Thermal Energy (Edexcel GCSE Physics)

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Energy, Temperature & Changes of State

  • 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
  • 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

Heating and Temperature Change

  • 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 in internal energy, downloadable AS & A Level Physics revision notes

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

Heating and Changes 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

Heating Curve

  • This graph shows how the temperature of a substance changes with time as it is heated
  • The substance is heated until it has melted to become a liquid, and then boiled to become a gas

Heating cooling curve for pure substance, downloadable IGCSE & GCSE Physics revision notes

Heating curve of a substance showing the energy changes as temperature is increased

The different sections of the graph show:

  • ORIGIN to A: Added heat energy is being used to increase the kinetic energy of the particles while it is a solid
  • A to B: Added heat energy is being used to break the bonds between the solid molecules, increasing the potential energy and melting the substance
  • B to C: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a liquid
  • C to D: Added heat energy is being used to break the bonds between the liquid molecules, further increasing the potential energy and boiling the substance
  • D to E: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a gas

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Ashika

Author: Ashika

Expertise: Physics Project Lead

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.