User:Anthony Argyriou/sandbox

In science, energy is a measurable property of a physical or chemical system, which means that the energy of a system may be expressed by a single real number, say A. It is possible and meaningful to state that the energy of a certain system is equal to A joule, with joule being an SI unit for amount of energy.

Roughly speaking, the energy of a system is a measure of the amount of work that the system is able to perform on its environment. However, the second law of thermodynamics expresses that some kinds of energy cannot be converted into work and hence this definition is not completely general and must be handled with care. An additional difficulty in defining energy is that energy has many, seemingly very different, manifestations; be it chemical energy of a certain amount of gasoline, the kinetic energy of a moving cannon ball, the heat stored in a steam boiler, the potential energy of water in a reservoir, the nuclear fusion energy contained in a hydrogen bomb, the electricity in a battery, etc. All these manifestations obey the same extremely important law: energy is conserved in conversion from one form of energy to the other. This law of conservation of energy is known as the first law of thermodynamics. This law pervades all of science, and is probably science's most important principle.

The concept of energy is best explained by means of examples. Assume, to that end, that we use a pump, running on gasoline, to pump water up to a reservoir, and when the reservoir is filled, we let the water flow down to drive an electrical generator. Doing this, we convert the chemical energy of the gasoline to (i) the mechanical energy of the pump to (ii) the potential energy of the water in the reservoir to (iii) the kinetic energy of the falling water, and finally to (iv) the electric energy generated by the generator. If we use the generated electric current for lighting, then the light bulbs convert the electric current to yet another form of energy, namely (v) light (electromagnetic radiation). During these energy conversion processes, the law of conservation of energy assures us that no energy is lost. To non-scientists the contrary may seem the case sometimes, because heat is generated (especially in burning the gasoline to drive the pump), and the heat will escape to the environment without any useful, or directly noticeable, effect. However, since heat is also a form of energy, it must be included in the energy balance of the first law.