Magnetism and Magnetic Fields

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CognitoMagnets and magnetic fields

Key facts & equations

Types of magnet

Permanent magnet: creates its own magnetic field. Made of hard magnetic materials (e.g. steel). Retains magnetism.
Temporary/induced magnet: magnetised by an external field; loses magnetism when field removed. Made of soft magnetic materials (e.g. iron).
Electromagnet: coil of wire carrying current. Field strength controlled by current. Can be switched off.
Magnetic poles: North and South. Like poles repel; unlike poles attract.
Only force between magnets that attracts AND repels — distinguishes from gravity (always attractive).

Magnetic field patterns

Field lines show direction a free North pole would move.
Lines go from North to South outside the magnet.
Closer lines = stronger field.
Bar magnet: field lines form closed loops, emerging from N and entering S.
Between two opposite poles: uniform field between them (parallel lines).
Two like poles: field lines curve away from each other, neutral point between.

Compasses and the Earth's field

Compass: freely suspended bar magnet. North-seeking pole aligns with magnetic field direction.
Earth has a magnetic field due to convection currents in the liquid iron outer core.
Geographic North Pole is actually a magnetic South Pole (attracts compass North-seeking pole).
Earth's field acts like a giant bar magnet (though the axis is tilted ~11° from rotation axis).
Magnetic declination: angle between true north and magnetic north.

Induced magnetism

Iron and steel become temporarily magnetised when placed near a magnet.
The near end is always attracted (opposite pole induced at near end).
Soft iron: easily magnetised and demagnetised. Used in transformers, electromagnets.
Hard steel: harder to magnetise and demagnetise. Used in permanent magnets.
Stroking with a magnet: aligns magnetic domains in the material.

Exam questions — 4 questions · 10 marks · AQA 8463 style

1 markMagnetic attraction/repulsionAQA 8463 style

Two bar magnets are brought near each other and repel. What does this tell us about their poles?

A Both are South poles facing each other

B Like poles are facing each other (both North or both South)

C Both magnets are permanent magnets

D One magnet is stronger than the other

3 marksDescribe field patternAQA 8463 style

Describe the magnetic field pattern around a bar magnet, including the direction of field lines and where the field is strongest. (3 marks)

Hint: Cover: where lines go (N to S outside). Density of lines (closer = stronger). Strongest at poles.

+30 XP

1 markSoft vs hard magnetic materialAQA 8463 style

Iron is described as a "soft" magnetic material. What does this mean?

A Iron is physically softer than other magnetic metals

B Iron is easily magnetised and easily demagnetised

C Iron produces only a weak magnetic field

D Iron is attracted to other soft magnetic materials only

3 marksInduced magnetismAQA 8463 style

A paper clip is held near (but not touching) a bar magnet. Explain why it is attracted to the magnet, and why the attraction decreases when the magnet is removed. (3 marks)