Thermal Expansion and Thermometers
Materials expand when heated. That fact causes engineering problems, provides useful applications, and gives us the commonest way of measuring temperature.
1. Why materials expand
When a substance is heated, its particles gain kinetic energy and vibrate or move more. They therefore occupy more space on average, so the substance expands.
The particles themselves do NOT get bigger. It is the spacing between them that increases. This is the explanation the mark scheme wants, and the misconception it tests.
The order of expansion:
Gases expand MOST. Liquids expand less. Solids expand LEAST.
The reason follows from the particle model: in a solid the particles are held by strong forces in fixed positions, so they can only vibrate a little further. In a gas the forces are negligible, so the particles spread out freely.
Expansion also means the density decreases, since the same mass now occupies a larger volume — which is what drives convection currents.
2. Consequences and applications
Problems caused by expansion:
| Situation | Solution |
|---|---|
| Railway lines buckling | small gaps between rails, or expansion joints |
| Bridges expanding | expansion joints and roller supports at one end |
| Concrete roads cracking | gaps filled with tar |
| Overhead cables | left slack in summer so they don’t snap when they contract in winter |
| Pipes | expansion loops built in |
Useful applications:
The bimetallic strip — two metals with different expansion rates bonded together. When heated, one expands more than the other, so the strip bends towards the metal that expands less.
Used in thermostats and fire alarms, where the bending strip makes or breaks an electrical contact.
Riveting — a hot rivet is inserted, then contracts on cooling to pull the plates tightly together. Fitting a metal rim to a wheel — heated to expand, fitted, then cooled to grip.
Water’s anomalous behaviour: water expands when it freezes, which is why pipes burst and ice floats.
3. Thermometers
A thermometer needs a physical property that changes measurably with temperature. The common choices are:
- The volume of a liquid (liquid-in-glass thermometer)
- The resistance of a thermistor
- The e.m.f. of a thermocouple
The liquid-in-glass thermometer
How it works: the liquid in the bulb expands up a narrow capillary tube when heated; the length of the liquid column indicates the temperature.
Calibration uses two fixed points:
Lower fixed point: 0 °C — the temperature of pure melting ice Upper fixed point: 100 °C — the temperature of steam above boiling water at normal atmospheric pressure
The scale between them is then divided into 100 equal divisions.
The upper fixed point is the steam, not the water — the water’s temperature can vary slightly with impurities, while the steam above it is reliable at standard pressure.
The three properties to know
RANGE — the difference between the highest and lowest temperatures it can measure. SENSITIVITY — how far the liquid moves per degree of temperature change. LINEARITY — whether equal temperature changes produce equal changes in the reading throughout the scale.
How to change them:
| Change | Effect |
|---|---|
| Narrower capillary tube | more sensitive (bigger movement per °C), but smaller range |
| Wider capillary tube | less sensitive, larger range |
| Larger bulb | more sensitive (more liquid to expand), but slower to respond |
| Thinner glass bulb wall | responds faster |
Sensitivity and range trade off against each other. A narrow tube gives a big movement per degree — but the liquid runs off the end of the scale sooner. Questions frequently ask you to explain this trade-off.
Choice of liquid:
Mercury — good conductor, responds quickly, doesn’t wet the glass, wide range — but toxic. Alcohol — expands more (so more sensitive), works at lower temperatures, safer — but has a lower boiling point, limiting its upper range.
4. Temperature scales
Celsius (°C) and kelvin (K). K = °C + 273
Absolute zero is 0 K = −273 °C, the temperature of minimum particle motion.
A temperature DIFFERENCE is the same number in °C and K, so for a change of temperature you can use either. Absolute temperatures must be in kelvin where a gas law requires it.
5. Mistakes that cost marks
Saying the particles themselves expand.
Getting the order of expansion wrong.
Explaining expansion without mentioning particle spacing.
Saying the bimetallic strip bends towards the metal that expands more — it bends towards the one that expands less.
Giving the upper fixed point as boiling water rather than the steam.
Confusing range with sensitivity.
Saying a narrow tube increases both range and sensitivity — it trades one for the other.
Forgetting the +273 in a kelvin conversion.
Frequently asked questions
Why do materials expand when heated? The particles gain kinetic energy and vibrate more, so the spacing between them increases.
Do the particles get bigger? No — only the spaces between them.
Which state expands most? Gases; then liquids; solids least.
Why do bridges have expansion joints? To allow for expansion in hot weather without buckling.
How does a bimetallic strip work? Two metals expand by different amounts, so the strip bends — towards the metal that expands less.
What are the fixed points of a thermometer? 0 °C (pure melting ice) and 100 °C (steam above boiling water).
What is the range of a thermometer? The difference between the highest and lowest temperatures it can read.
What is sensitivity? How far the liquid moves per degree of temperature change.
How do I make a thermometer more sensitive? Use a narrower capillary tube or a larger bulb — at the cost of range or response time.
How do I convert °C to K? Add 273.
Quick revision checklist
- I can explain expansion using particle spacing
- I know the particles themselves don’t change size
- I know gases expand most and solids least, and why
- I can give problems caused by expansion and their solutions
- I can explain the bimetallic strip and which way it bends
- I know how a liquid-in-glass thermometer works
- I know both fixed points, including “steam”
- I can define range, sensitivity and linearity
- I can explain the trade-off between range and sensitivity
- I can compare mercury and alcohol
- I can convert between °C and K
- I know a temperature difference is the same in both scales
These notes cover thermal expansion and thermometers in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons. These were among the least-covered subtopics in that set, so this page follows the syllabus closely rather than being padded. Always check the current syllabus for your own exam series.
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