# Engineering Physics Questions and Answers – Thermodynamics – 2

This set of Engineering Physics test focuses on “Thermodynamics – 2”.

1. In a given process of an ideal gas, dW=0 and the dQ<0. Then for the gas ___________
a) The temperature will decreases
b) The temperature will increase
c) The pressure will remain constant
d) The temperature will increase

Explanation: By first law of thermodynamics,
dQ = dU+dW
As dW = 0 and dQ<0, so dU<0
But for an ideal gas,
U is proportional to T
dT<0
Hence temperature of the gas decreases.

2. The temperature of reservoir of Carnot’s engine operating with an efficiency of 70% is 1000K. What is the temperature of source?
a) 300K
b) 400K
c) 500K
d) 700K

Explanation: 0.70 = 1-T2/1000
T2=0.30×1000=300K.

3. The temperature of sink of Carnot engine is 27°C. Efficiency is 25%. Then ___________
a) 227°C
b) 327°C
c) 127°C
d) 27°C

Explanation: 25/100=1-300/T1
300/T1 =1-1/4=3/4
T1=(300×4)/3=400K=127°C.

4. The freezer in a refrigerator is located at the top section so that ___________
a) The entire chamber of the refrigerator is cooled quickly due to convection
b) The motor is not heated
c) The heat gained from the environment is high
d) The heat gained from the environment is low

Explanation: The upper cold denser air goes down while the lower warmer air goes up. The convection currents so set up quickly cool the chamber of the refrigerator.

5. Which of the following is not a thermodynamics co-ordinate?
a) P
b) T
c) V
d) R

Explanation: P, T and V are thermodynamic variables while the gas constant R is not thermodynamics coordinate.
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6. A perfect gas is contained in a cylinder kept in vacuum. If the cylinder suddenly bursts, then the temperature of the gas ___________
a) Becomes zero K
b) Is decreased
c) In increased
d) Remains unchanged

Explanation: No work is done during the free expansion of a gas. There is no change in internal energy or temperature of the gas.

7. The increase in internal energy of a system is equal to the work done in the system. Which process does the system undergo?
a) Isochoric
c) Isobaric
d) Isothermal

Explanation: As work done on a system is negative, so ∆U=-∆W or ∆U+∆W=0 or ∆Q=0. Hence the system undergoes an adiabatic process.

8. If the temperature of the source is increased, the efficiency of a Carnot engine?
a) Increases
b) Decreases
c) Remains constant
d) First increases and then remains constant

Explanation: Efficiency,
Ƞ=1-T2/T1
When the temperature T1 of the source increases, the efficiency increases.

9. Assertion: In adiabatic compression, the internal energy and temperature of the system gets decreased.
Reason: The adiabatic compression is a slow process.
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. During adiabatic compression, work is done on the system. Both the internal energy and temperature of the system increases.

10. The efficiency of Carnot’s engine operating between reservoirs, maintained at temperature 27°C and -123°C is ___________
a) 50%
b) 24%
c) 0.75%
d) 0.4%

Explanation: Ƞ=1-T2/T1
Ƞ=1-(273-123)/(273+27)=1-150/300=1/2=50%.

11. The temperature of sink of Carnot’s engine is 27°C. If the efficiency of the engine is 25%, then the temperature of source must be ___________
a) 27°C
b) 127°C
c) 227°C
d) 327°C

Explanation: Ƞ=1 – T2/T1
0.25=1 – (273+27)/T1
T1=300/0.75=400K=127℃.

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