Physics Thermodynamics 3 — Questions and Answers
Question 1: According to the zeroth law of thermodynamics, if A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then:
- A and C are at different temperatures
- A and C are in thermal equilibrium (Correct answer)
- No conclusion can be drawn about A and C
- B must be hotter than both A and C
Correct answer: A and C are in thermal equilibrium
The zeroth law states that thermal equilibrium is transitive: A and C must also be in thermal equilibrium.
Question 2: Which statement best describes the third law of thermodynamics?
- Energy cannot be created or destroyed
- Heat flows from hot to cold spontaneously
- The entropy of a perfect crystal approaches zero as temperature approaches absolute zero (Correct answer)
- The internal energy of an ideal gas depends only on temperature
Correct answer: The entropy of a perfect crystal approaches zero as temperature approaches absolute zero
The third law states that as T → 0 K, the entropy of a pure perfect crystal approaches zero.
Question 3: In a P-V diagram, the area enclosed by a cyclic process represents:
- The change in internal energy
- The net work done by the system (Correct answer)
- The total heat absorbed
- The change in entropy
Correct answer: The net work done by the system
The enclosed area on a P-V diagram equals the net work done by the system per cycle.
Question 4: An ideal gas undergoes an isochoric (constant volume) process. Which of the following is true?
- No work is done on or by the gas (Correct answer)
- No heat is exchanged
- Internal energy stays constant
- Pressure stays constant
Correct answer: No work is done on or by the gas
At constant volume, ΔV = 0, so W = PΔV = 0; no work is done.
Question 5: The specific heat capacity at constant pressure (Cp) is always _____ the specific heat capacity at constant volume (Cv) for an ideal gas.
- Less than
- Equal to
- Greater than (Correct answer)
- Half of
Correct answer: Greater than
Cp = Cv + R for an ideal gas, because at constant pressure extra energy goes into doing work on the surroundings.
Question 6: Which of the following processes is spontaneous and results in an increase of entropy?
- Freezing of water at 0°C under 1 atm
- Compression of a spring
- Mixing of two different ideal gases (Correct answer)
- Charging a battery
Correct answer: Mixing of two different ideal gases
Mixing two different ideal gases is irreversible and increases the total entropy of the system.
Question 7: For a monatomic ideal gas, the ratio γ = Cp/Cv equals:
- 1.2
- 1.4
- 1.67 (Correct answer)
- 2.0
Correct answer: 1.67
For a monatomic ideal gas, Cv = (3/2)R and Cp = (5/2)R, so γ = 5/3 ≈ 1.67.
Physics Thermodynamics 3
According to the zeroth law of thermodynamics, if A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then: