Cyclic Process
ΔU = 0, W = ∮P dV — engine vs refrigerator direction.
Key Notes
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.
Formulas
Net first law over cycle
U is a state function.
Work done = enclosed area
Area inside the closed PV curve.
Efficiency (heat engine)
Fraction of input heat that becomes useful work.
Coefficient of performance (refrigerator)
Heat extracted from cold reservoir per unit work input.
Important Points
Over a cycle, ΔU = 0 always — state functions are PERIODIC.
Net work = NET heat absorbed (not just total heat in).
PV diagram area: clockwise ⇒ engine, counter-clockwise ⇒ refrigerator.
Real engines run on cycles — gasoline car: Otto cycle, jets: Brayton cycle.
Maximum efficiency for given hot/cold reservoirs: Carnot cycle.
Refrigerator COP can exceed 1 — heat extracted > work input (heat is also dumped from work).
Cyclic Process notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.