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How is energy converted?

Energy can be converted from one form into another in three basic ways:

  1. Through the action of forces. This category has several important special cases:
    1. Gravitational Forces - when gravity accelerates a falling object, its converts its potential energy to kinetic energy. Likewise, when an object is lifted the gravitational field stores the energy exerted by the lifter as potential energy in the earth-object system. 
    2. Electric and Magnetic Force Fields - Charged particles, upon which electrical fields exert forces, possess potential energy in the presence of an electric field in a way similar to that of an object in a gravitational field. These force fields can accelerate particles, converting a particle's potential energy into kinetic energy. Likewise, charged particles can interact via the electric and magnetic fields they create, transferring energy between them, and in the case of an electrical current in a conductor, cause molecules to vibrate, i.e. converting electrical potential energy into heat.  
    3. Frictional Forces - The macroscopic (large-scale) energy of an object, that is, the potential and kinetic energy associated with the position, orientation, or motion of the entire object, not counting the thermal or heat energy of the system, can be converted into thermal energy (heat),  whenever the object slides against another object. The sliding causes the molecules on the surfaces of contact to interact via electromagnetic fields with one another and start vibrating. 
  2. When atoms absorb or emit photons of light. When light falls on an object, an incident photon may either pass through the object, be reflected by the object, or be absorbed by the atoms making up the object. If most of the photons pass through, the object is said to be transparent. Depending on the smoothness of the surface on the scale of the photon's wavelength, the reflection may be either diffuse (rough surface) or coherent (smooth surface). 

    If the photon is absorbed, the photon's energy may also be split up and converted in the following ways:

    1. photothermal effect: the energy absorbed may simply produce thermal energy, or heat in the object. In this case the photon's energy is converted into vibrations of the molecules called phonons, which is actually heat energy.
    2. photoelectric effect: the energy absorbed may be converted into the kinetic energy of conduction electrons, and hence electrical energy.
    3. photochemical effect: the energy may bring about chemical changes which effectively store the energy. 
  3. When nuclear reactions occur, that is, when there are rearrangements of the subatomic particles that make up the nuclei of atoms. There are two basic types: Fission - when nuclei combine, and Fusion - when nuclei split apart.

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