6.4.1 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 contain both kinetic and potential energies
  • Kinetic energy is due to the speed of the molecules
  • Potential energy is due to the separation between the molecules

• 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 randomly distributed kinetic and potential energies of the particles in a body

• 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
  • Changing state from a gas to a liquid or liquid to a solid