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Electromagnetism: Fields, Waves & Theory

Both electric and magnetic interactions are elements of a single phenomenon called electromagnetism. There are four fundamental forces: the strong force, the weak force, gravitation and the electromagnetic force. The study of electromagnetism deals with how electrically charged particles interact with each other and with magnetic fields.

Electromagnetic fields

An electromagnetic field is generated by electrically charged objects. The field influences the behavior of other charged objects and extends infinitely in space. Stationary electric charges produce an electric field, while moving charges (electric currents) produce a magnetic field. An electromagnetic field consists of a combination of an electric field and a magnetic field.

This diagram shows the entire spectrum of electromagnetic waves.
This diagram shows the entire spectrum of electromagnetic waves. The scale at the bottom indicates representative objects that are equivalent to the wavelength scale. The atmospheric opacity determines what radiation reaches the Earth's surface.
Credit: UC Regents.

Electromagnetic radiation (waves)

Charged particles emit or absorb energy in a wavelike form known as electromagnetic radiation. This radiation consists of two components: an electric field and a magnetic field. These oscillate at right angles to each other and to the direction in which the wave is propagating.

EM radiation travels at the speed of light, in a vacuum. EM radiation is part of the electromagnetic field that is far enough away from its source (a moving electrical charge) that the charge doesn't absorb the radiation. EM radiation draws energy away from its source, and also has momentum. The basic unit of EM radiation is the photon, a discrete packet of light. Light is a form of EM radiation, and can behave as either a particle or a wave.

Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.
Electromagnetic waves are formed when an electric field (shown in red arrows) couples with a magnetic field (shown in blue arrows). Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.
Credit: NOAA.

Electromagnetic theory

James Clerk Maxwell developed a unified theory of electromagnetism in 1873. There are four main electromagnetic interactions:

The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them.

Magnetic poles come in pairs that attract and repel each other much as electric charges do.

An electric current in a wire produces a magnetic field whose direction depends on the direction of the current.

A moving electric field produces a magnetic field, and vice versa.

Maxwell also developed a set of formulas, called Maxwell's equations, to describe these phenomena.

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