A heat engine performs 100 J of work and at the same time rejects 400 J of heat energy to cold reservoirs.What is the efficiency of the engine?
Correct answer: A. 20%
- A. 20%
- B. 25%
- C. 4%
- D. 50%
Explanation
From the conservation of energy, the heat input (Q₁) must equal the work done (W) plus the heat rejected (Q₂). So, Q₁ = W + Q₂ = 100 J + 400 J = 500 J. Efficiency is η = W / Q₁ = 100 J / 500 J = 0.2, which is 20%.
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About First Law of Thermodynamics
The first law relates heat supplied, work done and the change in internal energy through energy conservation. Problems use sign conventions and apply the law to isothermal, adiabatic, isobaric and isochoric processes. Internal energy is a state function, while heat and work depend on the path followed.
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