MCQ
The heat capacity of one mole an ideal is found to be $C_V=3 R(1+a R T) / 2$, where $a$ is a constant. The equation obeyed by this gas during a reversible adiabatic expansion is
  • $T V^{3 / 2} e^{a R T}=$ constant
  • B
    $T V^{3 / 2} e^{3 a R T / 2}=$ constant
  • C
    $T V^{3 / 2}=$ constant
  • D
    $T V^{3 / 2} e^{2 a R T / 3}=$ constant

Answer

Correct option: A.
$T V^{3 / 2} e^{a R T}=$ constant
a
(A)

From the given data,

$C _{ V }=\frac{3 R ( a + aRT )}{2}$

So,

$C _{ p }= C _{ V }+ R =\frac{3 R ( a + aRT )}{2}+ R =\frac{3 R ( a + aRT )+2 R }{2}$

So,

$\gamma=\frac{C_{ P }}{C_{ V }}-1=\frac{\frac{3 R(a+a R T)+2 R}{2}}{\frac{3 R(a+a R T)}{2}}-1=\frac{2}{3(1+a R T)}=\frac{2}{3}(1+a R T)^{-1}$

Now, by putting this value of $\gamma-1$ in the adiabatic expression formula, we get $TV ^{\gamma-1}=$ constant $\Rightarrow TV ^{3 / 2} e ^{ aRT }=$ constant (we get this by binomial expansion).

Hence proved.

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