Internal energy of a substance – Definition and Different forms

To describe the total energy of the molecules in a substance, we need to take into account both the kinetic energies and the potential energies. This is called the internal energy of the substance and is defined as follows: The internal energy of a substance is the sum of the total random kinetic energies and total intermolecular potential energies of all the molecules inside it.

Moving molecules have kinetic energy. The molecules might be moving in different ways, which gives rise to three different forms of random molecular kinetic energy:

  • Molecules might be vibrating about fixed positions (as in a solid) − this gives the molecules vibrational kinetic energy.
  • Molecules might be moving from place to place (translational motion) − this gives the molecules translational kinetic energy.
  • Molecules might also be spinning (rotating) − this gives the molecules rotational kinetic energy.

How solids and liquids gain Potential Energy?

Molecules can have potential energy as well as kinetic energy. In solids and liquids, it is the electrical forces (between charged particles) that keep the molecules from moving apart or moving closer together. Wherever there are electrical forces there will be electrical potential energy in a system, in much the same way as gravitational potential energy is associated with gravitational force.

How Gases have negligible potential energy?

In gases, however, the forces between molecules are usually negligible because of the larger separation between molecules. This is why gas molecules can move freely and randomly. The molecules in a gas, therefore, usually have negligible electrical potential energy – all the energy is in the form of kinetic energy.

Why the word ‘random’ is used in the definition of internal energy

In the definition of internal energy given above, the word ‘random’ means that the molecular movements are disordered and unpredictable. That is, they are not linked in any way to each other, or ordered – as their motions would be if they were all moving together, such as the molecules in a macroscopic motion of a moving car. The molecules in a moving car have both the ordered kinetic energy of macroscopic movement together and the random kinetic energy of
internal energy.

differences between temperature, internal energy, and thermal energy

  • Internal energy is the total energy (random kinetic and potential) of all the molecules inside a substance.
  • If energy is transferred to a substance it gains internal energy and its molecules move faster. We say that it has become hotter and this is measured as an increased temperature.
  • Thermal energy (heat) is energy flowing from a higher temperature to a lower temperature. Thermal energy is transferred by conduction, convection, and radiation.


Internal energy of a substance – Definition and Different forms
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