Heat transfer: Convection, conduction and radiation

Philip and Kim were playing outside in the snow but they started freezing after a while. They decided to come back inside to warm up. Let’s stand close to the radiator! That’s a good way to warm up quickly - the radiator is much warmer than Kim and Philip. Because of this temperature difference, the heat from the radiator will travel to their bodies, which are colder.

We call this HEAT TRANSFER. But how is the heat transferred between things? The radiator has a metal casing which is filled with hot water or steam. When you touch the outside of the radiator, it feels hot. The heat is transferred from the water to the metal, and then to your hand by direct contact.

This is called THERMAL CONDUCTION. Materials like metal transfer heat very well. They are called CONDUCTORS. But I don’t feel anything... That’s because you are still wearing thick woollen gloves, Philip.

Wool doesn’t transfer heat very well - it has low conductivity. Materials like this are called INSULATORS. Air is another example of an insulator, so it doesn't conduct heat well either. So how is it possible that you feel warmth even if you’re not touching the radiator? You might have noticed that when things get really hot they start glowing - like fire or wires inside a toaster.

This glow is the visible part of electromagnetic waves. In fact, everything around us emits and absorbs such waves but normally we can’t see them, unless we use for example an infrared camera. Heat is carried by these electromagnetic waves. This is called THERMAL RADIATION. Radiation, just like light, doesn’t need any kind of substance - or medium - to travel through.

It can travel even through vacuum! That’s how the heat from the Sun reaches the Earth, or how heat from the radiator warms your hands even if you don’t touch it. But if you hold your hands above the radiator it will feel hotter than if you hold them to the side. This is because of how the air moves around the radiator. The air close to the radiator is heated up by conduction and radiation.

Hot air is less dense and lighter than cold air, so it floats upwards - above the radiator and up to the ceiling. There it slowly cools down and becomes heavier and then the cold air sinks to the bottom of the room. It gets heated by the radiator and floats up again. The heat spreads around the room because the air moves. This way of transferring heat is called THERMAL CONVECTION.

Heat moves through convection in gases, like air or steam, or liquids, for example in water. There are three ways of transferring heat: - conduction - when heat travels through direct contact between objects or substances, - radiation - when heat travels through space between different objects, - and convection - when heat flows within liquids and gases. Knowing how the heat is transferred helps us warm up our hands quickly. Can you think of any other examples of how we can use the knowledge about heat transfer in everyday life?