Overview

Electromagnetism is the study of electric and magnetic fields. It unified two seemingly different forces (electricity and magnetism) into one, and revealed that light is just an electromagnetic wave.

Core Idea

The core idea is the Field. Instead of particles touching each other, they create a “field” in space that pushes and pulls other particles.

Formal Definition

Governed by Maxwell’s Equations:

  1. Gauss’s Law: Electric charges produce electric fields.
  2. Gauss’s Law for Magnetism: There are no magnetic monopoles (magnets always have N and S).
  3. Faraday’s Law: Changing magnetic fields create electric fields (induction).
  4. Ampere-Maxwell Law: Electric currents and changing electric fields create magnetic fields.

Intuition

  • Electricity: Static charges (like a balloon on hair).
  • Magnetism: Moving charges (currents).
  • Light: A self-propagating wave where the electric field creates the magnetic field, which creates the electric field… forever.

Examples

  • Motors: Turning electricity into motion (using magnetic fields).
  • Generators: Turning motion into electricity (induction).
  • Radio/WiFi: Invisible electromagnetic waves carrying information.

Common Misconceptions

  • Misconception: Electrons move at the speed of light in a wire.
    • Correction: They actually drift very slowly (mm/s). The signal (field) moves at the speed of light.
  • Misconception: North poles attract North poles.
    • Correction: Opposites attract. N attracts S.

Applications

  • Electronics: The foundation of the modern world (computers, phones, power grid).
  • Medicine: MRI (Magnetic Resonance Imaging).

Criticism and Limitations

  • Classical Limit: Like mechanics, classical electromagnetism breaks down at the quantum scale (photons).

Further Reading

  • Introduction to Electrodynamics by David J. Griffiths
  • A Student’s Guide to Maxwell’s Equations by Daniel Fleisch