Internal Energy (AQA A Level Physics): Revision Note

Internal energy

• The internal energy of a substance includes both the kinetic and potential energy of the particles it contains

• The molecules of all substances have both kinetic and potential energies

  • Kinetic energy is due to the speed of the molecules

  • Potential energy is due to the intermolecular forces between the molecules and varies with the separation of particles

• 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

• This means internal energy is the total of all the kinetic energies plus the total of all of the potential energies

• 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 interactions between the particles 

• The internal energy of a system can increase by:

  • doing work on it

  • adding thermal energy to it (heating it)

• The internal energy of a system can decrease by:

  • losing thermal energy to its surroundings

  • the system doing work on its surroundings (e.g. a gas pushing a piston)