Cyclic Process (First Law) (ΔU = 0, Q = W (cyclic process))
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ΔU = 0, Q = W (cyclic process)
What each symbol means
| Symbol | What it stands for |
|---|---|
| ΔU | Change in internal energy over complete cycle = 0 (state function returns to initial value) |
| Q_net | Net heat absorbed by the system over the cycle (J) |
| W_net | Net work done by the system over the cycle (J) = area enclosed in PV diagram |
When to use this
System returns to its initial state after a complete cycle. ΔU = 0 because internal energy is a state function. Net heat absorbed = net work done. W_net = area enclosed by the PV curve: clockwise loop → W > 0 (engine), anticlockwise loop → W < 0 (refrigerator). Applies to all cyclic processes — reversible or irreversible.