From the time of Newton till the end of the nineteenth century the development of physics consisted essentially of the refinement and extension of the mechanical view of the Universe. There were many stages in this process but one of the most interesting came towards its end with the realization that the cosmic clockwork was inevitably unwinding and running down. The source of this realization was the development of thermodynamics.

The first half of the nineteenth century was a period of great economic and industrial growth. The steam engine, invented in the previous century, was becoming increasingly common in locomotives, mines and factories; power was becoming available on demand. A major priority for engineers was to produce more efficient engines, in order to deliver more useful power for less expenditure on fuel. Thermodynamics emerged as a study of the basic principles determining energy flows and the efficiency of machines.

This may seem like a big idea in engineering rather than a big idea in physics. Certainly, thermodynamics is important to engineers, and continues to guide the design of engines of all sorts, but thermodynamics is just as important to physicists. It explains a wealth of natural phenomena, from the freezing of water to the evaporation of a black hole, and casts light on concepts like temperature, heat and spontaneous processes, which do not fit naturally into the Newtonian world-view.

It is still instructive to return to the origins of the subject. Speaking very roughly, a steam engine is a device which uses fuel to convert water into steam and uses the resulting expansion in volume to drive a piston. The kinetic energy of the piston is exploited using a variety of mechanical devices - gears, drive belts, camshafts and so on, but thermodynamics concentrates on the early stages of the process, where heat is used to create kinetic energy.

Figure 1.13 A steam engine, in which energy stored in coal is used to create heat to vaporize water. The resulting increase in volume drives a piston, so allowing useful work to be done. Click here for larger image (13.79)

To begin with there was much dispute about the nature of heat. Many people thought of it as a sort of fluid which could flow from one body to another. Eventually, it became clear that no such fluid exists and that heat is best defined as energy transferred because of a temperature difference. This scientific definition of the word 'heat' is slightly different from everyday usage, so it may help to consider a specific example. Think of a hot steak (veggie-burger, if you prefer) resting on a cold plate. The steak cools down and the plate warms up as energy flows from the steak to the plate. The energy transferred in this way is called heat. By contrast, work is energy transferred by non-thermal means. For example, if you rub the plate vigorously with a cloth, the energy of the plate will increase and it will get slightly warmer. But, this energy transfer is not caused by a temperature difference between the plate and the cloth, so energy transferred by rubbing is classified as work rather than heat.
Continue on to Thermodynamics and entropy, part 1 of 3