Question

Discuss the source of electromagnetic waves.

Answer 1

Any stationary source charge produces only an electric field. When the charge moves with uniform velocity, it produces a steady current which gives rise to a magnetic field (not time-dependent, only space-dependent) around the conductor in which charge flows.

If the charged particle accelerates, in addition to the electric field it also produces a magnetic field. Both electric and magnetic fields are time-varying fields. Since the electromagnetic waves are transverse waves, the direction of propagation of electromagnetic waves is perpendicular to the plane containing electric and magnetic field vectors.

Any oscillatory motion is also an accelerating motion, so, when the charge oscillates (oscillating molecular dipole) about their mean position, it produces electromagnetic waves.

Suppose the electromagnetic field in free space propagates along z-direction, and if the electric field vector points along y-axis then the magnetic field vector will be mutually perpendicular to both electric field and the propagation vector direction,
which means
E_y = E₀ sin (kz – ωt)
B_x = B₀ sin (kz – ωt)
Where, E₀ and B₀ are amplitude of the oscillating electric and magnetic field, k is a wave number, ω is the angular frequency of the wave and k^ (unit vector, here it is called propagation vector) denotes the direction of propagation of electromagnetic wave.

Note that both electric field and magnetic field oscillate with a frequency (frequency of the electromagnetic wave) which is equal to the frequency of the source (here, the oscillating charge is the source for the production of electromagnetic waves). In free space or in a vacuum, the ratio between E₀ and B₀ is equal to the speed of the electromagnetic wave, which is equal to the speed of light c.

c = `"E"_0/"B"_0`

In any medium, the ratio of E₀ and B₀ is equal to the speed of electromagnetic wave in that medium, mathematically, it can be written as

v = `"E"_0/"B"_0` < c

Further, the energy of electromagnetic waves comes from the energy of the oscillating charge.