AQA A Level Physics

Revision Notes

6.4.1 Internal Energy

Internal Energy

  • Energy can be classified into two forms: kinetic or potential energy
  • The molecules of all substances contain both kinetic and potential energies
    • Kinetic energy is due to the speed of the molecules
    • Potential energy is due to the separation between the molecules


  • The amount of kinetic and potential energy a substance contains depends on its phase of matter (solid, liquid or gas)
    • This is known as the internal energy
  • The internal energy of a substance is defined as:

The sum of the randomly distributed kinetic and potential energies of the particles in a body

  • The symbol for internal energy is U, with units of Joules (J)
  • Particles are randomly distributed, meaning they all have different speeds and separations
  • The internal energy of a system is determined by:
    • Temperature (higher temperature, higher kinetic energy and vice versa)
    • The random motion of molecules
    • The phase of matter: gases have the highest internal energy, solids have the lowest
    • Intermolecular forces between the particles (greater intermolecular forces, higher potential energy and vice versa) – this is linked to the phase the matter is in


  • The internal energy of a system can increase by:
    • Doing work on it
    • Adding heat to it
  • The internal energy of a system can decrease by:
    • Losing heat to its surroundings
    • Changing state from a solid to a liquid or liquid to a gas

Exam Tip

When an exam question asks you to define “internal energy”, you can lose a mark for not mentioning the “random motion” of the particles or the “random distribution” of the energies, so make sure you include one of these in your definition!


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