Internal Energy (Edexcel A Level Physics)

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Katie M


Katie M



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 and gives the material its temperature
    • Potential energy is due to the separation between the molecules and their position within the structure

Internal Energy in Water, downloadable AS & A Level Physics revision notes

  • 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 kinetic and potential energies of all the molecules within a given mass of a substance 

kinetic energy and potential energy

  • 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 (solid, liquid, gas) that 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 gas to a liquid or a liquid to a solid

Exam Tip

Always remember internal energy is made up of both the kinetic and potential energy of the particles in a substance. 

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.