GCSE Physics What is the internal energy store of a substance? sum of thermal kinetic energy store plus potential energy store, examples explained defined IGCSE/GCSE Physics revision notes

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Thermal energy and particle theory: Part 4.2 What is the internal energy of a substance? What is the effect of temperature increase?

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[KEY POINTS and learning objectives for this page, after initial notes]

INDEX for my physics notes on particle model theory explaining state changes, latent heat, heating and cooling curves

4.2 What is the internal energy of a substance

(KE shorthand for kinetic energy)

The particles of solids, liquids and gases all have kinetic energy (KE).
In solids the particles vibrate with kinetic energy but can't move around to another position, but in gases and liquids the particles freely move from place to place with kinetic energy.
Particles also have energy in their potential energy stores due to their positions - the motion from their kinetic energy keeps them separated as it opposes the forces attracting the particles together.
- The particles in gases have the most potential energy because they are the farthest apart.
- In potential energy order: gases >> liquids > solids
- Remember there is a little space on average between liquid particles, but virtually non between the particles of a solid.
Therefore the internal energy store of a system stored by the particles (atoms, ions, molecules) is due to a thermal energy store (kinetic energy of particles) plus its potential energy store (spacing-position and bonds between molecules).
- Total internal energy of particle system =
- Thermal energy store + potential energy
- (see Part 4.1)
When you heat a system energy is transferred to the particles eg they move faster in gases and liquids (increase in KE of movement from one place to another) or the particles vibrate more strongly in a solid (increase in vibrational KE), so the internal energy is increased when you heat a material.
- Here, due to increase in temperature, the increase in the thermal energy store is effectively the increase in kinetic energy store of the particles.
This absorption of heat, i.e. increase in internal energy can cause an increase in temperature OR a change of state e.g. melting or boiling if the particles are given sufficient thermal energy.
Removing heat decreases the internal energy, so the material cools to a lower temperature OR undergoes a change of state e.g. condensing or freezing.
The size of the change depends on the energy input, the mass of substance involved and the specific heat capacity (which depends on the nature of the material).

INDEX of notes on Particle model theory state changes and latent heat

Key points Thermal energy and particle models - what is internal energy?

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks and syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured set of summary revision notes on Internal Energy and Thermal Energy Stores in the three states of matter, tailored to the GCSE/IGCSE Physics specifications for WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

Internal Energy and Thermal Energy Store

Definition of Internal Energy

Internal energy is the total energy stored in a system due to:
- Kinetic energy of particles (movement)
- Potential energy between particles (intermolecular forces)
It depends on:
- Temperature
- State of matter
- Mass of the substance

Thermal Energy Store

A thermal energy store refers to the energy held due to temperature.
It’s part of the internal energy, specifically the kinetic energy of particles.
When a substance is heated:
- Kinetic energy increases → particles move faster
- Potential energy may increase → particles move further apart (during state change)

Internal Energy in the Three States of Matter

State	Particle Motion	Kinetic Energy	Potential Energy	Internal Energy
Solid	Vibrate in fixed positions	Low	High (strong bonds)	Moderate
Liquid	Slide past each other	Medium	Medium	Higher
Gas	Move freely and rapidly	High	Low (weak bonds)	Highest

Heating a solid increases vibration → temperature rises.
Melting adds energy to break inter-particle bonds → potential energy increases.
Boiling adds energy to separate particles → large increase in potential energy.

Exam Tips

Know that internal energy = kinetic + potential energy
Understand that temperature only affects kinetic energy
During state changes, temperature stays constant
Label diagrams showing particle arrangements and energy changes

Common Misconceptions

Internal energy is only kinetic energy → It includes potential energy too
Temperature rises during melting/boiling → It stays constant
Particles gain mass when heated → Mass is conserved
Gases have no internal energy → They have the highest internal energy

Keywords, phrases and learning objectives for particle models and internal energy

Know the internal energy of a substance is the sum of the kinetic energy of the particles plus the potential energy.

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