Question

What would be the work done by the gas if the entire process is reversed (3 - 2 - 1)?

• A. -30 J
• B. 30 J
• C. -22.5 J
• D. 22.5 J

Answer 1

The correct option is A

-30 J

When the gas is taken through the process 3 - 2 - 1, it reduces in volume (from 700 cc to 250 cc).
Let us consider that this process happens on the system shown in the figure below.


When the piston is moving from the initial position (state marked by 1) to the final position (state marked by 3) it moves in the direction opposite to that of the force $F_g$ applied by the gas on it ($F_g$ = pressure of gas $\times$ area).
Since the displacement and force are opposite to each other, the work done by the gas will be negative.

The magnitude of the work done will remain the same. Hence the work done by the gas during the process 3 - 2 - 1 will be -30 J.