Explain the cyclic process with the help of a neat \(p\)–\(V\) diagram.

Answer: A cyclic process is one in which the system, after passing through a series of changes, returns to its initial state.

[Figure: closed loop on a \(p\)–\(V\) diagram. The system goes from state A to state B along the upper path (expansion) and returns from B to A along the lower path (compression); the enclosed area is shaded.]

• Since the system returns to its initial state, its internal energy is unchanged: \(\Delta U=0\).
• By the first law, \(Q=\Delta U+W\Rightarrow Q=W\): the net heat absorbed equals the net work done by the system in one cycle.
• The net work done in a cycle equals the area enclosed by the loop on the \(p\)–\(V\) diagram. Work done in expansion (area under the upper curve) is positive and in compression (area under the lower curve) is negative; the difference is the enclosed area.
• If the cycle is traversed clockwise, net work is done by the system (positive, as in a heat engine); if anticlockwise, net work is done on the system (as in a refrigerator).