2.2.1Thermal Physics

Thermal Expansion — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 17 of 45 2 quizzes

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

2.2.1 Thermal Expansion — IGCSE PHYSICS notes: Use the particle model to explain why solids, liquids and gases expand when heated and describe uses and problems of expansion.Energy transfer — heating gives each particle more kinetic energyIn a solid the particles vibrate with lar…

Fluid movement
Liquid-in-glass thermometer
Bimetallic strip
Loosening a lid
Hot-air balloon
Railway tracks

Part 1 of 6Particle explanation of expansion

Particle explanation of expansion

Energy transfer

Energy transfer heating gives each particle more kinetic energy
In a solid the particles vibrate with larger amplitude about fixed positions
In a liquid or gas the particles move faster

Result

Result the average separation increases, so the substance takes up more space

Unchanged

Unchanged the particles stay the same size, their number is unchanged and mass is constant

Cooling

Cooling the opposite happens and the substance contracts

Part 2 of 6Expansion in solids, liquids and gases

Solids

Solids expand least because strong forces hold the particles in a lattice
Larger vibrations push neighbours slightly further apart
Different materials expand by different amounts, for example aluminium expands more than steel

Liquids

Liquids expand more than solids because the forces between particles are weaker

Gases

Gases expand most because the forces between particles are almost zero

Order

Order gases greater than liquids greater than solids for equal temperature rise

Part 3 of 6Expansion and density

Density

Density density = mass divided by volume, so it falls when volume rises at constant mass

Fluid movement

Fluid movement warmer, less dense fluid rises above cooler, denser fluid, which gives convection currents

Water

Water between 0 °C and 4 °C water contracts on heating, which is why ice floats and lakes freeze from the top

Part 4 of 6Uses of expansion

1Liquid-in-glass thermometer

Liquid-in-glass thermometer the liquid expands up a narrow tube, so a small volume change gives a visible movement

2Bimetallic strip

Bimetallic strip two metals expand by different amounts, so the strip bends with the greater-expanding metal on the outside
Used in thermostats
Used in simple fire alarms

3Loosening a lid

Loosening a lid running a metal lid under hot water makes it expand more than the glass jar

4Hot-air balloon

Hot-air balloon heated air expands, becomes less dense than the cool air outside and rises

Part 5 of 6Problems caused by expansion

Bridges

Bridges expansion gaps or rollers stop them buckling in summer

Railway tracks

Railway tracks laid with small gaps or pre-stressed

Power cables

Power cables hung with slack so they do not snap when they contract in winter

Concrete pavements

Concrete pavements laid in slabs with joints filled by a soft material

Part 6 of 6More from the specification

Thermal energy quantified in joules

Thermal energy quantified in joules absorbing thermal energy measured in joules increases internal particle kinetic vibrations, driving volumetric expansion.

Thermal Expansion and Internal Energy

When a substance is heated, its particles gain kinetic energy and vibrate or move more vigorously,
causing them to push further apart and the material to expand;
- internal energy is the sum of the random kinetic and potential energies of the constituent molecules in the system.

Specific Heat Capacity and Electrical Calorimetry

Specific heat capacity c is the thermal energy required per unit mass to increase the temperature of a material by 1 °C without changing state (ΔE = m·c·ΔT); in laboratory investigations, an electrical immersion heater transfers energy to a known mass of water or metal block to determine its specific heat capacity.
- (E = P·t = I·V·t)

Evaporation vs Boiling and Phase Changes

Melting and boiling occur at fixed temperatures where supplied thermal energy breaks intermolecular bonds,
increasing molecular potential energy without increasing temperature;
- in contrast, evaporation occurs at any temperature from the liquid surface as energetic molecules escape into vapour.

Immersion heater

Electric heater placed in a liquid or block to supply energy

Potential energy

Energy stored by the configuration of a system.

Intermolecular force

An attractive force between particles.

Potential difference

Work done by a unit charge passing through a component.

Capacity

Charge a cell can supply, in A h or C.

Evaporation

Escape of the most energetic molecules from a liquid surface at any temperature

Boiling

Liquid becoming gas throughout the liquid at the boiling point

Molecule

Tiny particle of the fluid, too small to be seen.

Melting

Change from solid to liquid.

Heater

A device that transfers electrical energy to the thermal store.

Joule

Unit of energy, 1 kg·m²/s².

ΔE

ΔE mcΔθ [1 mark]

What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg⁻¹ °C⁻¹; water 4.18 J g⁻¹ °C⁻¹) · ΔT or Δθ = temperature change (°C or K)

Key terms

Term	Meaning
Thermal expansion	Increase in size of a substance when its temperature rises
Contraction	Decrease in size when the temperature falls
Bimetallic strip	Two metals joined together that bend when heated because they expand by different amounts
Density	Mass per unit volume; falls when a substance expands
Thermostat	Device that uses a bimetallic strip or similar to keep temperature steady

Avoid these mistakes

Saying the particles themselves expand — the spaces between them increase, not their size.

Thinking heating makes more particles or more mass — mass and particle number are unchanged.

Saying solids expand more than gases — gases expand most and solids least.

Forgetting to link expansion to a fall in density — volume rises while mass is constant.

Last updated: 15 October 2026