Army Medical College (AMC) Entry Test Physics Thermodynamics — Set 3

1. Question 1

   Q1. For an adiabatic expansion of an ideal gas, which quantity remains constant?

   • A) Internal Energy
   • B) Entropy
   • C) PV^\gamma
   • D) Temperature

   Answer: PV^\gamma

   Explanation: In adiabatic process, PV^\gamma = constant. Entropy is not exactly constant but for ideal gas it is.

2. Question 2

   Q2. A refrigerator's coefficient of performance is 4. How much heat is removed from the cold body if 100 J work is done?

   • A) 400 J
   • B) 500 J
   • C) 300 J
   • D) 200 J

   Answer: 400 J

   Explanation: COP = Q_c / W, so Q_c = COP * W = 4 * 100 = 400 J. Other options misinterpret COP or work done.

3. Question 3

   Q3. A system undergoes a cyclic process. What is the net change in internal energy?

   • A) Always positive
   • B) Always negative
   • C) Zero
   • D) Depends on the process

   Answer: Zero

   Explanation: In a cyclic process, \Delta U = 0 because initial and final states are the same.

4. Question 4

   Q4. For a given isothermal compression, which of the following is true?

   • A) Work done on the system is positive
   • B) Work done by the system is positive
   • C) Internal energy increases
   • D) Internal energy decreases

   Answer: Work done on the system is positive

   Explanation: In compression, work is done on the system, so it's positive. Internal energy doesn't change in isothermal process.

5. Question 5

   Q5. The entropy change of a free expansion is

   • A) Positive
   • B) Negative
   • C) Zero
   • D) Undefined

   Answer: Positive

   Explanation: In free expansion, entropy increases because disorder increases. Other options misinterpret entropy change.

6. Question 6

   Q6. A Carnot engine operates between 800K and 400K. What is its efficiency?

   • A) 25%
   • B) 50%
   • C) 75%
   • D) 100%

   Answer: 50%

   Explanation: Efficiency = 1 - (T_cold/T_hot) = 1 - (400/800) = 0.5 or 50%. Other options are wrong temperatures or calculations.

7. Question 7

   Q7. The molar specific heat at constant volume for a monatomic gas is

   • A) (3/2)R
   • B) (5/2)R
   • C) 3R
   • D) 2R

   Answer: (3/2)R

   Explanation: For monatomic gas, C_v = (3/2)R. Other options are specific heats for different types of gases.

8. Question 8

   Q8. A gas is compressed adiabatically. What happens to its temperature?

   • A) Decreases
   • B) Remains constant
   • C) Increases
   • D) Can't be determined

   Answer: Increases

   Explanation: In adiabatic compression, temperature increases because work done on the system increases its internal energy.

9. Question 9

   Q9. The first law of thermodynamics is a statement of

   • A) Conservation of energy
   • B) Conservation of momentum
   • C) Conservation of charge
   • D) Conservation of mass

   Answer: Conservation of energy

   Explanation: First law, \Delta U = Q - W, represents conservation of energy. Other options are unrelated conservation laws.

10. Question 10

    Q10. For an ideal gas, the internal energy during an isothermal expansion

    • A) Increases
    • B) Decreases
    • C) Remains constant
    • D) First increases then decreases

    Answer: Remains constant

    Explanation: Internal energy of ideal gas depends only on temperature, which remains constant in isothermal process.

11. Question 11

    Q11. A heat pump operates between 250K and 300K. What is its coefficient of performance?

    • A) 5
    • B) 6
    • C) 7
    • D) 8

    Answer: 6

    Explanation: COP = T_hot / (T_hot - T_cold) = 300 / (300 - 250) = 6. Other options misinterpret COP formula or temperatures.

12. Question 12

    Q12. In a thermodynamic process, the work done by the gas is 200 J and the heat supplied is 300 J. What is the change in internal energy?

    • A) 100 J
    • B) -100 J
    • C) 500 J
    • D) -500 J

    Answer: 100 J

    Explanation: \Delta U = Q - W = 300 - 200 = 100 J. Other options misapply the first law.

13. Question 13

    Q13. A thermodynamic system undergoes an isobaric expansion. What remains constant?

    • A) Pressure
    • B) Volume
    • C) Temperature
    • D) Internal Energy

    Answer: Pressure

    Explanation: In isobaric process, pressure remains constant. Other options change during isobaric expansion.

14. Question 14

    Q14. The change in entropy for a reversible adiabatic process is

    • A) Positive
    • B) Negative
    • C) Zero
    • D) Undefined

    Answer: Zero

    Explanation: For reversible adiabatic process, \Delta S = 0 because it's isentropic. Other options misinterpret entropy change.

15. Question 15

    Q15. A thermodynamic process occurs in which the volume remains constant. What type of process is this?

    • A) Isothermal
    • B) Adiabatic
    • C) Isobaric
    • D) Isochoric

    Answer: Isochoric

    Explanation: Isochoric process is one where volume remains constant. Other options are different types of thermodynamic processes.

16. Question 16

    Q16. Why does the temperature of a gas remain constant during an isothermal expansion?

    • A) Because the gas is ideal
    • B) Because heat is absorbed by the gas
    • C) Because the gas is insulated
    • D) Because the expansion is slow

    Answer: Because heat is absorbed by the gas

    Explanation: Heat is absorbed, maintaining constant temperature; 'ideal gas' is a property, not the reason for constant temperature.

17. Question 17

    Q17. In an adiabatic compression, what happens to the internal energy of the gas?

    • A) It decreases
    • B) It remains the same
    • C) It increases
    • D) It becomes zero

    Answer: It increases

    Explanation: Internal energy increases due to work done on the gas; no heat transfer occurs in adiabatic processes.

18. Question 18

    Q18. A refrigerator transfers heat from a cold body to a hot body. Which law of thermodynamics does this process violate if there is no external work?

    • A) Zeroth Law
    • B) First Law
    • C) Second Law
    • D) Third Law

    Answer: Second Law

    Explanation: Second Law is violated as spontaneous heat transfer from cold to hot is impossible without external work.

19. Question 19

    Q19. What is the primary function of a heat engine's 'sink'?

    • A) To supply heat to the engine
    • B) To absorb excess heat from the engine
    • C) To do work on the surroundings
    • D) To increase the engine's efficiency

    Answer: To absorb excess heat from the engine

    Explanation: The sink absorbs excess heat, allowing the engine to operate in a cycle; it doesn't supply heat or do work.

20. Question 20

    Q20. Why can't a real heat engine achieve 100% efficiency?

    • A) Due to friction
    • B) Due to heat loss
    • C) Because it violates the Second Law
    • D) Because it operates in a cycle

    Answer: Because it violates the Second Law

    Explanation: 100% efficiency violates the Second Law; friction and heat loss are practical limitations, not the fundamental reason.