Medium MCQ +4 / -1 PYQ · JEE Mains 2020

Under an adiabatic process, the volume of an ideal gas gets doubled. Consequently the mean collision time between the gas molecule changes from ${\tau _1}$ to ${\tau _2}$ . If ${{{C_p}} \over {{C_v}}} = \gamma$ for this gas then a good estimate for ${{{\tau _2}} \over {{\tau _1}}}$ is given by :

  1. A ${\left( 2 \right)^{{{1 + \gamma } \over 2}}}$ Correct answer
  2. B 2
  3. C ${\left( {{1 \over 2}} \right)^{{{1 + \gamma } \over 2}}}$
  4. D ${\left( {{1 \over 2}} \right)^\gamma }$

Solution

$\tau$ $\propto$ ${V \over {\sqrt T }}$ ....(1) <br><br>Also we know, PV<sup>$\gamma$</sup> = k <br><br>We know, PV = nRT <br><br>$\Rightarrow$ P $\propto$ ${T \over V}$ <br><br>$\therefore$ $\left( {{T \over V}} \right)$V<sup>$\gamma$</sup> = k <br><br>$\Rightarrow$TV<sup>$\gamma$ - 1</sup> = k <br><br>$\Rightarrow$ T $\propto$ V<sup>1 - $\gamma$</sup> <br><br>Using this value in equation (1) <br><br>$\tau$ $\propto$ ${V \over {{V^{{{1 - \gamma } \over 2}}}}}$ <br><br>$\Rightarrow$ $\tau$ $\propto$ ${{V^{1 - {{1 - \gamma } \over 2}}}}$ <br><br>$\Rightarrow$ $\tau$ $\propto$ ${{V^{{{\gamma + 1} \over 2}}}}$ <br><br>$\therefore$ ${{{\tau _2}} \over {{\tau _1}}}$ = ${\left( {{{2V} \over V}} \right)^{^{{{\gamma + 1} \over 2}}}}$ = ${\left( 2 \right)^{{{1 + \gamma } \over 2}}}$

About this question

Subject: Physics · Chapter: Thermodynamics · Topic: Zeroth and First Law

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