First slide

First law of thermodynamics

Question

In a thermodynamic process, pressure of a fixed mass of a gas is changed in such a manner that the gas molecules gives out 20 J of heat and 10 J of work is done on the gas. If the initial internal energy of the gas was 40 J, then the final internal energy will be

Moderate

A

30 J
B

20 J
C

60 J
D

40 J

Solution

$\begin{array}{l} ΔQ = ΔU + ΔW = (U_{f} - U_{i}) + ΔW \\ \Rightarrow 30 = (U_{f} - 40) + 10 \Rightarrow U_{f} = 60 J \end{array}$

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