How can one define the internal energy of a system?

The internal energy of a system is the sum of all the energies, i.e. mechanical, chemical, potential, kinetic etc., present in the system. The symbol U is used for the internal energy, and the unit of measurement is the joules (J).

Internal energy

Internal energy U of a system or a body with well-defined boundaries is the total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational motion and electric energy of atoms within molecules.

  • Internal energy is a state function of a system and is an extensive quantity.
  • The internal energy, ΔU, is a state function.
  • For a closed system, the internal energy is essentially defined byΔU = q + W


    • U is the change in internal energy of a system during a process
    • q is the heat
    • W is the mechanical work.

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