Q. Unit mass of a liquid volume $V_{1}$is completely charged into a gas of volume $V_{2}$at a constant external pressure p and temperature T. If the latent heat of evaporation for the given mass is L, then the increase in the internal energy of the system is
A) Zero
B) $p(V_{2}{- V}_{1})$
C) $L - p(V_{2} - V_{1})$
D) L
Explanation
(c) From first law of thermodynamics
$\mathrm{\Delta}Q = \mathrm{\Delta}U + p\mathrm{\Delta}V$
$\Rightarrow \ \ \ \ \ mL = \Delta U + p\left( V_{2} - V_{1} \right)$
$\Rightarrow \ \ \ \ \ \ \mathrm{\Delta}U = L - p\left( V_{2} - V_{1} \right)\ \ \ \ \ \ \ \ \ \ \ \ \ (\because m = 1)$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.