This is a straightforward application of the fixed relationship between magnetic and electric field
amplitudes in electromagnetic waves, given by Equation 25.17.
The electric and magnetic field amplitudes of an electromagnetic wave are related as
The electric field amplitude of the electromagnetic wave is
Because the magnetic field amplitude is much larger than the earth’s magnetic field, we expected a large electric
This is a straightforward application of the relationship between the intensity of an electromagnetic
wave and the magnetic and electric field amplitudes. This is described by Equation 25.18. Note that once we determine the
amplitude of one field type, the amplitude of the other is given by Equation 25.17.
The intensity of the microwaves is related to the amplitude of the oscillating electric field by
amplitude of the oscillating magnetic field is related to the amplitude of the oscillating electric field by
The amplitude of the oscillating electric field is
The amplitude of the oscillating magnetic field is
These are reasonable values for the amplitude of the oscillating electric and magnetic fields of microwaves.
We can use Equation 25.18 to express the intensity in terms of the electric field amplitude.
In the expression
we have all variables except the unknown electric field amplitude.
Rearranging Equation 25.18, we have
This electric field is, of course, always changing direction, and has no fixed orientation. So it does not have a net
effect on electric charge.
A radio wave is an electromagnetic wave. The power or energy transported per second by the radio
This energy is carried uniformly in all directions. Equation 25.18 connects power of a wave
that impinges on an area A (defined as its intensity) with the wave’s electric field amplitude.
Recall that the intensity of a point source free to spread in three dimensions (over a spherical surface) is
We will combine this Equation 25.18.
Using the above expression, the light intensity is
Using Equation 25.18,
These values are reasonable, in that they are small enough that they would not upset the normal operation of circuits