电磁波强度

When a capacitor is connected to an alternating current, the direction of the electric field between the plates changes with time, which induces a varying magnetic field.

The energy flow given by the Poynting vector is parallel to the plate. It varies rapidly with time.

Taking the time average of the Poynting vector gives the wave's intensity.

Consider a plane electromagnetic wave with the electric field oscillating along the y-axis and the magnetic field along the z-axis. Then, the Poynting vector points in the positive x-direction.

By replacing the fields with wave functions, the expression for the Poynting vector can be rewritten.

As the fields are mutually perpendicular, the Poynting vector magnitude equals the product of the peak amplitudes of the fields and a square of cosine term. By integrating the Poynting vector magnitude over a wave period, the expression for the intensity can be obtained.

Using the relationship between fields and the speed of light, the equivalent expressions for intensity can be defined in terms of the magnitudes of the electric and magnetic fields alone.