HEC USAT-CS (Computer Science) Physics Heat & Thermodynamics — Set 2

HEC USAT-CS (Computer Science) Physics Heat & Thermodynamics — Set 2

1. Question 1

   Q1. The first law of thermodynamics is based on the law of conservation of

   • A) Energy
   • B) Momentum
   • C) Charge
   • D) Mass

   Answer: Energy

   Explanation: The first law, ΔU = Q - W, represents energy conservation, where ΔU is internal energy change, Q is heat added, and W is work done.

2. Question 2

   Q2. In an adiabatic expansion, the temperature of an ideal gas

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

   Answer: Decreases

   Explanation: In adiabatic expansion, Q = 0, and W is positive, so ΔU is negative, causing temperature to decrease, as per ΔU = nCvΔT.

3. Question 3

   Q3. The efficiency of a Carnot engine is 1 / 2. If the sink temperature is 300 K, the source temperature is

   • A) 300 K
   • B) 400 K
   • C) 500 K
   • D) 600 K

   Answer: 600 K

   Explanation: Efficiency = 1 - (T_sink / T_source). Given efficiency = 1 / 2 and T_sink = 300 K, we get 1 / 2 = 1 - (300 / T_source), so T_source = 600 K.

4. Question 4

   Q4. The specific heat capacity at constant volume (Cv) for an ideal monatomic gas is

   • A) (3 / 2)R
   • B) (5 / 2)R
   • C) (7 / 2)R
   • D) 2R

   Answer: (3 / 2)R

   Explanation: For a monatomic ideal gas, Cv = (3 / 2)R, where R is the gas constant, derived from the kinetic theory of gases.

5. Question 5

   Q5. In a thermodynamic process, the pressure of a gas is kept constant. The process is

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

   Answer: Isobaric

   Explanation: An isobaric process is one where the pressure remains constant, as per the definition of isobaric processes.

6. Question 6

   Q6. The change in internal energy of a system is zero when the process is

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

   Answer: Isothermal

   Explanation: For an ideal gas, internal energy (U) depends only on temperature. In an isothermal process, temperature remains constant, so ΔU = 0.

7. Question 7

   Q7. The molar specific heat of a gas at constant pressure (Cp) is greater than at constant volume (Cv) because

   • A) At constant pressure, the gas expands and does work
   • B) At constant volume, the gas is compressed
   • C) Cp includes the energy required to do work
   • D) Cv is always less than Cp

   Answer: Cp includes the energy required to do work

   Explanation: Cp > Cv because Cp includes the energy required to do work against external pressure, as per Cp = Cv + R.

8. Question 8

   Q8. For a given amount of an ideal gas, which of the following is true?

   • A) PV = constant at constant T
   • B) P / T = constant at constant V
   • C) V / T = constant at constant P
   • D) All of the above

   Answer: All of the above

   Explanation: Ideal gas law, PV = nRT, implies that at constant T, PV = constant; at constant V, P / T = constant; and at constant P, V / T = constant.

9. Question 9

   Q9. The temperature of a body is a measure of its

   • A) Total energy
   • B) Internal energy
   • C) Kinetic energy
   • D) Potential energy

   Answer: Internal energy

   Explanation: Temperature is a measure of the average kinetic energy of particles, related to internal energy, not total, kinetic, or potential energy directly.

10. Question 10

    Q10. The second law of thermodynamics implies that

    • A) Heat always flows from a colder body to a hotter body
    • B) The total entropy of an isolated system always increases
    • C) The efficiency of a heat engine can be 100%
    • D) The internal energy of an ideal gas is a function of pressure

    Answer: The total entropy of an isolated system always increases

    Explanation: The second law states that the total entropy of an isolated system always increases or remains constant, never decreases.

11. Question 11

    Q11. A thermodynamic system undergoes a cyclic process. The change in internal energy is

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

    Answer: Zero

    Explanation: In a cyclic process, the system returns to its initial state, so ΔU = 0, as internal energy is a state function.

12. Question 12

    Q12. The heat capacity of a body is defined as

    • A) The amount of heat required to raise its temperature by 1 K
    • B) The amount of heat required to raise the temperature of 1 kg by 1 K
    • C) The amount of heat required to change its state
    • D) The amount of work done on the body

    Answer: The amount of heat required to raise its temperature by 1 K

    Explanation: Heat capacity is the amount of heat (Q) required to change the temperature (T) of a body by 1 K, given by C = Q / ΔT.

13. Question 13

    Q13. The ratio of the specific heat capacities (Cp / Cv) for a monatomic ideal gas is

    • A) 5 / 3
    • B) 7 / 5
    • C) 3 / 2
    • D) 4 / 3

    Answer: 5 / 3

    Explanation: For a monatomic ideal gas, Cp = (5 / 2)R and Cv = (3 / 2)R, so Cp / Cv = 5 / 3.

14. Question 14

    Q14. In an isothermal expansion of an ideal gas, the internal energy

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

    Answer: Remains constant

    Explanation: For an ideal gas, internal energy (U) depends only on temperature. In an isothermal process, T remains constant, so U remains constant.

15. Question 15

    Q15. For an ideal gas, the work done in an isothermal expansion from V1 to V2 is given by

    • A) nRT ln(V2/V1)
    • B) nRT ln(V1/V2)
    • C) P(V2 - V1)
    • D) nR(T2 - T1)

    Answer: nRT ln(V2/V1)

    Explanation: Work done in isothermal expansion is given by nRT ln(V2/V1), derived from the ideal gas law and the definition of work.

16. Question 16

    Q16. A gas is compressed adiabatically. The change in internal energy is

    • A) Zero
    • B) Positive
    • C) Negative
    • D) Depends on the gas

    Answer: Positive

    Explanation: In adiabatic compression, work is done on the gas, increasing its internal energy, so ΔU is positive.

17. Question 17

    Q17. For a given process, the heat absorbed is 100 J and the work done is 50 J. The change in internal energy is

    • A) 50 J
    • B) 150 J
    • C) 200 J
    • D) -50 J

    Answer: 50 J

    Explanation: ΔU = Q - W = 100 J - 50 J = 50 J, applying the first law of thermodynamics.

18. Question 18

    Q18. The root mean square speed of the molecules of an ideal gas is proportional to

    • A) √T
    • B) T
    • C) 1/√T
    • D) 1/T

    Answer: √T

    Explanation: v_rms = √(3RT/M), so v_rms ∝ √T, derived from the kinetic theory of gases.

19. Question 19

    Q19. The specific heat capacity at constant pressure (Cp) for an ideal gas is

    • A) Cv + R
    • B) Cv - R
    • C) R/Cv
    • D) Cv

    Answer: Cv + R

    Explanation: For an ideal gas, Cp - Cv = R, so Cp = Cv + R, a fundamental thermodynamic relation.

20. Question 20

    Q20. A thermodynamic system undergoes a cycle. The change in internal energy is

    • A) Positive
    • B) Negative
    • C) Zero
    • D) Depends on the cycle

    Answer: Zero

    Explanation: In a cyclic process, the initial and final states are the same, so ΔU = 0.