same cold reservoir for their exhaust heats. However, they use different hot reservoirs that have the following temperatures: (A) 1000 K, (B) 1100K, and (C) 900 K. Rank these engines in order of increasing efficiency (smallest efficiency first). Account for your answer.

| 15 | In Concept Simulation 15.1 at you can explore the concepts that are important in this question. Suppose you wish to improve the efficiency of a Carnot engine. Compare the improvement to be realized via each of the following alternatives: (a) lower the Kelvin temperature of the cold reservoir by a factor of four, (b) raise the Kelvin temperature of the hot reservoir by a factor of four, (c) cut the Kelvin temperature of the cold reservoir in half and double the Kelvin temperature of the hot reservoir. Give your reasoning.

| 16 | ssm A refrigerator is kept in a garage that is not heated in the cold winter or air-conditioned in the hot summer. Does it cost more for this refrigerator to make a kilogram of ice cubes in the winter or in the summer? Give your reasoning.

| 17 | Is it possible for a Carnot heat pump to have a coefficient of performance that is less than one? Justify your answer.

| 18 | Air conditioners and refrigerators both remove heat from a cold reservoir and deposit it in a hot reservoir. Why, then, does an air conditioner cool the inside of a house while a refrigerator warms the house?

| 19 | It has been said that heat pumps can’t possibly deliver more energy into your house than they consume in operating. Can they or can’t they? Explain.

| 20 | A refrigerator is advertised as being easier to live with during the summer because it puts into your kitchen only the heat that it removes from the food. Does this advertising claim violate the second law of thermodynamics? Account for your answer.

| 21 | On a summer day a window air conditioner cycles on and off, according to how the temperature within the room changes. Are you more likely to be able to fry an egg on the outside part of the unit when the unit is on or when it is off? Explain.

| 22 | An event happens somewhere in the universe and, as a result, the entropy of an object changes by . Which one (or more) of the following is a possible value for the entropy change for the rest of the universe: ? Account for your choice(s) in terms of the second law of thermodynamics.

| 23 | When water freezes from a less ordered liquid to a more ordered solid, its entropy decreases. Why doesn’t this decrease in entropy violate the second law of thermodynamics?

| 24 | In each of the following cases, which has the greater entropy: (a) a handful of popcorn kernels or the popcorn that results from them, (b) a salad before or after it has been tossed, (c) a messy apartment or a neat apartment? Why?

| 25 | A glass of water contains a teaspoon of dissolved sugar. After a while, the water evaporates, leaving behind sugar crystals. The entropy of the sugar crystals is less than the entropy of the dissolved sugar because the sugar crystals are in a more ordered state. Why doesn’t this process violate the second law of thermodynamics?

| 26 | A builder uses lumber to construct a building, which is unfortunately destroyed in a fire. Thus, the lumber existed at one time or another in three different states: (1) as unused building material, (2) as a building, and (3) as a burned-out shell of a building. Rank these three states in order of decreasing entropy (largest first). Provide a reason for the ranking.

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