Electromagnetism

Key principles and components

At its core are a few widely used ideas and equations: Coulomb's law governs the force between stationary charges; the magnetic force acts on moving charges; and the Lorentz force law combines both effects to give the total force on a charged particle. Electric and magnetic fields are vector fields that exist in space and mediate interactions. Maxwell's equations unify these elements into a consistent set of relations that also predict electromagnetic waves — oscillating fields that travel through space and include visible light.

History and development

Understanding of electricity and magnetism developed over centuries, from experiments with static charge and compasses to the 19th-century synthesis by scientists such as Ørsted, Ampère, Faraday and Maxwell. Ørsted first noticed that electric currents affect magnets; Faraday discovered electromagnetic induction; Maxwell combined experimental laws into a unified mathematical framework; later experiments by Hertz confirmed the existence of propagating electromagnetic waves. In the 20th century, classical electromagnetism was extended by quantum electrodynamics, a quantum field theory describing electromagnetic interaction at small scales.

Applications and examples

Practical uses of electromagnetism are ubiquitous: electric motors and generators convert between electrical and mechanical energy; antennas transmit and receive radio, television and cellular signals; transformers and power lines distribute electricity; magnetic resonance imaging (MRI) uses magnetic fields and radio waves for medical imaging; and everyday devices such as sensors, coils and capacitors rely directly on electromagnetic principles. Lightning and auroras are natural manifestations of electromagnetic processes in the atmosphere.

Distinctions and notable facts

Electrostatics studies stationary charges, while electrodynamics treats time-varying fields and currents. Magnetic effects arise only from moving charges or changing electric fields. Light and other electromagnetic radiation form a continuous spectrum from radio waves to gamma rays. On the microscopic scale, the quantum theory of electromagnetism describes how particles such as electrons and protons interact by exchanging photons, the force carriers of the electromagnetic field.

• Core equations: Maxwell's equations and the Lorentz force law.
• Classical vs quantum: classical theory works for many macroscopic applications; quantum electrodynamics is needed for microscopic precision.
• Everyday impact: power, communications, medical imaging, and electric machines.