Magnetism and Electromagnetic Induction

Moving charge is the source of magnetism

Every magnetic field you’ve ever encountered — from a refrigerator magnet to Earth’s own field — ultimately comes from moving electric charge. In a bar magnet, it’s the coordinated motion of electrons within atoms; in an electromagnet, it’s current flowing through a coil of wire.

A moving charge creates a magnetic field around it, and that field then exerts a force on other moving charges nearby — magnetism, unlike gravity or the electric force, only acts on charges that are already in motion.

The reverse trick: making electricity from magnetism

Here’s where it gets genuinely useful: if a moving charge can create a magnetic field, a changing magnetic field can push charges into motion — generating an electric current. This is electromagnetic induction, discovered by Michael Faraday, and it’s arguably one of the most economically important discoveries in the history of physics.

Move a magnet through a loop of wire (or move the wire through a magnetic field), and a current appears in the wire, with no battery required. The faster the magnetic field changes, the stronger the induced current.

Why this one idea powers civilization, almost literally

Nearly all electricity generated worldwide — from coal plants, nuclear plants, wind turbines, hydroelectric dams — works the same fundamental way: something spins a magnet (or coil of wire) to continuously change a magnetic field, inducing current. Solar panels are the major exception, working through a different (photoelectric) effect.

Why this matters

Electricity and magnetism turning out to be two faces of the same underlying force — called electromagnetism — was one of the great unifications in physics history, eventually shown mathematically by James Clerk Maxwell in the 1860s. That same unification is also what predicts that light itself is an electromagnetic wave, tying together electricity, magnetism, and optics into a single framework.

Check your understanding

3 practice questions in the style of AP Physics. Pick an answer to see if you got it, plus a short explanation. Where numbers are needed, use g = 9.81 m/s². These questions were written for this site and are not College Board questions.

1. A bar magnet is pushed into a coil of wire connected to a sensitive meter. When does the meter show a current?

ANeverBOnly while the magnet is movingCAll the time the magnet is inside, even when it is at restDOnly when the magnet is pulled out

2. A charged particle moves through a magnetic field. The magnetic force on the particle

Aspeeds it up.Bslows it down.Cchanges its direction but not its speed.Dis always zero.

3. Which situation induces an emf (voltage) in a loop of wire?

AThe magnetic flux through the loop changes.BThe loop sits next to a strong magnet that isn't moving.CA steady current flows in a nearby wire.DThe loop is heated.

Want the real thing? The College Board posts freepast AP Physics 1 exam questions ↗.