Electromagnetism

(the study of electric charge, electric fields, and magnetic fields, and how they relate)

What is electromagnetism?

Electromagnetism is the branch of physics studying electric charge, electric fields, magnetic fields, and how they relate to each other. Electricity and magnetism were once studied as separate topics, until 19th-century physics showed they are two aspects of the same underlying phenomenon.

Electric charge and Coulomb's law

Electric charge comes in two kinds, positive and negative. Like charges repel, opposite charges attract. Coulomb's law gives the exact strength of the force between two charges: F = k(q1q2)/r², where q1 and q2 are the charge amounts, r is the distance between them, and k is a fixed constant. Doubling the distance between two charges reduces the force to a quarter of its original strength, since r is squared in the denominator.

Electric fields

Rather than thinking of charges reaching across empty space to pull or push each other, physics describes a charge as creating an electric field around itself — a region where any other charge placed there would feel a force. This field exists whether or not a second charge is actually present to feel it, and it lets physicists calculate the force on any charge placed at any point, once the field is known.

Basic circuits: Ohm's law

In an electrical circuit, voltage (V) drives current (I) through a resistance (R), related by Ohm's law: V = IR. A higher resistance, for the same voltage, produces less current flowing — exactly as a narrower pipe restricts more water flow for the same water pressure.

Magnetism

A magnetic field is created by moving electric charges — including the organized electron motion inside a permanent magnet, and the current flowing through a wire. A compass needle aligns with a magnetic field because it is itself a small magnet, responding to the surrounding field's direction. A current-carrying wire creates a magnetic field circling around it — this can be demonstrated directly by placing a compass near a wire carrying current and watching the needle deflect.

Why electromagnetism matters

Electromagnetism explains and enables an enormous range of technology: every electric motor, generator, radio signal, and electronic device relies on the relationship between electric and magnetic fields. It is one of the most thoroughly tested and confirmed areas of physics, underlying almost all modern electrical technology.