Internal Energy (DP IB Physics: SL)

• 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

• Since temperature is a measure of the average kinetic energy of the molecules, 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)