Internal Energy (SL IB Physics)

• When a substance gains or loses thermal energy, its internal energy increases or decreases
• The internal energy of a substance is defined as:

The sum of the total kinetic energy and the total intermolecular potential energy of the particles within the substance

• As thermal energy is transferred to a substance, two things can happen:
  • An increase in the average kinetic energy of the molecules within the substance - i.e. the molecules vibrate and move at higher speeds
  • An increase in the potential energy of the molecules within the substance - i.e. the particles get further away from each other or move closer to each other

What is internal energy?

Kinetic energy and potential energy are the two energy stores that make up internal energy

• Temperature is a measure of the average kinetic energy of the molecules
  • Therefore, only an increase in the average kinetic energy of the molecules will result in an increase in temperature of the substance
  • Due to thermal expansion, when the temperature of a substance increases, the potential energy of the molecules also increases

• When only the potential energy of the molecules changes, the temperature of the substance does not change
  • This is the case for all state changes (e.g. melting, boiling)