Question

In a thermodynamic process, pressure of a fixed mass of a gas is changed in such a manner that the gas releases $20\, J$ of heat and $8\, J$ of work is done on the gas. If the initial internal energy of the gas was $30\, J$, the final internal energy will be:

• A. 18 J
• B. 9 J
• C. 4.5 J
• D. 36 J

Answer 1

Answer: (A)

Energy remains conserved in the process according to first law of thermodynamics.
When an amount of heat $Q$ is given to a system, a part of it will be used in increasing the internal energy $(\Delta U)$ of the system and the rest in doing work $(W)$ by the system,
hence $Q=\Delta U+W$
Heat released by system $Q=-20\,J$
Work done on the gas $ W=-8\,J $
$ \therefore \Delta U =Q-W$
$ =-20-(-8) =-12\,J$
Final internal energy
$U_{f}=U_{i}+\Delta U=30-12=18\,J$.