Internal Energy (AQA GCSE Physics: Combined Science)

• Internal energy is defined as:

The total energy stored inside a system by the particles that make up the system due to their motion and positions

• The molecules within a substance have energy in their:
  • Kinetic store (due to their random motion / vibration)
  • Potential store (due to their position relative to each other)

• Together, these stores form the total energy that makes up the internal energy of the system

Molecules in a substance have kinetic energy since they are in motion and potential energy from their position relative to each other

• Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles
  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules

• The higher the temperature, the higher the kinetic energy of the molecules and vice versa
  • This means they move around faster

• This increase in kinetic energy (and therefore internal energy) can:
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)

Change of State

• When a substance reaches a certain temperature, energy will stop being transferred to the kinetic store of the molecules and will be transferred to their potential store instead
• This energy goes into overcoming the intermolecular forces of attraction between the molecules, causing them to move further apart from one another leading to a change of state
  • For example, liquid to gas

• When a substance changes its state:
  • The potential energy of the molecules increases, allowing them to overcome the intermolecular forces of attraction 
  • The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated

An increase in internal energy from heating can cause a change of state