Physics · Class 11

Compare isothermal and adiabatic paths

An isothermal gas exchanges heat to keep its temperature fixed as it expands. A reversibly adiabatic gas receives no heat, so expansion work lowers its internal energy and temperature. The adiabatic pressure falls more steeply.

Open the interactive experiment →

Compare isothermal and adiabatic paths

சமவெப்ப நிகழ்வில் வெப்பநிலை மாறாது. வெப்பமாற்றமற்ற விரிவில் வாயு வேலை செய்வதால் வெப்பநிலை குறையும்.

Key notes, relationships and model limits

Key relationship

Isothermal pV=constant, W=nRTln(V₂/V₁). Reversible adiabatic pV^γ=constant, TV^(γ−1)=constant.

Remember

Common trap

Adiabatic does not mean constant temperature.

Model limits

Quasistatic ideal-gas processes with constant heat capacities. Adiabatic path is reversible; compression has Wby<0. Isothermal heat equals work because ΔU=0.

What you will learn

Compare thermodynamic paths, heat exchange, temperature and work.

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 1 mol at 300 K expanded to twice the volume, isothermal W=RTln2≈1728.9 J. With γ=1.4, adiabatic T₂=300×2⁻⁰·⁴≈227.36 K.

Why this matters

The model connects compare isothermal and adiabatic paths with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

In reversible adiabatic expansion of an ideal gas, what happens to temperature?

It stays fixed
It decreases
It increases

Read the explanation

Q=0 and Wby>0 give ΔU=−Wby<0; ideal-gas internal energy follows temperature.

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