This activity covers the following National Curriculum areas:

Key Stage 4 Science Sc1 Scientific Enquiry: Knowledge, Skills and Understanding: Ideas and Evidence in Science

When Joule began his work, people had a very strange idea about heat. They thought it was a kind of invisible fluid - a bit like a gas. Their idea was that, when you heat a solid, you drive this fluid into the solid and it heats up and expands. Then, when you cool it down, the fluid is squeezed out of the solid and disappears into the surroundings. Although this might sound very odd, it was what everybody thought.

However, the idea that friction can produce heating was not new. Even before Joule did his work the American, Count Rumford, had shown that by trying to drill into a brass canon he could produce an enormous amount of heat. A modern example of this can be seen when we use an electric power drill. If we are not careful, it is possible to melt the end of the drill bit - and certainly to make it too hot to touch - by continuous drilling. Another example of this is provided by skating on ice. It is often stated that we can skate on ice because the pressure of the skater lowers the melting point of the ice, thus allowing the skates to bite into the (otherwise solid) ice. However, this is not the reason. Whilst the lowering of the melting point does occur, it is not large enough to produce this result. The real reason is that friction heats, and thus melts, a thin layer of ice below the skate.

For A-level teachers: some more comments about the idea that heat is a kind of invisible fluid

We might think it rather strange, almost childish, to think that heat is an invisible fluid (called caloric) that can flow into and out of bodies. However, before we condemn it, let us look at this idea more closely. We often have problems trying to think in abstract terms and we therefore invent more practical models to represent the world around us. For example, when Maxwell developed the theory that electromagnetic waves were propagated through space with the speed of light, he used a model of the medium which was mechanical. Before Einstein produced his theory of relativity, it was thought that a vacuum was filled with an invisible aether, again to support the propagation of light. Even today, we have similar concepts. In nuclear physics, we talk of the vacuum levels being filled with electrons (particles) and of positive electrons (antiparticles) corresponding to vacancies in these levels. Finally, if we treat the caloric as representing entropy rather than heat itself, then it is possible to present a reasonable case for its adoption.