Energy Conversion and the Second Law:The Kelvin-Planck Statement

The Kelvin-Planck Statement

Even the—fully reversible—Carnot engine has a thermal efficiency ηC below unity: Not all heat received from the hot reservoir can be converted into work, some heat must be rejected to a colder reservoir. The Kelvin-Planck formulation of the second law states this as follows:

No steady state thermodynamic process is possible in which heat is completely converted into work.

Energy Conversion and the Second Law-0032

This statement is a direct consequence of the first and second law. For a steady state process with just one heat exchange the laws require

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hence heat and work must both be negative. Figure 5.3 shows the forbidden process, and also the—allowed—inverse process, the complete conversion of work into heat through friction. A typical example for the latter are resistance heaters in which electrical work is converted to heat through electric resistance.

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