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Question: Answered & Verified by Expert
A polyatomic gas has $f$ vibrational degrees of freedom, then the ratio of the specific heat at constant pressure to that at constant volume will be
PhysicsKinetic Theory of GasesJEE Main
Options:
  • A $\frac{4+f}{3+f}$
  • B $\frac{4-f}{3-f}$
  • C $\frac{3+f}{4+f}$
  • D $\frac{3-f}{4-f}$
Solution:
1359 Upvotes Verified Answer
The correct answer is: $\frac{4+f}{3+f}$
For a polyatomic atom value of specific heat at constant volume, $C_V=(3+f) R$ where, $f=$ vibration degree of freedom and $R=$ gas constant.
Now,
$$
\begin{aligned}
& C_p=C_V+R \\
& C_p=(3+f) R+R \\
& C_p=(4+f) R
\end{aligned}
$$
Now, ratio of $C_p$ and $C_V$
$$
\frac{C_p}{C_V}=\frac{4+f}{3+f}
$$

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