Electric charges create electromagnetic fields. If a charge distribution changes with time because charges accelerate, the changing fields do not rearrange everywhere instantaneously. The disturbance propagates outward at the speed of light and can carry energy away as electromagnetic radiation.

A charge at rest produces a static electric field. A steadily moving charge has electric and magnetic fields associated with its motion, but sustained radiation is tied to acceleration: the motion of the charge must change.

Why acceleration matters

Imagine a charge whose velocity suddenly changes. Nearby electromagnetic fields respond first. More distant regions cannot know about the change until an electromagnetic disturbance has had time to reach them.

The changing configuration therefore separates into a near field that remains strongly tied to the source and a propagating disturbance that can travel outward through space.

For a localized source, the radiative electric and magnetic field amplitudes far from the source fall approximately as

$$\boxed{E_{\rm rad},B_{\rm rad}\propto\frac1r}.$$

Because electromagnetic intensity is proportional to the square of field amplitude,

$$I\propto E^2,$$

radiation from a localized source spreads approximately as

$$\boxed{I\propto\frac1{r^2}}.$$

This is consistent with conservation of energy: spheres centered on the source have area $4\pi r^2$, so a fixed radiated power can cross larger spheres with correspondingly smaller intensity.

Oscillating charges

If a charge oscillates back and forth, it accelerates repeatedly and can emit a continuous electromagnetic wave.

An oscillating electric dipole is the simplest important example. Its radiation is directional: it is strongest in directions perpendicular to the oscillation axis and vanishes along the axis in the ideal dipole model.

The radiation frequency follows the oscillation frequency of the source. Driving charges sinusoidally at radio frequencies can therefore create radio waves; driving charge distributions at much higher frequencies can produce other parts of the electromagnetic spectrum.

Antennas

A transmitting antenna uses an alternating current to accelerate charges in a conductor. The time-varying charge and current distribution creates electromagnetic fields that detach into outward-propagating radiation.

A receiving antenna reverses the connection: an incoming electromagnetic wave drives charges in the conductor and produces a measurable alternating voltage or current.

Energy must come from the source

Radiated energy is not free. A source that continuously radiates must receive energy from somewhere—for example, from an electrical transmitter driving an antenna. That supplied energy flows outward in the electromagnetic field.

The detailed strength and angular pattern of radiation depend on the geometry and time dependence of the source. More advanced electromagnetic theory computes those fields using retarded potentials and multipole expansions. The essential introductory principle is simpler:

$$\boxed{\text{accelerating charge distributions can launch electromagnetic waves that carry energy away}.}$$