Physics · Thermodynamics · NEET
Equal temperature. A big body and a small body can hold very different amounts of heat energy, yet still be in thermal equilibrium if their temperatures are the same. Temperature (not total heat) decides the direction of heat flow. When temperatures become equal, the net heat flow becomes zero, and that is thermal equilibrium.
Yes. Mass and size do not matter for equilibrium. A cup of water and a swimming pool can be in thermal equilibrium if both are at the same temperature, say 25 degrees C. The pool holds far more internal energy, but no NET heat crosses between them because there is no temperature difference.
At equilibrium the NET heat flow is zero. In the molecular picture, energy is still exchanged both ways at the contact surface, but equal amounts go each way, so there is no overall transfer. For NEET, the working rule is simple: same temperature means no net heat flow and no more change in the macroscopic state (P, V, T stay constant).
Thermal contact means the two systems are connected by a conducting (diathermic) wall so heat CAN flow. Thermal equilibrium is the final state they reach after being in contact, when temperatures are equal and heat flow stops. Contact is the setup; equilibrium is the result.
No, but they are linked. Thermal equilibrium is just the condition of equal temperature with no net heat flow. The Zeroth Law is a separate statement: if A is in thermal equilibrium with C and B is in thermal equilibrium with C, then A and B are in thermal equilibrium with each other. That law uses the idea of equilibrium to DEFINE temperature.
A diathermic (conducting) wall lets heat pass through, so two systems separated by it can reach thermal equilibrium. An adiabatic (insulating) wall does not allow heat to pass, so systems on its two sides stay independent and need not reach the same temperature.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the state when two bodies in thermal contact reach the same temperature, so no net heat flows between them.
Temperature difference. Heat always flows from the body at higher temperature to the body at lower temperature, and it stops when their temperatures become equal.
Temperature. This is exactly why temperature is defined through thermal equilibrium in the Zeroth Law of Thermodynamics.
Yes. According to NCERT, during melting the temperature stays constant and the solid and liquid states coexist in thermal equilibrium at the melting point until all the solid has melted.
Yes, as a concept. It is the base for the Zeroth Law and the definition of temperature. NEET numericals usually appear on later topics like the first law, processes and Carnot engine, but understanding equilibrium first makes those easier.
