AQA GCSE Physics

Revision Notes

3.2.1 Internal Energy

Internal Energy

  • Internal energy is defined as:

The total energy stored inside a system by the particles that make up the system due to their motion and positions

  • The molecules within a substance possess two forms of energy:
    • Kinetic energy (due to their random motion / vibration)
    • Potential energy (due to their position relative to each other)
  • Together, these two form the total energy that makes up the internal energy of the system

Gas molecules in a box_2, downloadable AS & A Level Physics revision notes

Molecules in a substance have kinetic energy since they are in motion and potential energy from their position relative to each other


  • Heating a system changes a substance’s internal energy by increasing the kinetic energy of its particles
    • The temperature of the material, therefore, is related to the average kinetic energy of the molecules
  • The higher the temperature, the higher the kinetic energy of the molecules and vice versa
    • This means they move around faster
  • This increase in kinetic energy (and therefore internal energy) can:
    • Cause the temperature of the system to increase
    • Or, produce a change of state (solid to liquid or liquid to gas)

Change in internal energy, downloadable AS & A Level Physics revision notes

As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy

Change of State

  • When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead
  • This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state
    • For example, liquid to gas
  • When a substance changes its state:
    • The potential energy of the molecules increases, breaking the bonds between them and becoming further apart
    • The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated

Changing state, IGCSE & GCSE Chemistry revision notes

An increase in internal energy from heating can cause a change of state

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