Copyright © 2007 Dorling Kindersley
Heat energy can be transferred from one place to another by three main processes. In CONVECTION, heat energy is carried by the movement of particles of matter. In CONDUCTION, heat is transferred by particles vibrating. In RADIATION, heat is carried directly by electromagnetic waves. When a hot object touches a cool object, heat moves from the hot one to the cool one. When objects transfer heat, they cool down to a lower temperature unless the heat energy they lose is constantly replaced.
Another process by which heat is transferred is called evaporation. When a dog sticks out its tongue and breathes hard (pants), the moisture on the tongue turns into water vapour – it evaporates. Heat energy is needed to turn a liquid into a gas, so heat is removed from the dog’s tongue in the process. This helps to cool the dog down. People cool themselves down by sweating through pores (tiny holes) in their skin, which removes heat from their bodies in the same way.
Convection is like an invisible conveyor belt that can transfer heat through fluids (liquids and gases). When part of a fluid is heated up, it expands. This makes it lighter and less dense than the fluid around it, so it rises upwards. As it rises, it moves away from the source of heat. Then it starts to cool and move downwards, before starting the cycle over again.
A burner at the base of the balloon warms the air inside. As the air warms up, it moves upwards, cools, and moves round in a circular pattern known as a convection current. When the balloon is full of hot air, it lifts off the ground because the hot air inside it is less dense and lighter than the cold air outside it.
During the day, sunlight warms the land more quickly than the sea. Warm air rises from the land by convection, moves out to sea, and cools, creating a circular current. This is why, at ground level, sea breezes blow from sea to land during the day. At night, the land cools more quickly than the sea. Warm air rises from the sea, the convection current reverses, and the breezes blow from land to sea.
Explorer Alexander von Humboldt explained how the oceans circulate using convection. Water warms and rises at the Equator, where Earth is hottest, then flows along the surface before cooling and sinking at the poles. He gave his name to the Humboldt Current, which travels up the South American coast.
Heat travels through solids by conduction. If one end of a metal bar is heated, heat energy moves rapidly along the bar. The hot particles do not move along the bar, but vibrate and pass energy to their neighbours. Materials that conduct electricity are also good conductors of heat. Metals conduct heat well, but wood, plastics, and glass conduct heat only poorly.
Cooking pots are made from metal, often aluminium. This metal is a good conductor of heat, so it rapidly transfers heat energy from the stove to the food. The handles of cooking pots are often made of wood or plastic. These materials do not conduct heat very well and are called insulators.
All the light and heat energy we receive on Earth comes from the Sun, and travels through space in invisible electromagnetic waves known as radiation. Space is vast and empty, so heat energy cannot travel from the Sun by conduction or convection. Hot objects on Earth, such as fires and radiators, also radiate heat.