Thermal Properties of Matter

These two words are used interchangeably in everyday speech and mean quite different things in physics. Temperature is a measure of the average kinetic energy of the particles. Thermal energy, or internal energy, is the total energy of all of them added together.

A cup of tea at 80 °C is hotter than a swimming pool at 25 °C. But the pool holds far more thermal energy, because it contains vastly more particles. Temperature tells you the average; thermal energy depends on the average and on how much substance there is.

Energy always flows from higher temperature to lower temperature, never the other way of its own accord. That flow is what we call heating, and it continues until the two are at the same temperature — thermal equilibrium.

Different substances need very different amounts of energy for the same temperature rise. Water needs 4200 J to warm one kilogram by one degree; the same kilogram of copper needs only 385 J. Water is unusually hard to heat.

That high value has real consequences. It is why water is used in central heating systems and car radiators — it carries a great deal of energy for a modest temperature change. It is also why coastal areas have milder weather than inland ones: the sea warms and cools far more slowly than the land.

The word "specific" simply means "per kilogram". Multiply by the mass and the temperature change and you have the energy.

E

energy transferredJ

m

masskg

c

specific heat capacityJ kg⁻¹ °C⁻¹

Δθ

temperature change°C

Heat a solid steadily and its temperature climbs — until it starts to melt. Then, remarkably, the temperature stops rising even though energy is still going in. It stays put until every last bit has melted, and only then does it start climbing again.

The energy has not vanished. It is being used to break the forces holding the particles in their fixed positions, not to make them move faster. Since temperature measures average kinetic energy, and the kinetic energy is not changing, the thermometer does not move.

The same happens at the boiling point, and the energy required there is much larger — separating particles completely takes far more work than merely letting them slide past one another. For water, melting takes 334 000 J per kilogram and boiling takes 2 260 000 J per kilogram.

This is why a steam burn is so much worse than a burn from boiling water at the same temperature. The steam must first condense on your skin, releasing all of that latent heat before it even begins to cool.

E

energyJ

m

masskg

L

specific latent heatJ kg⁻¹

The two flat sections are melting and boiling. Energy is still being supplied throughout, but the temperature holds steady because that energy is breaking bonds rather than increasing the particles' speed. Notice how much longer the boiling plateau is.

Almost everything expands when heated. The particles vibrate more vigorously, so on average they sit slightly further apart, and the object grows very slightly in every direction.

The effect is small but the forces involved are enormous, so engineers must design for it. Bridges are built with expansion joints, and railway lines with small gaps, so that summer heat does not buckle them. Overhead power lines are hung with a deliberate sag in winter, because they contract and tighten in cold weather.

Gases expand most, liquids less, and solids least — a direct consequence of how strongly the particles are held. A bimetallic strip exploits the difference between two metals: they expand by different amounts, so the strip curls when heated, and that movement can switch a circuit. This is how a simple thermostat works.

Water is the famous exception. Between 0 °C and 4 °C it contracts as it warms, and ice is less dense than the water it forms from. That is why ice floats and why ponds freeze from the top down, leaving fish alive underneath.