Physics Questions and Answers – Thermodynamics – Heat Engines

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Thermodynamics – Heat Engines”.

1. In a heat engine heat is converted to work and the process need not be cyclic. True or False?
a) True
b) False
View Answer

Answer: b
Explanation: In heat engines heat is to be converted into work. The process is cyclic as we need the conversion of some heat into work for continuously carrying out our desired outcome.

2. If an engine takes 5J of heat energy from a reservoir and rejects 3J to the surroundings, what is its efficiency?
a) 2/5
b) 3/5
c) 1/5
d) 0
View Answer

Answer: a
Explanation: Efficiency is defined as work done/heat input.
Work done = 5-3=2J.
Heat input = 5J.
Thus, efficiency = 2/5.

3. An engine works between 2 reservoirs of temperature 500K and 300K. A company claims that this engine of theirs takes 200J of energy from the hot reservoir and rejects 110J of energy. Is this True or False?
a) True
b) False
View Answer

Answer: b
Explanation: The maximum possible efficiency is given by:
1 – Tcold/Thot = 1 – 300/500 = 0.4.
The company claims that their engine has an efficiency of (200-110)/200 = 0.45.
This is not possible as maximum efficiency is 0.4.
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