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 30 joules of heat and 10 joule of work is done on the gas. If the initial internal energy of the gas was 40 joule, then the final internal energy will be

Moderate

A

zero
B

80 joule
C

20 joule
D

-20 joule

Solution

Given that $dQ = â 30 J and dW = â 10 J$
$U_{i} = 40 J and let final internal energy = U_{f}$
So,
$\begin{array}{l} dQ = U_{f} â U_{i} + dW \\ â 30 = U_{f} â 40 â 10 \\ U_{f} = 20 J \end{array}$

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