Thermal Equilibrium & the Zeroth Law 😊

1. Thermodynamics in One Minute

Thermodynamics explores heat, temperature, and how heat turns into other kinds of energy. It speaks the language of whole systems using variables such as pressure (P), volume (V), temperature (T), mass, and composition. :contentReference[oaicite:0]{index=0}

While mechanics follows motion, thermodynamics focuses on what’s happening inside a body. When a bullet buries itself in wood, its motion energy becomes heat—raising the bullet’s temperature instead of its speed. :contentReference[oaicite:1]{index=1}

2. What “Equilibrium” Really Means 🛑

A system sits in equilibrium when every macroscopic variable stays constant over time. Think of a gas sealed in an insulated can—its P, V, and T don’t budge. :contentReference[oaicite:2]{index=2}

3. Meet the Two Magic Walls 🧱

• Adiabatic wall – perfect insulator; heat can’t pass.
• Diathermic wall – perfect conductor; heat flows freely.

Place two gases (A and B) on opposite sides of an adiabatic wall—nothing changes. Swap the wall for a diathermic one and heat zips across until both gases reach new states \((P_A’, V_A’)\) and \((P_B’, V_B’)\). At this moment A and B share the same temperature and are in thermal equilibrium. :contentReference[oaicite:3]{index=3}

4. Temperature: The Great Equalizer 🌡️

In thermal equilibrium the temperatures match:

\(T_A = T_B\)

That simple equality defines the whole concept! :contentReference[oaicite:4]{index=4}

5. Zeroth Law of Thermodynamics 🔄

Imagine three systems A, B, and C:

1. A and C touch through a conducting wall → they reach the same T.
2. B and C do the same.
3. Later, A meets B and nothing changes.

Zeroth Law: Two systems in thermal equilibrium with a third are in thermal equilibrium with each other. :contentReference[oaicite:5]{index=5}

This law lets a thermometer (the “third system”) give numbers to temperatures.

High-Yield NEET Takeaways 🚀

1. Thermodynamic equilibrium demands constant P, V, T, mass, and composition.
2. Adiabatic vs. diathermic walls—know which blocks heat and which allows it.
3. Thermal equilibrium ⇨ equal temperatures (\(T_A = T_B\)).
4. Zeroth Law links separate equilibria to give \(T_A = T_B\).
5. Temperature is the thermodynamic “badge” that proves equilibrium.