First slide

First law of thermodynamics

Question

Unit mass of a liquid with volume V₁ is completely changed into a gas of volume V₂ at a constant external pressure $ρ$ at temperature T. If the latent heat of evaporation is L, then the increase in the internal energy of the system is

Moderate

A

zero
B

$p (V_{2} - V_{1})$
C

$L - p (V_{2} - V_{1})$
D

L

Solution

First law of thermodynamics given, $Δ Q = Δ U + p Δ V$
$\Rightarrow m L = Δ U + p (V_{2} - V_{1})$
$\Rightarrow Δ U = L - p (V_{2} - V_{1})$ $(∵ m = 1)$

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