Thermal expansion

Last time

Before we start: last time, in Specific heat capacity, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why railway lines have gaps

On a hot summer day, a steel railway line gets a little longer. If it had no room to grow, it would buckle and twist. So engineers leave small gaps, or use special joints. The idea behind it is thermal expansion.

What is thermal expansion?

When a solid, a liquid or a gas is heated, it usually gets bigger. That is called thermal expansion. When it is cooled, it contracts, which means it gets smaller. The mass does not change. Only the volume does, so the density goes down when it expands. The material does not grow new particles. So why does it get bigger?

Cold and hot particles

In a cold solid the particles vibrate a little about fixed positions. When it is heated, the particles gain kinetic energy and vibrate with bigger movements. They push each other further apart. The particles do not get bigger. The gaps between them do.

The particle explanation

When a substance is heated, its particles gain kinetic energy. They vibrate more, or move faster, and they push against their neighbours. So the average distance between the particles increases. The particles themselves stay the same size. Only the spaces between them grow.

Solids, liquids and gases

For the same temperature rise, a solid expands the least. Its particles are held by strong forces. A liquid expands more than a solid, because its forces are weaker. A gas expands the most. Its particles are free, with almost no forces. Order, smallest to greatest: solid, then liquid, then gas.