NSAA NSAA Physics - Thermal Physics and Thermodynamics 2 — Questions and Answers
Question 1: In an adiabatic process, which of the following is true?
- Heat is exchanged freely with the surroundings
- Temperature remains constant throughout
- No heat is exchanged with the surroundings (Correct answer)
- Pressure remains constant throughout
Correct answer: No heat is exchanged with the surroundings
An adiabatic process is defined by Q = 0, meaning no heat is transferred between the system and its surroundings.
Question 2: A Carnot engine operates between a hot reservoir at T_H = 600 K and a cold reservoir at T_C = 300 K. What is its efficiency?
- 25%
- 50% (Correct answer)
- 75%
- 100%
Correct answer: 50%
Carnot efficiency η = 1 − T_C/T_H = 1 − 300/600 = 0.50 = 50%.
Question 3: Which statement correctly expresses the second law of thermodynamics?
- The total energy of an isolated system is constant
- The entropy of an isolated system tends to increase in irreversible processes (Correct answer)
- The entropy of a perfect crystal at absolute zero is zero
- Systems in equilibrium have the same pressure
Correct answer: The entropy of an isolated system tends to increase in irreversible processes
The second law states that natural processes in isolated systems proceed in the direction of increasing entropy.
Question 4: The specific latent heat of vaporization of a substance is the energy required to:
- Raise 1 kg of the substance by 1°C
- Convert 1 kg of solid to liquid at constant temperature
- Convert 1 kg of liquid to gas at constant temperature (Correct answer)
- Cool 1 kg of gas by 1°C at constant pressure
Correct answer: Convert 1 kg of liquid to gas at constant temperature
Latent heat of vaporization is the energy per unit mass needed to change phase from liquid to gas at constant temperature and pressure.
Question 5: According to the kinetic theory of gases, pressure is caused by:
- Gravitational attraction between gas molecules
- Collisions of gas molecules with the container walls (Correct answer)
- Intermolecular repulsion between adjacent molecules
- The weight of gas molecules pressing downward
Correct answer: Collisions of gas molecules with the container walls
Gas pressure arises from the rate of momentum transfer when molecules collide elastically with the container walls.
Question 6: Over one complete thermodynamic cycle, the net work done by the system equals:
- The total heat absorbed by the system
- The heat absorbed minus the heat rejected (Correct answer)
- The change in internal energy over the cycle
- Zero, because all state variables return to their initial values
Correct answer: The heat absorbed minus the heat rejected
Over a complete cycle ΔU = 0, so by the first law W_net = Q_in − Q_out (heat absorbed minus heat rejected).
Question 7: Which thermodynamic process occurs at constant pressure?
- Isothermal
- Adiabatic
- Isobaric (Correct answer)
- Isochoric
Correct answer: Isobaric
An isobaric process is defined by constant pressure throughout; the prefix 'iso-bar' (equal weight/pressure) indicates this.
NSAA NSAA Physics - Thermal Physics and Thermodynamics 2
In an adiabatic process, which of the following is true?