Why is gamma usually larger for a monatomic gas than for a diatomic gas?
Adding heat to a monatomic gas mainly feeds three translational modes, so f = 3 and gamma = 5/3.
Adding heat to a monatomic gas mainly feeds three translational modes, so f = 3 and gamma = 5/3. A diatomic gas at ordinary temperatures can also rotate, giving f = 5 and gamma = 7/5. Those extra storage modes raise C_V; since gamma = 1 + R/C_V, a larger C_V makes gamma smaller.
Key point
More active degrees of freedom lower gamma.
Common mistake
memorising 5/3 and 7/5 without connecting them to degrees of freedom.
Memory tip
more places to store heat means a smaller gamma.
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