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"One day man will connect his apparatus to the very wheelwork of the universe... and the very forces that motivate the planets in their orbits and cause them to rotate will rotate his own machinery," Nikola Tesla
Energy is notoriously difficult to define. It is often defined as the ability to do work, but this is an incomplete definition. American Supreme Court Justice Potter Stewart famously said of pornography that he could not define it, "but I know it when I see it." Thus it is with energy. It is easy to recognize in most of its several forms: mechanical (including kinetic and gravitational potential), chemical, nuclear, electromagnetic (including light and other non-nuclear radiations), elastic, and thermal. The SI unit of energy is the joule, named in honor of James Prescott Joule, who discovered the equivalence of heat and mechanical energy. The joule is a derived unit, equivalent to kg•m²/s² . It can also be expressed as newton-meters (Nm), but is not usually done to avoid confusion with the units of Torque (N x m) The word energy comes from the Greek words en and ergon which means at work. energy at etymonline.com
Work and Energy
Work is also measured in joules, which shows their close relationship. Energy can be used to perform work; work can be made to increase the energy of a system. This is one way of stating an important principle, the work-energy theorem. The two quantities are not the same, any more than mass and energy are the same thing, but one can be converted into the other. Unlike the situation with mass and energy however, there is not 100% conversion. The system will always lose some energy as heat, which will increase the entropy, or disorder, of the system. Note that this does not mean that some of the energy somehow disappears. It is merely lost from the human point of view, but not in terms of the Universe. It is no longer in a form that is useful to us. Heat is a form of energy and "counts" in the conservation law, and entropy is a measure of the amount of disorder that is created by the dissipation of heat. See the second law of thermodynamics.
Energy is a conserved quantity—so long as we are not considering nuclear changes. For many purposes it is sufficient to treat energy conservation as if it were a law of nature. This approach is taken in thermodynamics, chemistry, and most of high school physics (except in dealing with nuclear changes).
A more rigorous treatment of energy conservation requires that mass and energy be considered together: mass-energy is conserved using E=mc². This equation was discovered by Albert Einstein in 1905 as a corollary to his theory of special relativity.
Transfer and Transformation
Energy is particularly susceptible to measurement when being transferred (one object to another) or transformed (one type to another).
Examples of transfer include the flow of electricity from the outlet to your computer through the power cord and heat from a hot cup of coffee to your mouth. We have invented many devices that transform energy. For example, microphones change sound (a form of mechanical energy) into electromagnetic energy and loudspeakers change it back. Telephones have both. Nuclear power plants convert nuclear energy to thermal energy; the heat boils water into high pressure steam. The mechanical energy of the steam drives a generator that converts it to electromagnetic energy. You may use some of that energy at your house to recharge your laptop or camera battery, converting the electromagnetic energy to chemical energy. None of these processes is 100% efficient, so energy is converted into waste heat, more properly known as entropy, at every step along the way. When you use incandescent light bulbs, as much as 95% of the electrical energy is converted to heat (entropy) instead of light.
To normal perception energy seems to flow in uneven amounts however at a subatomic level all energy moves in packets called quanta, or singular, quantum. Each quantum has the same amount of energy.