Heat transfer

Heat always flows from hot to cold

Heat is energy being transferred because of a difference in temperature. It always flows from a hotter object to a colder one, until they reach the same temperature. A cold drink doesn't "give out cold" to your hand: your hand gives heat to the drink.

Heat can move in three ways: conduction, convection and radiation. A pot on a campfire shows all three at once.

Conduction

In conduction, heat passes through a material from particle to particle. Particles at the hot end vibrate faster and bump into their neighbours, passing the energy along. The material itself doesn't move.

Metals are excellent conductors because they contain free electrons that carry energy quickly through the metal. Materials that conduct heat poorly are insulators: wood, plastic, glass, wool and especially trapped air. Most good insulators work by trapping air in small pockets: think of a puffer jacket, foam cups, home ceiling batts, or the fluffed-up feathers of a bird on a cold morning.

This explains the seatbelt buckle. Metal conducts heat into your skin much faster than fabric does, so it feels hotter even at the same temperature. On a cold morning, a metal railing feels colder than a wooden bench for the same reason.

Convection

Liquids and gases (fluids) can carry heat by moving. When part of a fluid is heated, its particles spread out, so it becomes less dense and rises. Cooler, denser fluid sinks to take its place, gets heated, and rises in turn. This circulating flow is a convection current.

Convection explains why heaters work best near the floor and air conditioners near the ceiling, why the top shelf of an oven is hottest, and why sea breezes blow in from the ocean on hot afternoons. Glider pilots and wedge-tailed eagles ride rising columns of warm air called thermals without flapping.

Radiation

Radiation is heat transferred by electromagnetic waves, mainly infrared radiation. It needs no particles at all, so it can travel through empty space. That's how the sun's energy crosses 150 million kilometres of vacuum to reach Earth.

Everything gives off some infrared radiation; hotter objects give off more. Thermal imaging cameras detect it, which is how firefighters find people in smoke and how vets spot inflamed joints in horses.

The surface of an object matters a lot. Dark, dull surfaces absorb and give off radiation well. Light, shiny surfaces reflect it. That's why houses in hot parts of Australia often have white roofs, why emergency blankets are shiny, and why a black car gets so much hotter than a white one.

Stopping heat transfer: the vacuum flask

A vacuum flask (often called a Thermos) keeps hot drinks hot and cold drinks cold for hours by blocking all three methods of heat transfer.

Heat transfer and living things

Reptiles such as bearded dragons can't make much of their own body heat. In the morning they bask in the sun to absorb radiation, and press their bellies on warm rocks to gain heat by conduction. In the midday heat they move into shade or burrows. Kangaroos lick their forearms, where blood vessels run close to the skin, so evaporation cools the blood. Emperor penguins huddle in tight groups to reduce the surface area they lose heat from.

People use heat transfer on a much larger scale. In Iceland, which sits on a volcanic hot spot, hot water from deep underground heats almost every home and produces about a quarter of the country's electricity. This is geothermal energy.