Internal energy

When a material is heated or cooled, two changes may happen to the particles within the material:

  • Chemical bonds between the particles may form, break or stretch. There is a change in the chemical potential store of energy in the material.
  • The material will heat up or cool down as the particles within it gain or lose speed. There is a change in the thermal store of energy within the material.
The internal energy is the total amount of kinetic energy and potential energy of all the particles in the system.
Flow chart showing the particle model changes between solid, liquid and gas, as internal energy is increased.

When energy is given to raise the temperature, particles speed up so they gain kinetic energy.

When the substance melts or boils, energy is put in to breaking the bonds that are holding particles together, which increases the potential energy.

The conservation of energy means that, assuming no energy is lost to the environment, any energy transferred to a material will be distributed between the chemical store and the thermal store of the internal energy.

Whether the energy breaks bonds, increases the speed of the particles to stretch bonds, or just increases the speed of the particles depends on the temperature and state of the material.