Question

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

• A. 18J
• B. 9J
• C. 4.5J
• D. 36J

Answer 1

Answer: (A)

18J

Answer 2

Given $\Delta Q = - 20J,$ $\Delta W = - 8J$and $U_{1} = 30J$

$\Delta Q = \Delta U + \Delta W$ ⇒ $\Delta U = (\Delta Q - \Delta W)$

⇒ $(U_{f} - U_{i})$ = $(U_{f} - 30) = - 20 - ( - 8)$ ⇒$U_{f} = 18J$