Cyclic Process

Cyclic process: gas returns to its initial state after a series of processes.

Net change in any state function over a cycle is ZERO: ΔU_cycle = 0, ΔT_cycle = 0, ΔP_cycle = 0, ΔV_cycle = 0.

First law for a cycle: Q_net = W_net. Net heat input = net work output.

On PV diagram: closed curve. Area enclosed = net work done by gas.

Clockwise cycle on PV: net work > 0 (heat engine).

Counter-clockwise: net work < 0 (refrigerator/heat pump).

Examples: Carnot, Otto (gasoline), Diesel, Brayton, Stirling cycles — all engine/refrigerator cycles.

Efficiency of a heat engine cycle: η = W_net/Q_hot — fraction of input heat converted to useful work.