Thermal expansion
Explain why bridges need room, thermometers rise and two bonded metals bend.
Core: expansion at constant pressure and its everyday uses. Supplement / Extended: explaining the usual order with particles. This is Cambridge IGCSE, separate from Egyptian Baccalaureate / Thanaweya Amma and not Year 3.
Core 2.2.1.1–2 · Supplement 2.2.1.3 · PDF pp.165–170 / printed pp.157–162.
Heating usually makes a sample larger
Thermal expansion means an increase in size as temperature rises. Cooling usually causes contraction. A metal rod becomes slightly longer and wider; a liquid takes up more volume; a gas can take up much more volume if its boundary can move. No extra material has been added.
Worked example
A rod grows from 1000.0 mm to 1000.6 mm. Its increase is 0.6 mm, not 1000.6 mm. Its mass is unchanged. This is a reading exercise, not a new formula to memorise.
Think first: Does expansion mean every particle gets bigger?
No. The whole object changes size. Individual particle size and particle number stay the same in this model. Average separation changes.
Build an explanation: change → consequence → solution
A hot-water pipe is clamped so that neither end can move. Why might it be damaged, and what could a designer change?
- Heating makes the pipe try to lengthen. [1 mark]
- Rigid supports prevent free expansion, so compressive forces develop. [1 mark]
- The pipe or supports can bend or be damaged. [1 mark]
- An engineered expansion loop or suitable sliding arrangement can allow movement while keeping the pipe sealed. [1 mark]
A final check is useful, but “check your answer” is not a separate physics mark. Each mark above rewards a distinct idea.
Worked data example: remaining space
A different deck has 12 mm space at 20 °C. Its supplied growth to 60 °C is 8 mm. Remaining space = 12 − 8 = 4 mm. Read the reference temperature carefully. If expected free growth exceeds the gap, report contact and a movement shortfall rather than a negative physical gap.
Bimetallic-strip prediction
Predict the outside of a bend
Core. Two bonded metals are straight at the reference temperature. Uniform warming/cooling is small and reversible. The diagram exaggerates the bend; it does not calculate curvature.
Expansion-gap explorer
Will the deck still have room?
Core data-reading practice. One end is fixed; all supplied free growth moves the other end towards a fixed stop. Data are illustrative, not real bridge specifications. Use the table, not a memorised expansion formula.
| Temperature / °C | Growth towards stop / mm |
|---|---|
| 20 | 0 |
| 30 | 2 |
| 40 | 4 |
| 50 | 6 |
| 60 | 8 |
| 70 | 10 |
Enter 0 if contact has been reached. Objects do not pass through each other.
Changing a control clears the prediction. Neither activity records a lasting score.
Practise, then explain
Four multiple-choice questions, three numerical data-reading responses and five written tasks. Applications and data reading are Core; particle explanations of the usual expansion order are Supplement / Extended. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
A freely supported metal rod is warmed gently, with no change of state. What usually happens?
Optional hint
Separate the size of the whole object from the size and number of its particles.
Source clarifications and safe boundaries
The original PDF is unchanged. Its cable question Q3(b) says “tight in winter”, but its answer describes subsequent cooling without specifying a colder temperature. Our question explicitly starts with a cable installed tight on a warm day, then cooled. Cooling further from winter conditions could also increase tension.
A balloon is only an approximate illustration, not a guarantee of constant pressure. The gas model needs a movable boundary and controlled external pressure. Gas–liquid–solid is the usual relative-volume comparison for equal starting volumes, the same temperature rise and no state change; material exceptions exist. The liquid column depends on expansion relative to its glass container.
The source’s bridge/railway example is a simplification: continuous welded rails use different engineering controls. Use sealed expansion joints or loops for pipes, never an open break. Boiling water in cold glass can cause unequal expansion and cracking; do not try this at home. All apparatus investigations require teacher-approved equipment and safe temperatures.
Figures 23.1–23.2 informed the explanations; the strip diagram is a deliberately exaggerated schematic. The deck table is original illustrative data, not a real bridge measurement or an engineering design rule. An expansion-coefficient formula is not required for these objectives.
Source alignment: supplied Physics-0625 Volume A, PDF pp.165–170 / printed pp.157–162, Figures 23.1–23.2; official Cambridge 2026–2028 syllabus, section 2.2.1. Original teaching, activities and questions. Existing source-download access is unchanged.