# Magnetism

> Poles, magnetic fields, Earth's own magnet, electromagnets and the motor effect.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/magnetism/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The lost hiker
Tom was hiking in the Blue Mountains when thick fog rolled in. He could not see the track. His phone had no signal. His battery was nearly flat.

Then he remembered the small compass his grandmother had clipped to his bag. He held it flat. The needle swung, wobbled, and settled pointing north. His map showed the car park was to the north-east. He walked carefully in that direction and found the road an hour later.

A tiny magnet, floating on a pin, had found its way by feeling the invisible magnetic field of the whole Earth.

**In short:** A magnet has a north and a south pole: like poles repel and unlike poles attract. Iron, nickel and cobalt are magnetic, and a current in a coil of wire makes an electromagnet that can be switched on and off.

## Magnets and poles

A **magnet** is an object that attracts certain metals and has a magnetic field around it.

- Every magnet has two ends called **poles**: a **north (N) pole** and a **south (S) pole**.
- **Like poles repel**: N pushes N away, S pushes S away.
- **Unlike poles attract**: N and S pull together.
- If you cut a magnet in half, you get **two smaller magnets**, each with its own N and S. You can never get a pole on its own.

**Magnetic materials** are attracted to magnets. The main ones are **iron, nickel and cobalt**, and steel (which is mostly iron). Copper, aluminium, gold, plastic and wood are **not** magnetic.

### Watch out: Not all metals are magnetic
A magnet will not pick up an aluminium can or a copper coin. Only iron, nickel, cobalt and their alloys are attracted.

## Magnetic fields

A **magnetic field** is the region around a magnet where a magnetic force acts. We draw it with **field lines**.

- Field lines go **from N to S** outside the magnet.
- Lines **never cross**.
- Where lines are **closer together**, the field is **stronger**. This is near the poles.

*Diagram: Bar magnet with curved field lines running from the north pole around to the south pole*
*Field lines leave the north pole and enter the south pole.*

You can see a field by sprinkling iron filings around a magnet, or by moving a small **plotting compass** around it.

## Earth is a giant magnet

Earth's core of molten iron makes Earth act like a huge bar magnet. A compass needle is a small magnet. Its north pole points towards Earth's **geographic North Pole**.

Since unlike poles attract, this means the magnetic pole near the geographic North Pole is actually a magnetic **south** pole. Earth's magnetic field also protects us by steering charged particles from the Sun away from the planet. Where some get through near the poles, they make the glowing **auroras**.

## Electromagnets

An **electric current** in a wire makes a magnetic field around it. Coil the wire around an iron nail and you have an **electromagnet**.

You can make an electromagnet **stronger** by:
1. Increasing the **current**
2. Adding **more turns** of wire to the coil
3. Using a **soft iron core** inside the coil

The big advantage: you can **switch it off**. Scrapyard cranes use electromagnets to lift cars and drop them. Electromagnets are also in electric bells, speakers, MRI machines and door locks.

### Key idea: Right-hand grip rule
Grip a wire with your right hand, thumb pointing in the direction of the current. Your fingers curl the way the magnetic field lines go around the wire.

**Mind map**

```text
Magnetism
  Poles
    North and south
    Like repel
    Unlike attract
  Materials
    Iron, nickel, cobalt
    Steel
    Not copper or aluminium
  Fields
    N to S
    Close lines = strong
    Compass, iron filings
  Earth
    Giant magnet
    Compass points north
    Auroras
  Electromagnets
    Current makes a field
    More current, more turns
    Iron core
    Can switch off
```

### Year 11: The motor effect
A wire carrying a current inside a magnetic field feels a **force**. This is the **motor effect**:

$$F = BIl\sin\theta$$

- $B$ = magnetic field strength (tesla, T)
- $I$ = current (A)
- $l$ = length of wire in the field (m)
- $\theta$ = angle between wire and field

The force is biggest when the wire is at 90° to the field, and zero when parallel. Use the **right-hand palm rule**: fingers along the field, thumb along the current, palm pushes in the direction of the force.

A DC motor is a coil in a magnetic field. Forces on opposite sides of the coil point in opposite directions, making it turn. A **split ring commutator** reverses the current every half turn so the coil keeps spinning the same way.

### Year 12: Electromagnetic induction
Moving a magnet into a coil, or changing the field through it, **induces** a voltage. Faraday's law:

$$\varepsilon = -N\frac{\Delta\Phi}{\Delta t}$$

where $\Phi = BA\cos\theta$ is the **magnetic flux**. The minus sign is **Lenz's law**: the induced current flows to oppose the change that caused it.

This is how generators make electricity and how **transformers** change voltages:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

## Quick summary

- Magnets have N and S poles. Like poles repel, unlike poles attract.
- Iron, nickel and cobalt are magnetic. Most other metals are not.
- Field lines go from N to S. Closer lines mean a stronger field.
- Earth behaves like a giant bar magnet, which is why compasses work.
- A current in a wire makes a magnetic field. A coil with an iron core makes a strong electromagnet that can be switched off.
- Electromagnets get stronger with more current, more turns and an iron core.

## Quiz

Try these questions from the chapter quiz.

1. Two north poles are placed near each other. They will:
   - A. Attract
   - B. Repel
   - C. Do nothing
   - D. Attract then repel
   - Answer: B. Repel. Like poles repel.

2. Which of these is a magnetic material?
   - A. Copper
   - B. Aluminium
   - C. Nickel
   - D. Plastic
   - Answer: C. Nickel. Iron, nickel and cobalt are the magnetic metals.

3. Magnetic field lines outside a bar magnet go:
   - A. From north to south
   - B. From south to north
   - C. Straight up
   - D. In circles around the middle
   - Answer: A. From north to south. By convention, field lines leave the N pole and enter the S pole.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/magnetism/#quiz