# Engineering Physics Questions and Answers – Thermal Equilibrium

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Thermal Equilibrium”.

1. Any process in which the system returns to its initial state after undergoing a series of changes is known as ___________
a) Clockwise process
b) Anticlockwise process
c) Cyclic process
d) Thermodynamic process

Explanation: Any process in which the system returns to its initial state is known as a cyclic process. The change in internal energy after a complete cycle is zero because the system returns to its initial state.

2. A gas does work during isothermal expansion. The source of mechanical energy so produced is the internal energy of the gas itself.
a) True
b) False

Explanation: By the first law of thermodynamics,
∆Q=∆U+∆W.
But for an isothermal process, ∆U is zero, so ∆Q=∆W. Thus the energy required for doing mechanical work during an isothermal process is obtained as heat by the gas from the surroundings.

3. An ideal gas is pressed at a constant temperature. Its internal energy ___________
a) Decreases
b) Increases
c) First increases and then decreases
d) Remains the same

Explanation: When an ideal gas is pressed at a constant temperature, its internal energy remains the same because the internal energy of the gas depends only on its temperature.

4. When a gas is suddenly compressed, its temperature decreases.
a) True
b) False

Explanation: Sudden compression of gas gives an adiabatic process. The work done in compressing the gas increases the internal energy of the gas. Hence the temperature of the gas rises.

5. A monoatomic ideal gas, initially at temperature T1 is enclosed in a cylinder fitted with a frictionless piston. The gas is allowed to expand adiabatically to a temperature T2 by releasing the piston suddenly. L1 and L2 are the lengths of the gas column before and after expansion respectively, then T1/T2 is given by?
a) (L1/L2)(2/3)
b) L1/L2
c) L2/L1
d) (L2/L1)(2/3)

(TV)(γ-1)=constant
$$T_1 V_1^{γ-1}=T_2 V_2^{γ-1}$$
For a monoatomic gas, γ=5/3
If A is the area of cross-section of the piston, then
T1/T2 =(V2/V1)(γ-1)
T1/T2 =((AL2)/(AL1))(2/3).

6. An ideal gas heat engine is operating between 227°C and 127°C. It absorbs 104 J of heat at the higher temperature. The amount of heat converted into work is?
a) 2000 J
b) 4000 J
c) 8000 J
d) 5600 J

Explanation:
Ƞ=1-T2/T1 = 1-(273+127)/(273+227)=1-400/500=1/5
W=ȠQ1=1/5×104=2000J.

7. Internal energy change, when a system goes from state A and B is 40 kJ mole-1. If the system goes from A and B by a reversible path and returns to state A by an irreversible path, what would be the net change in internal energy?
a) 40 kJ
b) Greater than 40 kJ
c) Later than 40 kJ
d) Zero

Explanation: As the system returns to the initial state A, the change in internal energy is zero.

8. Even Carnot engine cannot give 100% efficiency, because we cannot ___________
b) Find ideal sources
c) Reach absolute zero temperature
d) Eliminate friction

Explanation: The efficiency of a Carnot engine will be 100% when its sink is at 0K. But the temperature 0K cannot be realised in practice, so the efficiency is never 100%.

9. Assertion: The temperature of a gas does not change, when it undergoes an adiabatic expansion.
Reason: During an adiabatic process, heat energy is exchanged between a system and its surrounding.
a) Both assertion and reason are true and the reason is the correct explanation of the assertion
b) Both assertion and reason are true but the reason is not a correct explanation of the assertion
c) Assertion is true but the reason is false
d) Both assertion and reason are false

Explanation: Both the assertion and reason are false. Temperature decreases during adiabatic expansion while it increases during adiabatic compression.

10. We consider a thermodynamic system. If ∆U represents the increase in its internal energy and W, the work done by the system; which of the following statements are true?
a) ∆U=-W in an isothermal process
b) ∆U=W in an isothermal process
c) ∆U=-W in an adiabatic process
d) ∆U=W in an adiabatic process

∆Q=0
∆U=∆Q-∆W=0-W=-W.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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