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Thermal equilibrium and Zeroth law of Thermodynamics

  • Thermodynamics deals with heat and temperature, and the inter-conversion of heat and other energy forms.

  • Thermodynamics is a macroscopic science. It deals with bulk system, and does not go into the molecular constitution of matter.

  • Thermal equilibrium: State of a system is an equilibrium state if the macroscopic variables that characterise the system do not change. Two systems at the same temperature are said to be in thermal equilibrium with each other.

  • Adiabatic wall: It is an insulating wall that does not allow flow of heat

  • Diathermic wall: It is a conducting wall that allows flow of heat. In this case, thermal equilibrium can be attained.

Zeroth Law of Thermodynamics

  • When two systems are separately in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.

  • The physical quantity which is required to be in thermal equilibrium is temperature

  • Consider three systems, X, Y and Z.

X and Y are separately in thermal equilibrium with Z.


TX = Temperature of X

TY = Temperature of Y

TZ = Temperature of Z


The systems X and Y are also in thermal equilibrium with each other.


  • It is a form of energy.

  • Heat is a transfer of energy due to the temperature difference between a system and its surroundings.

  • The flow of heat between two bodies stops when their temperatures equalise; the two bodies are then in thermal equilibrium.

Internal Energy

  • Work is transfer of energy brought about by other means, such as moving the piston of a cylinder containing gas.

  • Internal energy of a system is the sum of the kinetic and potential energies of the molecular constituents of the system.

  • It includes the energy associated with the random motion of molecules of the system.

  • Internal energy (as a state variable) depends on the given state of the system, and not on the path taken to reach the state.

  • Internal energy and work are equivalent.

Difference between heat and internal energy

In thermodynamics, heat and work are not state variables; however, internal energy is a state variable.

First Law of Thermodynamics:

  • The first law of thermodynamics is based on the law of conservation of energy .
  • According to the first law…

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