What is the speed of temperature?


The Sunday Nation


08 May 2005


Joseph Mwanzia has asked an interesting question: What is the speed of temperature? Presumably, he doesn’t mean how quickly temperature rises, but how fast it moves.

To answer that, we must first ask whether temperature moves. If one end of a steel rod in placed on a fire, the temperature at that end will start to rise immediately. Then gradually, the rest of he rod will also become hot. When this happens, can we say that the temperature from the hot end has moved to the cooler side?

When something moves from point A to B, it is no longer found in A. Thus, clearly, the temperature has NOT moved. However heat energy is moving through the rod continuously. So, perhaps the right question should be: what is the speed of heat?

That is also not a straightforward question because there are three ways by which heat can move from one point to another – convection, radiation and conduction. In convection, heat is transported by the material in which it is contained. For example, as hot air is rising, it takes its heat energy along. So the speed of heat in this case is the speed of the material.

Radiation refers to the movement of heat energy through vacuum. The energy of a hot object is emitted in the form of electromagnetic waves and when these land on another object some distant away, the second object becomes hot as well. In this case the speed is that of electromagnetic waves (or light) which is about one billion kilometres per hour.

Strictly speaking, radiation is NOT transfer of heat – it is the movement of energy in the form of electromagnetic waves. Heat cannot exist in vacuum! Heat is the internal energy is a material substance and in vacuum, there is nothing.

Finally, conduction refers to movement of heat energy through a material. In school they say it is transfers of heat through solids. This is not accurate but it keeps thing simple for the children. Gases and liquids also conduct.

Nevertheless, the rate of heat flow through a substance depends on the nature of the material, its dimensions and the temperature difference between its ends. When one end of a steel rod is placed in a flame, the temperature of that end rises quickly to that of the flame.

After some time, the rod will be very hot at one end and cool at the other but its temperature will not be changing any more. However, the fact that the temperature difference remains constant does not mean that heat is not flowing.

If one end of a steel rod measuring, say, one centimetre in diameter and 50cm in length were placed on a flame at 1500 degrees celsius, and the cool end stabilised at about 100 degrees. In that situation, the rod would be transferring heat at the rate of 11joules per second (only 11watts). Not very much, is it?

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