Annaleise Faint of Riverdale School asks :-

How does heat travel?

John Campbell, a physicist at the University of Canterbury, responded.

There are three main ways; conduction, convection and radiation. Atoms in all materials are in rapid motion and jostling their neighbours in a fashion compatable with the temperature of the material. The higher the temperature, the greater the jostling. In solids each atom is bound to its position in the solid by the electrical forces between atoms. However at any given instant the atom can be travelling at very high speed in any direction, until it collides with a neighbour and changes speed and direction. After a collision the atom can occasionally be stationary, but only for the very short interval of time until an impacting neighbour affects it. On average, its position doesn't change.

The amplitude of the vibration increases with temperature, as shown by the facy that most objects expand when heated. If you heat one end of a wooden spoon in stirring a cooking stew, the atoms in the hot end vibrate more and this increase in vibrational energy (heat) is transferred to the cooler neighbouring atoms but only very slowly. The handle of the spoon stays cool because the heat transfer is so slow the handle looses the heat energy through other processes.

If you use a silver spoon to stir your stew or tea, the handle gets warm much more quickly. This is because metals are good electrical conductors which means they have about one electron per atom which is free to move throughout the metal. This is a much faster process than random atomic collisions. Hence if you want a metal spoon then have it made out of a metal that is a poor electrical conductor, such as stainless steel. If you want to keep a computer chip cool then mount it on an electrical insulator which conducts heat very well, ie something made from light atoms. Diamonds are one of the best.

Fluids are materials in which the atoms can move about in the material, for example liquids and gases. The above mechanism also applies to fluids but usually the main heat transfer is though convection. Warmer air is less dense than colder air so it rises and colder air sinks, giving rise to a lot of our weather. We stir the pot because that is the quickest way to get the the whole stew to the same temperature rather than burn the bottom, where the heat is normally applied, whilst the top doesn't. Stirring just brings hot atoms into contact with cooler ones far more quickly than the diffusion mechanism of conduction does.

The third mechanism is radiation. A radio signal comes from a cell phone or radio transmitter because electrons are caused to oscillate to and fro in the aerial. Every object radiates heat energy because every object consists of atoms and electrons that are continually changing velocity. As you sit reading this you should be at a comfortable temperature because your body is in equilibrium with the room. It radiates, conducts and convects heat energy away at the same rate it receives it from the surrounds.

If there are no atoms between two objects, such as us and the Sun, then radiation is dominant. Because the Sun is far hotter than us and the Earth, the net heat radiation flow is to us. The Sun gives us about 1 kiloWatt of free energy for every square metre at the equator. This heating keeps our planet habitable and also generates a lot of our weather.