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When an ideal monatomic gas is heated at constant pressure, what fraction of the heat energy supplied is used to increase the internal energy of the gas?

1. 2/5
2. 3/5
3. 3/7
4. 3/4

Correct answer: 3/5

Solution

For a monatomic ideal gas: degrees of freedom f=3; Cv=(3/2)R; Cp=(5/2)R; gamma=5/3. At constant pressure, heat supplied Q=nCpdT. Change in internal energy dU=nCvdT. Fraction = dU/Q = Cv/Cp = (3/2)/(5/2) = 3/5.

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