5.4 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

• 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 

• 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