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In physics, energy (from the Greek ἐνέργεια - energeia, "activity, operation", from ἐνεργός - energos, "active, working") is a quantity that can be assigned to every particle, object, and system of objects as a consequence of the state of that particle, object or system of objects. Different forms of energy include kinetic, potential, thermal, gravitational, sound, elastic, light, andelectromagnetic energy. The forms of energy are often named after a related force. German physicist Hermann von Helmholtz established that all forms of energy are equivalent - energy in one form can disappear but the same amount of energy will appear in another form. Energy is subject to a conservation law. Energy is a scalar physical quantity. In the International System of Units (SI), energy ismeasured in joules, but in some fields other units such as kilowatt-hours and kilocalories are also used.
Energy transfer
Because energy is strictly conserved and is also locally conserved (wherever it can be defined), it is important to remember that by the definition of energy the transfer of energy between the "system" and adjacent regions is work. A familiar example is mechanical work. Insimple cases this is written as the following equation:
ΔE = W             (1)
if there are no other energy-transfer processes involved. Here E  is the amount of energy transferred, and W  represents the work done on the system.
More generally, the energy transfer can be split into two categories:
ΔE = W + Q             (2)
Where Q represents the heat flow into the system.
There are other ways inwhich an open system can gain or lose energy. In chemical systems, energy can be added to a system by means of adding substances with different chemical potentials, which potentials are then extracted (both of these process are illustrated by fueling an auto, a system which gains in energy thereby, without addition of either work or heat). Winding a clock would be adding energy to a mechanicalsystem. These terms may be added to the above equation, or they can generally be subsumed into a quantity called "energy addition term E" which refers to any type of energy carried over the surface of a control volume or system volume. Examples may be seen above, and many others can be imagined (for example, the kinetic energy of a stream of particles entering a system, or energy from a laser beamadds to system energy, without either being either work-done or heat-added, in the classic senses).

ΔE = W + Q + E             (3)
Where E in this general equation represents other additional advected energy terms not covered by work done on a system, or heat added to it.
Energy is also transferred from potential energy (Ep) to kinetic energy (Ek) and then back to potential energy constantly.This is referred to as conservation of energy. In this closed system, energy cannot be created or destroyed; therefore, the initial energy and the final energy will be equal to each other. This can be demonstrated by the following:
Epi + Eki = EpF + EkF
The equation can then be simplified further since Ep = mgh (mass times acceleration due to gravity times the height) and (half mass timesvelocity squared). Then the total amount of energy can be found by adding Ep + Ek = Etotal.
Energy and thermodynamics
Internal energy is the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules; it comprises the following types of energy:
Type |Composition of internal energy (U) |
Sensible energy | The portion of the internal energy of a system associated with kinetic energies (molecular translation, rotation, and vibration; electron translation and spin; and nuclear spin) of the molecules. |
Latent energy | The internal energy associated with the phase of a system. |
Chemical energy | The...
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