Carnot Engine: Proof of Carnot Theorem...

“It is impossible for any heat engine to be more efficient than a Carnot engine when operating between two given temperatures”.

The Carnot engine is a conceptual engine that achieves the most efficient conversion of heat to work permitted by Kelvin’s statement. In general, efficiency is defined as the ratio of work out to heat in:

For a Carnot engine, the efficiency is found in terms of the temperature of the reservoirs the engine operates between:

Carnot’s Theorem

It is impossible for any heat engine to be more efficient than a Carnot engine when operating between two given temperatures:

Consider a heat engine drawing heat Q1 from a heat reservoir at temperature T1​, delivering work W and dumping heat Q2​ into a heat sink at temperature T2.

The heat engine operates in cycles, that is, it takes in heat Q1, does work W, dumps heat Q2, and in the end returns back to its original unchanged state.

Consider the net change in entropy ΔS of the universe:

The heat reservoir releases a heat Q1 at a constant temperature T1. Thus the change in its entropy is

The heat sink accepts heat Q2 at a constant temperature T2. Thus the change in its entropy is

Thus the net change in entropy of the universe is

Using the second law of thermodynamics, ΔS≥0, which implies

Since the left-hand side represents the efficiency of the given heat engine (η) and the right-hand side is the efficiency of a Carnot engine,

Hence proved!