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In physics, energy (from Greek ἐνέργεια - energeia, "activity, operation", from ἐνεργός - energos, "active, working"[1]) is a quantity that is often understood as the ability a physical system has to produce changes on another physical system.[2][3]

The changes are produced when the energy is tranferred from a system to another. A system can transfer energy by means of three ways, namely: physical or thermodynamical work, heat transfer, or mass transfer.

This quantity can be assigned to any physical system. The assigned energy, according to Classical Physics, depends on its physical state relative to the frame of reference used to study it.

On the other hand, in Relativistic Physics, when using an inertial reference frame, invariant mass energy is independent of such kind of reference frames. The invariant mass of a system is the same in all the inertial reference frames, it means that its energetic equivalent (invariant mass energy) would be the same in all the inertial reference frames, too.

All the forms of energy that a system has can belong to one of two great components: the internal energy and the external energy (not to be confused with the energy of the surroundings which is outside the system). All kinds of internal and external energies can, additionally, be classified as kinetic energy or potential energy. Kinetic energy considers the mass and the motion of a system. If the system is studied as a whole, it is called external kinetic energy. The thermal energy is the internal kinetic energy and it considers the motion of every constitutive particle of the system (molecules, atoms, electrons, etc.). The gravitational potential energy is an external potential energy and so is the electrostatic potential energy. The elastic energy is an internal potential energy. The forms of energy are often named after a related force, as in the previous examples.

Some forms of energy are associated to the particle-like behaviour of the system. But, there might be cases like that of sound energy in which the energy overall effect is related to the wave-like behaviour of the system. In the specific case of sound, there is a transmission of oscillations in the pressure through the system. The energy associated to the sound wave converts back and forth between the elastic potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium of which the system is made up.

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.[4] A restatement of this idea is that energy is subject to a conservation law over time.

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