Electromagnetism

Electric and magnetic fields describe different aspects of the electromagnetic interaction. An electric field specifies the force that would act per unit charge on a test charge; electric charges are its sources. Magnetic fields act on moving charges and are associated with electric currents and magnets. In classical electromagnetism, the fields are related rather than independent: a changing magnetic field induces an electric field, while a changing electric field contributes to the production of a magnetic field. These relationships explain why electrical and magnetic effects often appear together in devices and natural phenomena.

Maxwell’s equations express how electric and magnetic fields are generated and how they change. James Clerk Maxwell synthesized earlier experimental laws and added an important term to the relationship between electric current and magnetic field. The resulting theory showed that coupled, changing fields can travel outward as waves. Heinrich Hertz later generated and detected radio waves, providing experimental evidence for Maxwell’s prediction. Unlike sound waves, electromagnetic waves do not require a material medium and can travel through a vacuum.

Electromagnetic radiation spans a continuous range of wavelengths and frequencies. Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays are regions of this spectrum, not separate kinds of force. The human eye detects only the visible portion. As wavelength becomes shorter, frequency and photon energy increase. Different parts of the spectrum interact with matter in different ways: some radiation passes through the atmosphere, while much of the higher-energy ultraviolet, X-ray and gamma radiation is absorbed. X-rays, gamma rays and some ultraviolet radiation are ionizing and can damage living cells, although controlled radiation also has useful applications such as cancer treatment.

Electromagnetism underlies much of modern technology. Electric motors convert electrical energy into motion, generators use electromagnetic induction to produce electric current, and transformers transfer electrical energy between circuits. Radio, television, mobile communications and satellite links use electromagnetic signals; optical instruments and astronomy use visible and other wavelengths to observe objects. The same framework also helps explain lightning, magnets and the behavior of charged particles. The broad reach of electromagnetism makes it essential to physics, engineering, communications and the study of the universe.