Electromagnetic energy

The forces electrostatic S and magnetic can make move remote objects, it thus seems obvious that with any electromagnetic phenomenon a potential energy is associated, i.e. that if a load is there, the electromagnetic source accelerates it, but if no load is there, the action is only potential. This potential energy is called electromagnetic energy.

According to the Maxwell's equations, the electromagnetic Densité of energy in the vacuum is

w = \ frac {1} {2} \ varepsilon_ {0} E^ {2} + \ frac {1} {2 \ mu_ {0}} B^ {2}
where ε0 is the electric permittivity of the vacuum (or “constant of influence”), μ0 is the magnetic permeability of the vacuum, \ vec {E} is the vector electric field and \ vec {B} is the vector magnetic field.

In the case of a electromagnetic Wave, this energy moves; starting from the Maxwell's equations, one deduces that the flow of energy is given by the Vecteur of Poynting

\ vec {\ pi} = \ frac {1} {\ mu_ {0}} \ cdot \ vec {E} \ wedge \ vec {B} .
By electromagnetic Wave, one understands the waves radio, Radar, Micro-onde S, the Lumière, the x-rays and gamma.

Work of max Planck on the black Body and of Heinrich Hertz on the photoelectric Effet showed that the energy exchanges were done per defined quantities, or quanta . Albert Einstein, in 1905, introduced the concept of Photon, electromagnetic Quantum of energy. The Quantum of exchanged energy (or “energy of the photon”) depends on the wavelength, according to the formula of Planck:

E = H .ν
where ν is the frequency of the wave (expressed in hertz Hz) and H is the Planck's constant. For any wave, one can write
λ = C /ν
where λ is the Wavelength (in meter m) and C is speed of light.

The wavelengths visible with the human eye lie between 380 Nm and 780 Nm (in lower part Ultraviolet above Infrarouge).

One can recover this electromagnetic energy thanks to the photovoltaic panels (purple radiation and ultra-violet) or by the solar-fired heaters (infra-red). It is also this energy which is recovered by the plants for the Photosynthèse, and by the photographic film. The Photon S can cause Ion isations of the Atome S, the excited atoms then yield their energy which can be recovered in another form (for example electricity, photosynthesis), or that modifies the structure of the matter which “is thus printed” (for example photographhic film. The photons can also yield their energy by causing the agitation of the atoms of the matter (for example water-heater, furnace microwave). In the case of the Hertzian waves (radio, television, cellphones), electromagnetic energy causes the circulation of a Electric current in the antenna which is transformed into its or in image.

Constants

  • Constant of Planck: H = 6,626  068  76 × 10-34 J.s (experimental value, precision 5,2.10-41  J.s)

  • electric Permittivity of the vacuum: ε0 = 8,854  187  817… × 10-12 F.m-1 (exact value)
  • magnetic Permeability of the vacuum: μ0 = 4π × 10-7 = 12,566  370  614… × 10-7 N.A-2 (exact value)
  • Speed of light in the vacuum: C = 299  792  458 m.s-1 (exact value)

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