Heat, temperature and how heat travels

Touch a steel glass of hot milk and you say it is hot; touch an ice cube and you say it is cold. But our sense of touch is not reliable. A room at the same temperature feels warm when you come in from the winter cold and cool when you come in from the summer sun. To measure how hot something is, we need a number: its temperature.

Heat and temperature are different

Heat is a form of energy. It flows from a hotter object to a colder one. Temperature is a measure of how hot or cold an object is. A cup of boiling water and a bucket of boiling water are at the same temperature, but the bucket holds far more heat.

Temperature is measured with a thermometer, usually in degrees Celsius (°C). The normal temperature of the human body is about 37°C. Water freezes at 0°C and boils at 100°C. Doctors use a clinical thermometer, which is today mostly digital.

How heat travels

Heat always moves from a hot place to a cooler one. It does so in three ways.

1. Conduction

Leave a steel spoon in hot dal and soon its handle becomes hot. Heat has passed along the spoon from particle to particle, though the particles themselves have not moved from their places. This is conduction, and it is the main way heat travels through solids.

Materials that let heat pass easily are good conductors: metals such as copper, aluminium and iron. That is why cooking vessels are made of metal. Materials that do not are poor conductors, or insulators: wood, plastic, wool, air. That is why pans have plastic or wooden handles.

2. Convection

When water is heated in a pan, the water at the bottom becomes hot, expands, grows lighter and rises. Cooler, heavier water sinks to take its place and gets heated in turn. This circular movement carries heat through the whole liquid. It is called convection, and it is how heat travels through liquids and gases.

Convection in air causes winds. Near the coast, land heats up faster than the sea during the day. The warm air over the land rises and cool air from the sea blows in: the sea breeze. At night the land cools faster and the breeze reverses.

3. Radiation

The sun's heat reaches us across empty space, where there is nothing to conduct or carry it. Heat that travels without any medium is called radiation. We also feel radiation when we sit near a fire or a hot iron.

Dark surfaces absorb more radiant heat than light ones. So we prefer light-coloured clothes in summer and dark ones in winter.

Using the idea

• Woollen clothes keep us warm because wool traps air, and air is a poor conductor.
• Two thin blankets are warmer than one thick one, because of the layer of air between them.
• A thermos flask keeps tea hot by cutting down all three ways of heat loss.