Electromagnetism Basics: Why Electricity and Magnetism Are the Same Thing

Electricity and magnetism look like two separate topics. They're the same phenomenon seen from different angles. Once that clicks, half of A Level physics gets easier.

The four big ideas

1. A static charge creates an electric field (Coulomb's law)
2. A moving charge creates a magnetic field (right-hand rule)
3. A changing magnetic field creates an electric field (Faraday's law)
4. A changing electric field creates a magnetic field (Maxwell's correction to Ampere)

Idea 3 is how generators work. Idea 4 is how light waves propagate.

The right-hand rule

For a current-carrying wire: point your right thumb in the direction of conventional current. Your fingers curl in the direction of the magnetic field.

For a force on a charge in a magnetic field: F = qv × B. Use the right-hand rule for the cross product.

Faraday's law in one sentence

The electromotive force (EMF) induced in a loop equals the negative of the rate of change of magnetic flux through the loop.

EMF = -dΦ/dt

This explains how power stations work, how transformers work, how dynamo lights on bikes work.

Lenz's law

The minus sign in Faraday's law has a name: Lenz's law. The induced current opposes the change that caused it. If the flux is increasing, the induced current creates a field opposing the increase. If decreasing, opposing the decrease.

This is conservation of energy in disguise.

What questions ask

• Find the magnetic field at a point (use Biot-Savart for a wire, B = μ₀I / (2πr))
• Find the force on a current-carrying wire (F = BIL)
• Find the EMF in a coil rotating in a field (E = NBA ω sin(ωt))

Common pitfalls

• Wrong direction from the right-hand rule (always use right hand, not left)
• Forgetting the minus sign in Faraday's law
• Mixing up the field of a current loop with the field of a single wire

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