Unit 12. The conservation and the transformation of energy

Energy in a system may take on various forms (e.g. kinetic, potential, heat, light). Einstein’s work on the theory of relativity which he put forward early in the last century has altered our ideas regarding mass and energy as being separate and distinct from one another. Einstein has simplified our picture of the universe by showing that the mass and energy can be converted from one to the other. This view has been supported by a number of experiments. Until this theory was put forward two fundamental laws were accepted by physicists.

Firstly, the law of conservation of matter, which states that the total quantity of matter in the universe is fixed and cannot be increased or decreased. And secondly, the law of conservation of energy which affirms that the total quantity of energy in the universe is likewise constant and can be neither created nor destroyed.

Nowadays, in the light of the theory of relativity, these two laws have become fused into one. We have seen a number of cases where one type of energy has been transformed into another. Whenever energy in one form is expended, an equal amount of energy in some other form takes its place. In every situation, the transformation of energy is inefficient because we are unable to harness all the energy available. It is not possible, for example, to convert the chemical energy of coal directly into electrical energy. It must first be burned in order to heat water and create steam, which then turns a turbine to produce electricity. In such a transformation, energy efficiency is reduced. Old-fashioned steam engines had an efficiency of 6 to 7%, due to heat loss from the sides of the locomotive and loss of steam and heat up the stack. Another example is driving in a car. In this case, the petrol in the car's fuel tank provides energy to push the car forward. Once that energy has been used, however, it cannot be recovered. The energy is transformed into heat and sound through friction with the air and the ground and combustion within the engine. Some cars, such as hybrid cars, are able to transform some of the excess energy into potential energy, but the transfer is not complete. Some energy is still lost to heat and sound. The transformation of energy is never 100% efficient because we cannot usefully capture all the energy supplied by one source.

Energy can be defined as the capacity for doing work. It may exist in a variety of forms and may be transformed from one type of energy to another. However, these energy transformations are constrained by a fundamental principle, the Conservation of Energy principle. One way to state this principle is "Energy can neither be created nor destroyed". Another approach is to say that the total energy of an isolated system remains constant.

So, a fundamental law that has been observed to hold for all natural phenomena requires the conservation of energy – i.e., that the total energy does not change in all the many changes that occur in nature. The conservation of energy is not a description of any process going on in nature, but rather it is a statement that the quantity called energy remains constant regardless of when it is evaluated or what processes –possibly including transformations of energy from one form into another – go on between successive evaluations.