Heat and How It Travels

Heat is energy on the move

When you hold a mug of hot chocolate, your hands feel warm. That warmth is heat — energy flowing from the hot mug into your cooler skin. Heat is a form of thermal energy, and the most important rule is simple:

Heat always travels from a hotter object to a cooler one.

It keeps flowing until both reach the same temperature. This is one of the energy transfers you can read about in the many forms of energy.

Heat is not the same as temperature

People mix these up, but they are different.

• Temperature measures how hot something is, in degrees Celsius (°C). It tells you the average energy of the particles.
• Heat measures the total thermal energy moving between objects, in joules (J).

A sparkler burns at over 1000°C, yet it carries very little heat energy, so it does not burn down a whole room. A warm bath is far cooler but holds enormous heat energy because it contains so many particles.

The three ways heat travels

Heat moves in exactly three ways: conduction, convection, and radiation.

Conduction — heat through contact

In conduction, heat passes through a material by direct contact. The particles at the hot end vibrate fast and bump into their slower neighbours, passing the energy along, particle by particle.

Conduction works best in solids, especially metals, where particles are packed tightly. This is why a metal spoon in hot soup soon burns your fingers. Materials that conduct heat poorly — like wood, plastic, air, and wool — are called insulators.

Convection — heat through moving fluids

Convection happens in liquids and gases, which can flow. When part of a fluid is heated, it expands, becomes less dense, and rises. Cooler, denser fluid sinks to take its place. This circular movement is a convection current.

Convection currents explain why warm air rises to the ceiling, why a saucepan of water heats evenly, and how sea breezes form.

Radiation — heat through waves

Radiation is heat carried by infrared waves. Unlike the other two, it needs no material at all — it can cross empty space. That is how the Sun's heat reaches Earth across millions of kilometres of vacuum.

Dark, dull surfaces absorb radiation well, while shiny, light surfaces reflect it. This is why solar panels are black and why emergency blankets are shiny silver.

Worked example: trace the heat

Imagine a pot of water boiling on a gas hob in a warm kitchen. Where is each type of heat transfer?

1. The flame heats the metal pot base by radiation.
2. Heat moves up through the solid metal by conduction.
3. Water at the bottom warms, rises, and circulates — convection.
4. The hot pot warms the air and your face nearby by radiation again.

All three working at once, in one everyday moment.

Try it yourself! 🧪

See a convection current with your own eyes.

1. Fill a clear glass with cold water and let it settle for a minute.
2. Gently drop in one teabag, or add a single drop of food colouring at the surface, directly above where you will warm it.
3. Carefully place the glass on a warm spot, or hold a warm hand against one side of the base.
4. Watch the colour. Where the water is warmed it rises in a plume, spreads across the top, cools, and sinks down the sides.

You have just made a convection current visible — the same motion that heats your home and stirs the oceans.