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Physics · Class 11

Trace a pressure-volume cycle

Thermodynamics

Internal energy returns to its starting value over a cycle, but net work can be nonzero. The signed area inside a PV loop measures that work. A clockwise loop produces positive work; reversing it requires work input.

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Trace a pressure-volume cycle

சுழற்சியில் உள் ஆற்றல் மாற்றம் பூஜ்யம்; ஆனால் p–V வரைபடத்தின் பரப்பளவு நிகர வேலையைக் காட்டும்.

Key notes, relationships and model limits

Key relationship

Wcycle=∮p dV=±(ph−pl)(Vmax−Vmin); ΔUcycle=0.

Remember

Internal energy returns to its starting value over a cycle, but net work can be nonzero. The signed area inside a PV loop measures that work. A clockwise loop produces positive work; reversing it requires work input.

Common trap

A closed PV loop can have nonzero heat and work even though ΔU is zero.

Model limits

Rectangular quasistatic idealized PV cycle. Changing direction changes net work sign. Reversible means vanishing dissipative gradients; finite heat transfer or friction makes real cycles irreversible.

What you will learn

Read signed cycle work and connect cyclic and irreversible-process ideas.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

For 100–300 kPa and 2–6 L, clockwise work=(200000 Pa)(0.004 m³)=800 J. Net heat into the gas is also 800 J.

Why this matters

The model connects trace a pressure-volume cycle with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

What is net ΔU for a complete cycle?

Read the explanation

Internal energy is a state function and the gas returns to its initial state.

Course connection: Class XI Chapter 11. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.