Maxwell’s equations are:
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In lih materials with  we have: |
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Maxwell’s equations are linear equations and the principle of superposition holds.
,
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Poynting’s theorem,
is a statement of energy
conservation.
(SI units),
(Gaussian units)
is the energy density and
(SI units),
(Gaussian units)
is the energy flux in the electromagnetic field. We define the momentum density as
(in SI
and Gaussian units).
If in electrodynamics we choose the Lorentz gauge defined through
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| then f and each Cartesian component of
A satisfy the inhomogeneous wave equation |
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Choosing
is called
choosing the Coulomb gauge.