From the perspective of physics, every physical system contains (alternatively, stores) a certain amount of a continuous, scalar quantity called energy; exactly how much is determined by taking the sum of a number of special-purpose equations, each designed to quantify energy stored in a particular way. There is no uniform way to visualize energy; it is best regarded as an abstract quantity useful in making predictions.
The first sort of prediction energy allows one to make is how much work a physical system could be made to do. Performing work requires energy, and thus the amount of energy in a system limits the maximum amount of work that a system could conceivably perform. In the one-dimensional case of applying a force through a distance, the energy required is ∫ f(x) dx, where f(x) gives the amount of force being applied as a function of the distance moved.
Note, however, that not all energy in a system is stored in a recoverable form; thus, in practice, the amount of energy in a system available for performing work may be much less than the total amount of energy in the system.
Energy also allows one to make predictions across problem domains. For example, if we assume we are in a closed system (i.e. the conservation of energy applies), we can predict how fast a particular resting body would be made to move if a particular amount of heat were completely transformed into motion in that body. Similarly, it allows us to predict how much heat might result from breaking particular chemical bonds.
The SI unit for both energy and work is the joule (J), named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton metre, and in terms of SI base units, 1 J equals 1 kg m2/s2. (Conversions. In cgs units, one erg is 1 g cm2/s2. The imperial/US unit for both energy and work is the foot pound.)
Noether’s theorem relates the conservation of energy to the time invariance of physical laws.
Energy is said to exist in a variety of forms, each of which corresponds to a separate energy equation. Some of the more common forms of energy are listed below.
Feynman, Richard. Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher. Helix Book. See the chapter “conservation of energy” for Feynman’s explanation of what energy is, and how to think about it.
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