User:Anthony Argyriou/sandbox
In science, energy is a measurable physical quantity of a system which can be expressed in joules (the SI unit for a quantity of energy) or other measurement units such as ergs, calories, watt-hours or Btu.
Energy is commonly defined as the amount of work that a system is capable of performing. However, the second law of thermodynamics states that some kinds of energy cannot be converted into work. Hence, this definition is not completely general and should be used with care.
An additional difficulty in defining energy is that energy has many seemingly very different forms such as the kinetic energy of a moving cannon ball, the potential energy of water stored in an elevated tank, the chemical energy stored in gasoline or other fuels, the heat stored in steam, the electrical energy in a battery, the nuclear fusion energy in a hydrogen bomb, etc. While each form of energy may be transformed into another, the total amount of energy is conserved (remains constant). In other words, energy cannot be created or destroyed. This conservation of energy is expressed by the first law of thermodynamics that pervades all of science and is probably science's most important principle.
The concept of energy is best explained by means of examples. To that end, assume that we use a gasoline-fueled engine driving a pump to pump water up to an elevated reservoir, and when the reservoir is filled, we let the water flow down to drive an electrical generator. In doing this, we convert the chemical energy of the gasoline to (a) the mechanical energy of the pump to (b) the potential energy of the water in the reservoir to (c) the kinetic energy of the falling water, and finally to (d) 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 (e) 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 required by the first law of thermodynamics.