Y9 Physics - Magnets and Magnetic fields

Permanent magnet (A magnet…all times…stay aligned)

A magnet that produces its own magnetic field at all times because its magnetic domains are aligned and stay aligned

Induced magnet

A material that becomes magnetic only when placed in an external magnetic field because its domains are temporarily forced to align

Magnetic domains

Tiny regions within a material where atoms act like small magnets, all pointing in the same direction within that region

Iron vs steel magnetism

Iron loses induced magnetism quickly because domains easily randomise, whereas steel keeps magnetism longer because its distorted alloy structure locks domains in place

Why alloys retain magnetism

Alloys like steel have different

Why induced magnets always attract

The external magnet creates an opposite pole at the nearest end, so the force is always attraction

Like poles

Repel because their magnetic fields push against each other

Unlike poles

Attract because their field lines connect smoothly from north to south

Non

contact force

Magnetic field

A region around a magnet where magnetic forces act

Field strength

The strength of a magnetic field, shown by how close the field lines are together

Field strength at poles

Strongest at the poles because field lines are closest together there

Field strength with distance

Decreases as distance increases because the field spreads out

Magnetic field lines

A model used to show the direction, strength, and shape of a magnetic field

Direction of field lines

Always from north to south outside the magnet

Why field lines go north to south

A north pole placed in the field would move in that direction, so this defines the convention

Field line spacing

Closer lines indicate a stronger magnetic field, wider spacing indicates a weaker field

Plotting a magnetic field

Using a compass to trace the direction of field lines step by step around a magnet

Why a compass works

The needle is a tiny magnet that aligns with the magnetic field direction at its position

Earth’s magnetic field

The Earth acts like a giant magnet with a magnetic field surrounding it

Why a compass points north

The compass’s north pole is attracted to the Earth’s magnetic south pole near the geographic North Pole

Earth’s magnetic poles

The geographic North Pole is actually a magnetic south pole

Dynamo effect

The process by which moving molten iron in Earth’s outer core generates a magnetic field

Bar magnet field pattern

Field lines leave the north pole, curve around, and enter the south pole, forming continuous loops

Two magnets (like poles)

Field lines push apart, showing repulsion

Two magnets (unlike poles)

Field lines join together, showing attraction

Field lines rule

Field lines never cross because this would mean two directions at once

Exam trap (field direction)

Field lines go from north to south outside the magnet, not the other way around

Exam trap (induced magnets)

Induced magnets cannot repel because they always form opposite poles

Exam trap (Earth magnetism)

Earth’s geographic north is a magnetic south

Exam trap (field lines meaning)

Field lines show direction, while spacing shows strength.