Which of the following statements regarding Carnot engine is/are correct? 1. It is an irreversible engine operating between two temperatures T1 (source) and T2 (sink). 2. Its efficiency is given by η = 1 − T2/T1, where T1 is the temperature of source and T2 is the temperature of sink. Select the answer using the code given below.
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Correct answer
Explanation
The correct answer is B, 2 only. A Carnot engine is an ideal reversible engine, not an irreversible one, and its efficiency is 1 − T2/T1 with temperatures in kelvin.
Sadi Carnot described this engine in 1824. Its working substance goes through a reversible cycle of four steps: isothermal expansion at the source temperature T1, adiabatic expansion, isothermal compression at the sink temperature T2, and adiabatic compression. Carnot's theorem says no engine working between the same two temperatures can be more efficient than a reversible one, so this efficiency is the upper limit for all real engines. For example, between 500 K and 300 K the best possible efficiency is 1 − 300/500 = 40%. It can reach 100% only if the sink were at absolute zero.
A is wrong because statement 1 calls the engine irreversible. C is wrong because statement 1 is false. D is wrong because statement 2 gives the correct formula.
Exam tip: Carnot = reversible, η = 1 − T2/T1 (kelvin), the maximum for any heat engine.
Practice Questions
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