Physics Questions and Answers – Thermodynamics – Refrigerators and Heat Pumps

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses 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
View Answer

Answer: c
Explanation: 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.
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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
View Answer

Answer: a
Explanation: 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
View Answer

Answer: b
Explanation: Efficiency by definition can never be greater than 1. Else the law of energy conservation will be violated.
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4. The coefficient of a refrigerator can be greater than 1. True or False?
a) True
b) False
View Answer

Answer: a
Explanation: 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
View Answer

Answer: a
Explanation: 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.
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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter