Section 1 of 5Why Materials Expand
Expansion is the result of particle motion, and the same idea explains all three states of matter.
1
When a material is heated, its particles gain kinetic energy and vibrate or move more vigorously.
2
The particles push one another further apart, so the space the material occupies increases.
3
The particles themselves do not get bigger; only the spacing between them changes.
4
This increase in separation makes the whole material larger in volume.
Section 2 of 5Solids, Liquids and Gases
Solids
- Solids expand only slightly, because their particles are held firmly by strong forces and cannot move far apart.
Liquids
- Liquids expand more than solids, because their particles have enough energy to partly overcome the forces holding them together.
Gases
- Gases expand the most, because their particles are already far apart and spread into any space that is available.
Amount of expansion
- The amount a material expands depends on the material itself and on the size of the temperature change.
Section 3 of 5Hazards and Uses
Engineers and designers make use of thermal expansion, and also allow for it, in many everyday situations.
Hazards and Uses
Railway tracks
- Metal rails are laid with small gaps so they can expand in hot weather without buckling.
Bridges and roads
- Expansion gaps and joints allow bridges and road surfaces to move as the temperature changes, preventing damage.
Liquid thermometer
- The liquid expands up a narrow capillary tube, so the length of the liquid column shows the temperature.
Bimetallic strip
- Two metals expand by different amounts, so the strip bends and can close a circuit at a set temperature.
Section 4 of 5Everyday Observations
Everyday Observations
Loose jar lids
- A metal lid can be loosened by warming it, because the metal expands more than the glass rim of the jar.
Expanding balloon
- A balloon grows larger when warmed, because the gas inside expands and pushes the rubber outwards.
Paving slabs
- Gaps between paving slabs allow them to expand in summer without pushing against one another and cracking.
Section 5 of 5Exam Checklist & Tips
4.2.5 – Key Points for the Exam
- Thermal expansion The increase in size of a material when its temperature rises.
- Capillary tube A narrow glass tube that makes small changes in liquid volume easy to see.
- Bimetallic strip A strip made from two metals with different expansion rates, used in temperature-activated switches.
- Buckling Bending of a solid structure caused by expansion when no space is allowed for it to grow.
- Expansion gap A space left in a structure to allow for thermal expansion.
Common Mistakes
- Saying the molecules themselves expand; the space between the molecules increases.
- Stating that gases expand less than solids when heated.
- Ignoring the need for gaps in railway tracks, bridges and road surfaces.
- Claiming that a liquid thermometer works because the glass expands; the liquid expands up the tube.