Conduction, convection and radiation
Conduction is the process of handing on energy from one bit of stuff to the next, rather like a message in a letter being handed along a line of students from one neighbour to the next.
Before beginning any other experiments, students could touch a number of objects around the room and classify them into those which feel warm to the touch and those which feel cool to the touch. Unless sunlight is falling on them or they are near to a heater:
- all the materials are likely to be at the same temperature
- that temperature is likely to be lower than the temperature of the body.
Thermal energy will flow from the body to the object. In this way students will be able to draw up a crude list of good and bad thermal conductors of energy.
Energy can be carried by wholesale movement of the medium: a warmer fluid moves, displacing a colder fluid and thus transfers thermal energy in convection currents. This is rather like a student carrying a message in a letter to others rather than just passing it on down the line, as is the case with conduction.
Radiation is quite different from conduction and convection. It is not a matter of something hot carrying the energy itself, or of atoms handing the energy on from one to the next. Hot things produce electromagnetic waves and so they cool down, unless we keep on supplying them with energy. When electromagnetic waves hit something, they normally warm it up.
The energy transferred by each photon of electromagnetic radiation is given by hƒ (Planck’s constant multiplied by the frequency of the radiation). All frequencies transfer quanta of energy. The energy transferred by a quantum of ultra-violet radiation is greater than for a quantum of infra-red radiation. However, there is more infra-red radiation emitted from a hot body than a cooler body. One watt of green light gives just as much heating as one watt of infra-red light. There are no special kinds of heat rays or heat radiation. The electromagnetic waves only produce thermal energy when they are absorbed; they transfer no energy as they travel through a completely transparent medium or when being reflected from a perfectly reflecting mirror.
To continue the analogy of transfer of energy with passing on a message: in the case of radiation, the message must be taken to a radio station where it can be transmitted as a radio message from a radio transmitter; you don't expect to see a piece of paper whizzing along with its message in a radio wave.