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Ratio of specific heats

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The ratio of specific heats (also known as adiabatic index), usually denoted by \gamma, is the ratio of specific heat at constant pressure to the specific heat at constant volume.

\gamma \equiv \frac{C_p}{C_v}

The adiabatic index always exceeds unity; for a polytropic gas it is constant. For monatomic gas \gamma=5/3, and for diatomic gases \gamma=7/5, at ordinary temperatures. For air its value is close to that of a diatomic gas, 7/5 = 1.4.

Sometimes \kappa is used instead of \gamma to denote the specific heat ratio.

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