Thermal Expansion of Solids, Liquids and GasesCambridge IGCSE Physics: Revision notes
Section 1
What Is Thermal Expansion?
Thermal expansion is the increase in size (length, area or volume) of a substance when its temperature rises, at constant pressure.
- All three states of matter — solids, liquids and gases — expand when heated.
- The reverse also happens: substances contract when they cool.
- The amount of expansion differs greatly between the three states for the same temperature rise.
Section 2
Why Do Solids, Liquids and Gases Expand by Different Amounts?
The size of the expansion for a given temperature rise depends on how strongly the particles are held together and how much extra space their increased motion needs:
| State | Expansion for a given temperature rise | Why |
|---|---|---|
| Solid | Smallest | Particles are held in fixed positions by strong forces, only vibrating more vigorously about those positions |
| Liquid | Larger than solid | Particles are more loosely arranged and can move further apart as they gain energy |
| Gas | Largest | Particles are already far apart and move freely, so extra kinetic energy increases their average separation the most |
In all three states, a rise in temperature increases the average kinetic energy of the particles, making them move more vigorously and, on average, take up more space.
Do not say solids 'don't expand' — they do expand on heating, just by a much smaller amount than liquids or gases for the same temperature rise.
Section 3
Internal Energy and Temperature
A rise in the temperature of an object increases its internal energy. An increase in temperature corresponds to an increase in the average kinetic energy of all the particles in the object — this is the underlying reason why substances expand as they warm up: more energetic particles vibrate or move over a larger average space.
Section 4
Everyday Applications and Consequences of Thermal Expansion
Thermal expansion has many practical effects that engineers must account for:
- Expansion gaps are left between sections of railway track and in bridges, so they do not buckle as they expand in hot weather.
- A bimetallic strip (two different metals joined together) bends when heated because the two metals expand by different amounts; this is used in thermostats to switch a circuit on or off.
- Overhead power cables are hung with some slack, as they expand and sag more in hot weather.
- Sealed containers of liquid should not be overfilled or heated strongly, as the liquid could expand and cause the container to burst.
A thermostat uses a bimetallic strip that bends away from an electrical contact as it heats up, breaking the circuit and switching off a heater.
If asked for an application, name a specific device or structure (railway track gaps, bimetallic strip thermostat) rather than a vague statement like 'things get bigger'.
Must Know
- Thermal expansion: substances get bigger when heated (at constant pressure), and smaller when cooled.
- Order of expansion for the same temperature rise: gases expand most, then liquids, then solids expand least.
- This order exists because gas particles are freest to spread out, while solid particles are held rigidly in place by strong forces.
- A temperature rise increases the average kinetic energy of particles, which is why they need more space (in liquids and gases) or vibrate more (in solids).
- Applications include expansion gaps in railways/bridges, bimetallic strip thermostats, and slack overhead cables.
- A rise in temperature increases an object's internal energy.
That's the notes covered.
Carry on to the next subtopic.