For traveling electromagnetic waves, the electric and magnetic fields E and B are in phase. In cgs,
and in MKS,
where
is the permeability of free space. The average flux is then
For standing EM waves, E and B are 90° out of phase. In cgs,
and in MKS,
For an electromagnetic wave represented in complex notation as
the following identities hold:
The electromagnetic wave equations follow from the telegraphy equations. In free space in cgs
and
In MKS
where is the permeability of free space and 
is the permittivity of free space, and
In a dielectric in cgs
so using
where c is the speed of light,
is the electric permittivity,
is the magnetic permeability, and n is the index of refraction gives
In MKS,
so using
where
is the dielectric constant and 
is the relative permeability gives
(1)
(2)
(3)
(4)
and in MKS,
(5)
(6)
(7)
where
is the permeability of free space. The average flux is then(8)
For standing EM waves, E and B are 90° out of phase. In cgs,
(9)
(10)
(11)
(12)
and in MKS,
(13)
(14)
(15)
(16)
For an electromagnetic wave represented in complex notation as
(17)
the following identities hold:
(18)
(19)
(20)
The electromagnetic wave equations follow from the telegraphy equations. In free space in cgs
(21)
(22)
and
(23)
(24)
In MKS
(25)
(26)
where
is the permeability of free space and is the permittivity of free space, and(27)
(28)
In a dielectric in cgs
(29)
(30)
so using
(31)
where c is the speed of light,
is the electric permittivity, is the magnetic permeability, and n is the index of refraction gives (32)
(33)
In MKS,
(34)
(35)
so using
(36)
where
is the dielectric constant and is the relative permeability gives (37)
(38)
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