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Statements of the Second Law

Clausius Statement

:
A system operating in a thermodynamic cycle can not transfer energy (heat) from a cooler body to a hotter body and produce no other effect on the universe.

Kelvin - Planck Statement

:
A system operating in a thermodynamic cycle can not produce a net amount of work while receiving heat from a single thermal reservoir.

Statements of the Second Law of Thermodynamics

The Kelvin-Planck Statement tells us that the efficiency of a heat engine must be less than 100%.

What is the maximum efficiency for a heat engine?

Lesson 6D: page 1 of 11

Previous LESSON

We introduced the Clausius and Kelvin-Planck Statements of the 2nd Law in the previous lesson.
In this lesson, we are going to begin to learn why the 2nd Law is so important to the study of thermodynamic cycles.
We begin with the Kelvin-Planck statement, which tells us that the efficiency of a heat engine must be less than 100%.
The question that leaps to mind is, exactly how much less ? Can a heat engine have an efficiency of 99% or is 83% the limit ?
What is really the upper limit on the efficiency ?
We will begin to resolve this question in this lesson, but we won’t determine the complete answer until the last lesson of this chapter.
You’ve got to be patient because there are several new ideas that you need to understand before we can state the upper limit on the efficiency of a heat engine.
That’s enough foreshadowing for now. Let’s get started.

Statements of the Second Law

Clausius Statement

Kelvin - Planck Statement

Ch6, Lesson D, Page 1 - Statements of the Second Law