3.2.1 Internal Energy

Internal Energy

• 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 possess two forms of energy:
• Kinetic energy (due to their random motion / vibration)
• Potential energy (due to their position relative to each other)
• Together, these two 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

• 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)

As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy

Change of State

• When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead
• This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state
• For example, liquid to gas
• When a substance changes its state:
• The potential energy of the molecules increases, breaking the bonds between them and becoming further apart
• 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

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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