Educator

Heat Engines and Second Law of Thermodynamics

• The heat flow statement of the second law of thermodynamics: Heat flows spontaneously from a substance at high temperature to a substance at lower temperature; it does not flow spontaneously in the reverse direction.
• Essential features of a heat engine:
1. Heat is supplied to the engine at high T
2. Part of the input heat is used to do work
3. The remainder is rejected at a lower T.
• Efficiency of a heat engine e = 1 – Qc/QH, where QH is the input heat taken in at the higher T, and Qc is the heat rejected at the lower T.
• Kelvin-Planck statement of the second law: It is impossible to construct a heat engine, operating in a cycle, with efficiency e equal to 1.
• Heat pump: It takes energy from a cold object, along with energy input by work, and it rejects heat to a hot reservoir.
• Clausius statement of the second law: It is impossible to construct a machine that transfers energy from a cold object to a hot object without the input of energy by work.