Magnetic poles and fields
Every magnet has two poles: north and south. Like poles repel; opposite poles attract — you've felt magnets shove each other away. Break a magnet in half and you don't get a lone north: you get two smaller complete magnets. Poles always come in pairs.
A magnetic field is the invisible region of influence around a magnet, drawn as field lines leaving the north pole and curving around to the south. Iron filings sprinkled on paper over a bar magnet trace these lines — one of the best five-minute physics demos there is. The field is strongest where the lines crowd together: at the poles.
- North–south attract; north–north and south–south repel
- Breaking a magnet gives two smaller magnets, never a lone pole
- Field lines run out of north and into south, outside the magnet
- Where lines crowd close, the field is strongest (the poles)
- Iron, nickel and cobalt (ferromagnetic) are the magnetic materials
Electromagnets: magnetism from electricity
In 1820, Hans Oersted noticed a compass needle twitch beside a current-carrying wire — electricity and magnetism are two faces of one thing. Wrap a wire into a coil and send current through it, and you get a magnetic field like a bar magnet's. Add an iron core and the field concentrates: that's an electromagnet.
Electromagnets beat permanent magnets because you can switch them on and off, reverse them, and tune their strength. Strength grows with more turns of wire, more current and a better core. Junkyard cranes, speakers, doorbells, MRI machines and maglev trains all run on that controllability.
- Electric current always creates a magnetic field around it
- Electromagnet = coil of wire + current, usually with an iron core
- More turns, more current, better core = stronger
- On/off and reversible — the superpower permanent magnets lack
For a science project, an electromagnet with 50 coils picks up 12 paper clips. You double the turns to 100, and it picks up about 24 — strength scales roughly with the number of turns, so coil count is the cheapest upgrade. Then bump the current with a second battery and watch the count climb again (and the wire warm up).
How motors work
A motor is the loop-the-loop child of electricity and magnetism. Put a current-carrying wire inside a magnetic field, and the field pushes the wire sideways — that's the motor effect. Bend the wire into a loop and the push twists one side up and the other down, spinning the loop.
The trick that keeps it spinning: just as the loop lines up with the field and would stop, the commutator — a split-ring contact — flips the current direction, which flips the push, and the loop keeps rotating. Add more loops and you get smoother, stronger rotation. Every fan, drill, EV wheel and fridge compressor is this idea industrialized.
- Current + magnetic field = sideways force on the wire
- Loop of current in a field = torque = rotation
- Commutator flips the current every half-turn to keep the spin going
- More loops = smoother, stronger rotation
- Same physics in fans, drills, hard drives and electric cars
Key concepts to memorize
🎯 Study tips for this topic
- Draw field lines from north to south on every magnet sketch — direction is half the credit.
- Remember the strength trio for electromagnets: turns, current, core.
- Trace the motor story out loud: push, spin, flip, repeat — until the commutator's job is obvious.
- Build the classic nail-and-wire electromagnet once; hands beat diagrams for retention.
- Pair this cluster with electricity-basics — Ohm's law plus the motor effect explains most everyday tech.
