Physics Multiple Choice Questions on “Thermodynamics – Refrigerators and Heat Pumps”.
1. What is the coefficient of performance of a refrigerator? Let Q1 be the heat released to hot reservoir, Q2 be the heat extracted from a cold reservoir & W be the work done on the refrigerator.
a) Q1/W
b) Q1/Q2
c) Q2/W
d) Q2/Q1
Answer: c
Clarification: Coefficient of performance of a refrigerator is defined as the ratio of heat extracted from the cold reservoir to the work done on the refrigerator. It is denoted by α = Q2/W.
2. Select the correct order of steps in which a working substance(gas) goes through in a refrigerator.
a) Expansion of gas, absorption of heat, heating of vapour, release of heat
b) Absorption of heat, expansion of gas, heating of vapour, release of heat
c) Heating of vapour, expansion of gas, absorption of heat, release of heat
d) Heating of vapour, absorption of heat, expansion of gas, release of heat
Answer: a
Clarification: In a refrigerator, the working substance(gas) first gets expanded from high to low pressure which cools it into a vapour-liquid mixture. Then heat is absorbed from the region to be cooled. Then the external work done on the system heats up the vapour. Finally, the vapour’s heat is released to the surroundings.
3. Efficiency of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: b
Clarification: Efficiency by definition can never be greater than 1. Else the law of energy conservation will be violated.
4. The coefficient of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q2/W,
where Q2 is heat extracted from a cold reservoir and W is work done.
α = Q2/( Q1 – Q2).
Therefore α can be less than or greater than one.
5. 20J of heat energy is extracted from a cold reservoir when 30J of work is done on a refrigerator. What is the coefficient of the refrigerator?
a) 2/3
b) 5/3
c) 3/5
d) 1/3
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q2/W,
where Q2 is heat extracted from a cold reservoir and W is work done.
α = 20/30 = 2/3.