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Thermodynamics Practice Questions

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Updated on Sep 27, 2011

Review these concepts at: Thermodynamics Study Guide

Practice Questions

  1. What do you think: Is work a quantity of state or of process?
  2. If the temperature and number of moles remain constant and the volume of an ideal gas increases, what does the pressure do? Explain briefly.
  3. A piston is slowly acted on by a force of 20.0 N such that the pressure inside the piston remains constant at all times. The section of the piston is 201 cm2, and its final volume is 20% of the initial volume. If the piston's final volume is 4.02 · 10–3 m3, find out the work performed by the force.
  4. For the information given in practice problem 3, what is the sign of the work and what is its meaning?
  5. Find the piston's initial height, final height, and change in height for the process described in practice problem 3.
  6. Consider the PV-diagram in Figure 12.3, and find the work performed. The arrows indicate the direction of the process.
  7. Thermo-dynamics

  8. Can you apply the W = –P · ΔV formula to all parts of the graph? Why or why not? Explain briefly.
  1. On a cold winter morning, you cool off a cup of coffee by leaving it outside. Is it reasonable to say that the change in temperature when thermal equilibrium is reached is drastic for the coffee but not for the surrounding air? Explain your answer.
  2. Consider room-temperature air in a closed container. The container is in thermal contact with a heat source. Consider the air behaving as an ideal gas. At thermal equilibrium, is the pressure of the air in the container larger or smaller than it was initially?
  1. Consider a process in which the internal energy of a system increases by 125 J and the heat exchange is Q = –32 J. Find out what the work is and if it is performed by or on the system.
  2. In practice problem 10, is the heat absorbed or released by the system? Explain.
  1. If a system is thermal isolated (no heat exchange) and no work is performed by or on the system, how does the temperature of this system vary? Explain.
  2. A container of dimensions 12 × 16 × 6 cm holds a fluid at a temperature T = 300 K. The fluid is heated through an isothermal process (Δ U = 0 J) and an energy of 150 J is absorbed. Find the work produced in the process. Is the system expanding or contracting? Explain.
  3. In practice problem 13, consider an ideal gas inside the container, and the initial pressure of the gas is of the final pressure. Find the final volume.
  4. You are constructing a machine that works on a cycle similar to the one in Figure 12.7. Is any work being done by the system or on the system through the entire process? If yes, calculate the work. The arrows show the evolution of the parameters for this system.
  5. In practice problem 15, what is the amount of heat exchanged by the fluid?
  6. Is the heat absorbed or released by the system?
  7. Thermo-dynamics

Answers

  1. Of process, since it relates to a mechanical change.
  2. Decrease by the same ratio as the volume increases since the P, V product is constant.
  3. We will consider that all work done by the external force is used upon compressing the piston and so the work is W = P · Δ V = (F/area) · Δ V = 16 J.
  4. The work done by the force is negative since it's done on the outside, and the work received by the system is positive since it is performed on the system.
  5. Δ V = 16.1 · 10–3 m3, Vi = 1 m3, and Vi = 0.2 m3
  6. W = –2.9 · 102 J
  7. No, since the pressure is not constant along the whole process.
  8. Yes, due to the difference in sizes.
  9. Larger because the temperature increases at constant volume and then the pressure has to increase.
  10. W = + 157 J and it is performed on the system since it is positive.
  11. Negative heat means the system releases heat to the surroundings.
  12. It doesn't since internal energy has to be constant.
  13. W = – 150 J and the system is expanding.
  14. 768 cm3
  15. Yes, W = –270 J.
  16. The initial and final temperatures are the same. Hence, the change in internal heat is zero and W = – Q; then Q = 270 J.
  17. The system absorbs heat through this process since Q > 0.
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