Magnetism

Overview

Magnets produce magnetic fields that exert forces on other magnets and magnetic materials. Every magnet has two poles (north and south) that follow predictable rules of attraction and repulsion. Understanding magnetic fields, field lines, and the difference between permanent and induced magnets is fundamental to electromagnetism, motors, and generators.

Magnetic Poles

Every magnet has two poles:

• **North pole (N)**: North-seeking pole
• **South pole (S)**: South-seeking pole

Rules of magnetic poles:

• **Like poles repel**: N-N repel, S-S repel
• **Unlike poles attract**: N-S attract

The force between poles decreases with distance (inverse square law - though not required for IGCSE detail).

Pole naming: If a bar magnet is suspended freely, the north pole points toward Earth's geographic North (because Earth's magnetic North is actually a south magnetic pole).

Magnetic Materials

Magnetic materials: Materials attracted to magnets

• **Iron** - easily magnetized, easily demagnetized
• **Steel** - harder to magnetize, retains magnetism longer
• **Cobalt**
• **Nickel**

Non-magnetic materials: Not affected by magnets

• Copper, aluminum, brass, wood, plastic, paper, glass

Magnetic Fields

Magnetic field: The region around a magnet where a magnetic force is experienced

Properties:

• Invisible but can be detected
• Strongest near the poles
• Extends in all directions around magnet
• Weaker further from magnet

Field direction: From north pole to south pole (by convention)

Magnetic Field Lines

Magnetic field lines (lines of force) show:

• **Direction** of magnetic field
• **Strength** of magnetic field

Rules for field lines:

1. Point from **North to South** outside the magnet
2. Never cross each other
3. **Closer together** = stronger field
4. **Further apart** = weaker field
5. Form closed loops (continue through magnet from S to N)

Patterns:

Bar magnet:

• Field lines curve from N pole to S pole
• Densest (closest together) at poles
• Field strongest at poles

Two magnets - attracting (N-S):

• Field lines join between unlike poles
• Concentrated between magnets

Two magnets - repelling (N-N or S-S):

• Field lines push away from each other
• Neutral point between poles (no field)

Uniform field:

• Between two flat magnets (N facing S)
• Parallel, evenly spaced field lines
• Same field strength everywhere

Plotting Magnetic Fields

Method 1: Iron filings

1. Place magnet on paper
2. Sprinkle iron filings around magnet
3. Tap paper gently
4. Iron filings align with field lines, showing pattern

Method 2: Plotting compass

1. Place magnet on paper, draw outline
2. Place small compass near N pole
3. Mark direction compass needle points
4. Move compass so S pole is where previous N pole was
5. Repeat to trace field line
6. Draw multiple field lines starting from different points

Induced Magnetism

Induced magnetism: When a magnetic material becomes a magnet when placed in a magnetic field

Process:

• Magnetic material (e.g., iron nail) brought near permanent magnet
• Domains in iron align with magnet's field
• Iron becomes temporarily magnetized
• Always attracted to permanent magnet (induced pole opposite to nearest pole)
• Loses magnetism when magnet removed

Example: Iron nail near bar magnet's N pole:

• Nail's near end becomes S pole (attracts)
• Nail's far end becomes N pole
• When magnet removed, nail loses magnetism (if soft iron)

Permanent vs Temporary Magnets

Permanent magnets:

• Stay magnetized
• Made from hard magnetic materials (steel, neodymium)
• Difficult to magnetize
• Difficult to demagnetize
• Retain magnetism for long periods
• Uses: Compasses, fridge magnets, speakers

Temporary (induced) magnets:

• Easily magnetized
• Easily demagnetized
• Made from soft magnetic materials (soft iron)
• Lose magnetism quickly when removed from field
• Uses: Electromagnet cores, transformer cores

Magnetization Methods

How to magnetize:

1. Stroking method:

• Stroke steel bar with permanent magnet
• Always stroke in same direction
• Lift magnet away between strokes
• Repeat many times
• End near starting point becomes opposite pole to magnet's pole used

2. Electrical method:

• Place steel bar inside solenoid (coil of wire)
• Pass DC current through solenoid
• Magnetic field magnetizes bar
• More current/turns = stronger magnetization

Demagnetization Methods

How to demagnetize:

1. Heating:

• Heat magnet to high temperature (above Curie point)
• Vibration disrupts domain alignment
• Magnet loses magnetism

2. Hammering:

• Hit magnet repeatedly while pointing east-west
• Vibration randomizes domains
• Loses magnetism

3. AC electrical method:

• Place magnet in solenoid
• Pass decreasing alternating current through solenoid
• Alternating field randomizes domains
• Magnet demagnetized

Magnetic Domains

Domain theory explains magnetism:

• Magnetic materials contain tiny regions called **domains**
• Each domain is a group of atoms with aligned magnetic fields
• In **unmagnetized** material: Domains point in random directions, cancel out
• In **magnetized** material: Domains aligned in same direction, creating overall magnetic field
• **Hard materials** (steel): Domains difficult to align/realign
• **Soft materials** (soft iron): Domains easily align/realign

Uses of Magnets

Compasses:

• Freely suspended magnet aligns with Earth's magnetic field
• Points North-South
• Used for navigation

Magnetic storage:

• Hard drives, magnetic tape
• Data stored as magnetized regions

Sorting:

• Separate magnetic materials (iron, steel) from non-magnetic (aluminum, copper)
• Recycling plants

Maglev trains:

• Magnetic levitation
• Reduces friction

Loudspeakers:

• Permanent magnet + electromagnet create sound

Credit/debit cards:

• Magnetic stripe stores information

Earth's Magnetic Field

• Earth acts like giant bar magnet
• Magnetic field extends far into space
• Geographic North Pole is near magnetic South pole (confusing!)
• Compasses align with Earth's field
• Protects Earth from solar wind

Exam Tips

• Always state field direction: N to S outside magnet
• Draw field lines with arrows, never crossing
• Remember like poles repel, unlike attract (easy to confuse in exam stress)
• Induced magnetism always causes attraction (induced pole is always opposite)
• Distinguish permanent (steel, hard to magnetize/demagnetize) from temporary (soft iron, easy both ways)
• Field strongest where lines closest together (at poles)
• Use iron filings to show pattern, plotting compass to show direction
• Explain demagnetization disrupts domain alignment
• For questions about attraction vs repulsion, identify which poles are facing