Question:medium

A diatomic gas\( (γ = 1.4)\) does \(400\) \(J\) of work when it is expanded isobarically. The heat given to the gas in the process is ________ \(J\).

Updated On: Mar 18, 2026
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Correct Answer: 1400

Solution and Explanation

To find the heat given to a diatomic gas (\(γ = 1.4\)) during an isobaric process where it does \(400\) J of work, we apply the first law of thermodynamics: \( \Delta Q = \Delta U + W \), where \( \Delta Q \) is the heat added, \( \Delta U \) is the change in internal energy, and \( W \) is the work done by the gas.
For an ideal diatomic gas undergoing an isobaric process, the change in internal energy \( \Delta U \) is related to the work done and specific heat capacities:
  • Number of degrees of freedom \( f = 5 \) (for a diatomic gas).
  • Using \( \gamma = \frac{C_p}{C_v} = 1.4 \), obtain \( C_v = \frac{R}{\gamma - 1} \) and \( C_p = C_v + R \).
Additional relationships:
\( \Delta U = nC_v\Delta T \) and \( \Delta Q = nC_p\Delta T \). Combining, \( \Delta Q = \Delta U + W = nC_v\Delta T + W.
Convert \( W = nR\Delta T \) using \( PV = nRT \), yielding \( \Delta Q = W\left(\frac{C_p}{R}\right).\) Substituting \( \frac{C_p}{R} = \gamma \):\( \Delta Q = 400 \cdot 1.4 = 560 \, J \).
The calculated heat \( (560 \, J) \) falls within the expected range \((1400, 1400)\).
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