When materials are heated, their particles vibrate more vigorously and move slightly further apart. This causes the material to expand.
Expansion in solids, liquids and gases
- Solids expand the least (strong forces keep particles close).
- Liquids expand more than solids.
- Gases expand the most (weak forces, large spaces to fill).
Useful applications
Bimetallic strip: Two metals with different expansion rates are bonded together. When heated, the strip bends (the metal that expands more is on the outside of the curve). Used in thermostats, fire alarms and some thermometers.
Liquid-in-glass thermometer: The liquid (mercury or alcohol) expands when heated and rises up a narrow tube. The narrow bore makes the expansion visible as a large change in length.
Riveting: A hot rivet is inserted through metal plates. As it cools and contracts, it pulls the plates tightly together.
Expansion joints in bridges: Gaps are left between sections of bridge decks and railway tracks. When the metal expands on hot days, it fills the gap instead of buckling.
Problems caused by expansion
- Railway tracks can buckle in extreme heat if expansion gaps are insufficient.
- Overhead power lines sag in hot weather and tighten in cold weather.
- Glass can crack when heated unevenly (one part expands faster than the rest).
- Concrete roads crack if expansion joints are not provided.
Common errors and how to correct them
- Saying particles get bigger when heated. The particles themselves do not change size; they move further apart.
- Forgetting that different materials expand at different rates (this is what makes the bimetallic strip work).
How to apply this in an exam
Name the application. Explain which material expands and why. Describe the practical consequence of the expansion or contraction.
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