10.4 - Thermal Expansion

Thermal expansion

• When most substances are heated, they expand. This is called thermal expansion.
• It happens in solids, liquids and gases, as long as the surrounding pressure stays the same.

Why it happens

• Heating gives the particles more energy, so they move about more and push further apart, causing the substance to expand.
• Cooling reverses this: the particles lose energy, move less, settle closer together, and the substance contracts.

Why solids, liquids and gases expand by different amounts

• How much a substance expands depends on the forces of attraction between its particles.
• The stronger the forces, the less the particles can move apart, so the less the substance expands.

Comparing the three states

• Solids - Have the strongest forces between particles, so they expand the least.
• Liquids - Have weaker forces than solids, so they expand more than solids.
• Gases - Have almost no forces between particles, so they expand the most.

Uses and drawbacks of thermal expansion

Rivets

• A rivet fastens metal plates together using contraction on cooling.
• A hot metal rod is put through holes in the plates, and its ends are hammered flat.
• As the rivet cools, it contracts and pulls the plates tightly together.

Loosening a jar lid

• When a solid ring is heated, it expands and the opening in its centre gets bigger.
• Running a stuck metal jar lid under hot water causes the lid to heat up and expand, making it easier to turn.

Expansion joints

• If a structure is fixed in place when it expands, large forces build up that can damage it.
• Expansion joints leave small gaps so the material can expand into them without buckling or cracking.

Sagging power lines

• Power lines are hung with a slight sag rather than pulled tight.
• In cold weather they contract; if they were strung tight, they could snap or pull on pylons.
• In hot weather they undergo thermal expansion and sag further.