Answer: A cyclic process is one in which the system, after going through a series of changes, returns to its initial state. For it \(\Delta U = 0\) and \(Q = W\) = area enclosed by the loop on the p-V diagram.
[Figure: p-V diagram with pressure p on the vertical axis and volume V on the horizontal axis. A closed loop A → B → C → D → A: the gas expands from A to B along the upper curve and is compressed from C back to A along the lower curve. Arrows show a clockwise direction and the area inside the loop is shaded and marked “net work W”.]
- Since the system returns to the same state, its internal energy (a state function) is unchanged: \(\Delta U = 0\).
- By the first law \(Q = \Delta U + W\), so \(Q = W\): the net heat absorbed equals the net work done.
- Work done during expansion (area under the upper curve) is positive and during compression (area under the lower curve) is negative. Net work = area enclosed by the loop.
- If the cycle is traversed clockwise, net work done by the system is positive (heat engine); if anticlockwise, it is negative (work is done on the system, as in a refrigerator).
