Magnetism and Electromagnetism

Magnets and magnetic fields

A magnet has two poles: north and south. The rule of attraction:

• Like poles repel (N–N, S–S); unlike poles attract (N–S).
• These are non-contact forces — they act at a distance through a magnetic field.

A magnetic field is the region around a magnet where a magnetic force acts. Field lines go from north to south outside the magnet, and are closest together where the field is strongest (at the poles). A compass needle lines up with the field (and points to Earth's magnetic north, showing Earth has its own field).

Permanent vs induced magnets

• Permanent magnets produce their own field all the time.
• Induced magnets become magnetic only when placed in a magnetic field (e.g. a paperclip near a magnet). They lose their magnetism when removed and are always attracted.
• Magnetic materials: iron, steel, cobalt, nickel.

Electromagnetism

When a current flows through a wire, it creates a magnetic field around the wire (circular field lines). Coiling the wire into a solenoid concentrates the field, making it like a bar magnet — much stronger inside the coil.

An electromagnet = a solenoid with an iron core. Its advantages: it can be switched on and off, and its strength changed by:

• increasing the current;
• adding more turns to the coil;
• adding an iron core.

Uses: scrapyard cranes, electric bells, relays, loudspeakers.

The motor effect

When a current-carrying wire is placed in a magnetic field, the field of the wire interacts with the external field, producing a force on the wire. This is the motor effect — the basis of electric motors.

• The force is greatest when the wire is at 90° to the field.
• Fleming's left-hand rule predicts the direction (thumb = force/motion, first finger = field, second finger = current).

force = magnetic flux density × current × length   (F = B × I × L)

Worked example

Give three ways to make an electromagnet stronger.

• Increase the current, add more turns to the coil, and add an iron core. ✓

Common mistakes

• Getting attraction/repulsion wrong — like poles repel, unlike attract.
• Forgetting field lines run north to south (outside the magnet).
• Saying an electromagnet can't be switched off — the big advantage is that it can.

Exam tips

• Learn how to strengthen an electromagnet (current, turns, iron core).
• Know the motor effect and that the force is largest at 90° to the field.
• Be able to describe drawing a magnetic field with a compass.

Key facts to remember

• Like poles repel, unlike attract; field lines go N→S, strongest at the poles.
• A current makes a magnetic field; a solenoid + iron core = electromagnet (can be switched on/off; strengthened by more current, more turns, iron core).
• Motor effect: a current-carrying wire in a magnetic field feels a force (F = BIL), greatest at 90°; direction from Fleming's left-hand rule.