3. Internal Energy

Internal Energy of a Closed System

For a closed system the internal energy is essentially defined by

ΔU = q + W


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

If an energy exchange occurs because of temperature difference between a system and its surroundings, this energy appears as heat otherwise it appears as work. When a force acts on a system through a distance the energy is transferred as work. The above equation shows that energy is conserved.

The different components of internal energy of a system is given below.

Thermal energySensible heatEnergy change of a system associated with:Molecular translation, rotation, vibration.Electron translation and spin.Nuclear spin of molecules.
Latent heatEnergy required or released for phase change, change from liquid to vapour phase requires heat of vaporization.
Chemical energyEnergy associated with the chemical bonds in a molecule.
Nuclear energyThe large amount of energy associated with the bonds within the nucleus of the atom.

The physical and chemical processes that can change the internal energy of a system is given below.

Transferring energy across the system boundary byHeat transferEnergy transfer from a high temperature to low temperature state.
Work transferEnergy transfer driven by changes in macroscopic physical properties of a system such as compression or expansion work.
Mass transferEnergy transfer by mass flowing across a system boundary.
Change through internal processesMixingHeat releases upon components mixing that may lead to lower internal energy.
Chemical reactionHeat required or released during a chemical reaction that changes chemical energy.
Nuclear reactionHeat released during a nuclear reaction that changes nuclear energy.

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