Conduction, convection and radiation

Thermal conduction

Conduction is the way in which energy is transferred (through heating by contact) from a hot body to a cooler one (or from the hot part of an object to a cooler part). It is the result of particle motion: fast or vigorously moving particles bumping into less energetic particles and making them move faster or vibrate more vigorously.

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.

Because the objects are at a lower temperature, energy will be transferred from the students' hands to the object. However, even though they are all at the same temperature, some of the materials will feel colder. These are the ones that are better conductors. The reason that they feel colder is because, being good conductors, they will transfer the energy quickly across the whole object. Or, put differently, the good conductor cannot maintain a temperature difference between the piece that the students is holding and the rest of the object. Therefore, the student has to raise the temperature of the whole object, not just the bit that they are holding.

Thermal convection

Energy can be carried from one place to another by wholesale movement of the medium: a warmer fluid moves, displacing a colder fluid and thus transfers 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

Radiation is quite different from conduction and convection. It is not a matter of something hot carrying the energy itself, or of atoms transferring the energy on from one to the next. Hot things produce electromagnetic waves and so they cool down, unless we keep them hot. When electromagnetic waves hit something, they are absorbed and can raise its temperature.

The energy transferred by each photon of electromagnetic radiation is given by {hf} (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 increase the energy stored thermally by an object 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.