Magnetism

Magnetism is a force that acts at a distance. A magnet has two poles: a north pole (N) and a south pole (S). Like poles repel (push apart). Unlike poles attract (pull together).

Magnetic and Non-magnetic Materials

Magnetic materials are attracted by magnets and can be magnetised. Common examples: iron, steel, nickel, cobalt. Non-magnetic materials (like wood, plastic, copper, aluminium) are not attracted and cannot be magnetised.

Magnetised vs Unmagnetised

In a magnetised piece of iron, many tiny magnetic regions (domains) point the same way, giving it N and S poles. In an unmagnetised piece, domains point in different directions, so their effects cancel out.

Induced Magnetism

When a magnet is brought near a magnetic material (like an iron paperclip), the material becomes a temporary magnet. The end nearest the magnet becomes the opposite pole, so it is attracted. This is why magnets always attract magnetic materials, even though two magnets can repel.

Temporary and Permanent Magnets

• Temporary magnets (soft iron): Easy to magnetise, easy to lose magnetism; useful for electromagnets and relays.
• Permanent magnets (steel): Harder to magnetise, keep magnetism well; useful for fridge magnets and door latches.

Magnetic Fields

A magnetic field is a region where a magnetic pole experiences a force. The direction of the field at a point is the direction of the force on a north pole at that point.

Field lines show the shape and strength of the field. Around a bar magnet they form closed loops from N to S outside the magnet. Lines never cross. Where lines are close together, the field is stronger (strongest near the poles).

Showing Field Patterns

• Iron filings: Sprinkle on paper over a magnet and tap gently. Filings line up along field lines.
• Plotting compass: Place a small compass near the magnet, mark where the needle points, move it step by step to trace lines from N to S.

Using a Compass

A compass needle is a tiny bar magnet. It lines up with the Earth’s magnetic field, so it points approximately towards the Earth’s magnetic north.

Everyday Uses

• Permanent magnets: fridge doors, magnetic cupboard catches, bicycle speed sensors, compasses.
• Electromagnets (temporary): cranes to lift scrap iron, electric bells, relays, speakers.

Common Misconceptions

• Cutting a magnet gives two smaller magnets, each with N and S. You cannot get a single pole by cutting.
• A magnet and a piece of iron never repel; attraction occurs due to induced magnetism.
• Field lines are a model; they are not physical threads.