Anything which is able to do work, as defined above, is said to possess energy, and therefore I nergy is the capacity to perform work. Work and energy are, of course, both measured in the same units, namely, joules. The world we live in provides energy in many different forms, of which the most important has been chemical energy. The utilization of the latent chemical energy in oal, oil, and gas, released in the form of heat to drive steam turbines and internalombustion engines, has been a major factor in the development of modern civilizaion. Many of the material comforts which we enjoy today come from the use of electric energy. The first electricity generating plants were powered by coal-fired steam engines, but by the middle of the twentieth century large hydro-electric power installations had been built in countries all over the world. “Hydro-electric” means the production of electricity by generators driven by water turbines.
The rapid flow of water required for this purpose comes from big reservoirs formed by building dams across valleys and large rivers (Fig. 7.1). Windmills which transfer the energy in wind to mechanical energy in machinery have long been in use for working water pumps as well as for milling grain or sawing timber. See also Fig. 7.2. In some parts of the world where the sun shines uninterruptedly for long periods, large concave mirrors have been set up to collect energy directly from the sun by focusing its rays on to special boilers which provide power for running electric generators. Fig. 7.3 shows a space telescope solar cell array. The utilization of atomic energy, which began after the middle of the twentieth century, has made available a new source of heat as a link in the production of electricity.