Pressure-Volume Paths and Cycles
Pressure-Volume Paths and Cycles
- Recent Paper 4 questions apply the first law to several changes shown on one pressure-volume graph.
- Read each path separately before considering the complete cycle.
Read one path at a time
Swipe left or right to see the whole diagram →
A to B
Volume decreases.
Work done on the gas is positive.
B to C
Volume is constant.
No pressure-volume work is done.
C to D
Volume increases.
Work done on the gas is negative.
D to A
Volume is constant.
No pressure-volume work is done.
- A vertical path has no volume change, so no pressure-volume work is done.
- The high-pressure expansion transfers more work out than the low-pressure compression transfers in.
- The enclosed area is the net work done by the gas. For the clockwise cycle shown, the first-law value of work done on the gas is negative.
Return to the starting state
- A complete cycle returns the gas to its starting pressure, volume and temperature. It therefore returns to its starting internal energy.
This is the exact chain used in the latest 2025 cycle question (9702/42/O/N/25 Q4(d)).
Your turnquick check
A gas completes a cycle and does 400 J of net work on its surroundings. Find the net thermal energy supplied to the gas.
Show worked answer
- The cycle returns to its starting state.
- Work done on the gas is negative.