Physics · Class 11

Compare three thermal states

The zeroth law makes temperature a consistent comparison: two bodies that separately equilibrate with the same thermometer are at the same temperature as each other. Set both bodies to the thermometer reading to establish the relation.

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Compare three thermal states

மூன்றாவது பொருளுடன் தனித்தனியாக வெப்பச் சமநிலையில் உள்ள இரு பொருட்கள் ஒன்றுடனொன்று வெப்பச் சமநிலையில் இருக்கும்.

Key notes, relationships and model limits

Key relationship

If A is in thermal equilibrium with C, and B with C, then A and B are in equilibrium.

Remember

Common trap

Equal internal energies are not required for equal temperatures.

Model limits

Three independently controlled uniform temperatures. Equality tests the zeroth law; this diagram does not simulate actual contact dynamics.

What you will learn

Use a third body to establish temperature equality.

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

If C reads 30°C and A and B each equilibrate with it, both are at 30°C. If B is 40°C, B cannot be in equilibrium with that unchanged C.

Why this matters

The model connects compare three thermal states with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

A and B each equilibrate with thermometer C. What follows?

A and B must have equal internal energies
A and B have equal temperatures
A and B must have equal masses

Read the explanation

The zeroth law makes thermal-equilibrium comparison transitive.

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