Energy It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units not part of the SI, such as ergs, calories, British Thermal Units, kilowatt-hours and kilocalories, which require a conversion factor when expressed in SI units. The SI unit of energy rate (energy per unit time) is the watt, which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The CGS energy unit is the erg and the imperial and US
Elastic energy of compressible fluids, and especially gases, demonstrates the essence of elastic energy with negligible complication. The simple thermodynamic formula: formula_2 where dU is an infinitesimal change in recoverable internal energy "U", "P" is the uniform pressure (a force per unit area) applied to the material sample of interest, and "dV" is the infinitesimal change in volume that corresponds to the change in internal energy. The minus sign appears because "dV" is negative under compression by a positive applied pressure which also increases the internal energy. Upon reversal, the work that is done "by" a system is the negative of the