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Cyclic process
Intermediate
Returns to initial state, net ΔU=0.
The system returns to its initial state: ΔU = 0, so net heat equals net work. The area enclosed in the PV diagram is the net work.
Engines and refrigerators run in cycles.
Clockwise cycles produce work.
In more detail
A cyclic process returns a system to its initial state. Over a full cycle the internal energy change is zero, so the net work equals the net heat added.
Real-world uses
- Heat engines run in cycles to produce continuous work.
- Refrigerators also operate in cycles, moving heat from cold to hot regions.
- A heat engine runs in a cycle, returning to its starting state after each stroke sequence, so its internal energy does not change over one cycle.
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Frequently asked questions
Why is the change in internal energy zero?
Because the system returns to its starting state, and internal energy is a state function.
Does a cycle produce work?
Yes, if the cycle encloses an area on the PV diagram in the right direction.
Does a cycle mean the engine does no net work?
No. The area enclosed by the cycle on a P-V diagram is the net work done, which is nonzero for a useful engine.
Related topics in Heat & Thermodynamics
Adiabatic process · Isobaric process · Isochoric process · Heat capacities C_p and C_v · Ratio of specific heats · P–V diagrams