Magnet Mastery
Poles, forces and fields: learn why like poles repel, tell a permanent magnet from an induced one, name the magnetic materials, and read the field lines around a bar magnet.
Work through it, step by step
Work through it free and interactively, with each step checked before the next.
Start revising freeWhat you'll cover
Magnet Mastery
Every magnet has two poles, a north and a south, and they exert a non-contact force on each other and on magnetic materials. Along the way this module answers a question most people have never thought to ask: a fridge magnet sticks to the door whichever way round you turn it, yet two bar magnets will happily shove each other apart. Why the difference?
The words for it
Six terms. The rule for the first is short: like poles repel, unlike poles attract, and the force acts without anything touching:
Why does it always stick?
A bar magnet picks up a steel paperclip whichever of its poles you offer. Two bar magnets, though, sometimes attract and sometimes repel. What explains the difference?
- The paperclip is not already a magnet. It becomes an induced magnet, and the pole induced in the end nearest the magnet is always the opposite one, so the force is always attraction
- The paperclip is too weak a magnet to push the bar magnet away
- Steel is attracted to magnets by a different force altogether, which has no direction
- The paperclip has both a north and a south pole at each end at once
Permanent and induced
You have just worked out the last row. It is the one examiners ask about most:
Permanent or induced?
A magnet that produces its own field all the time is a _____ magnet, and it can attract or repel. A piece of iron that is magnetic only while it sits in a field is an _____ magnet, and it is always _____. Only a few materials behave this way at all: iron, steel, cobalt and _____.
Label the magnet
This bar magnet is surrounded by its magnetic field. Drag each label to the right place: the red end, the blue end, one of the curved lines, and the arrow showing its direction.
Reading the field
Field lines are a drawing convention, and every part of the convention carries information: • They run from north to south on the outside of the magnet. • Where they are closer together, the field is stronger. So the field is strongest at the poles, where they crowd in. • They never cross, because the field at any point has one direction, and a plotting compass placed there lines itself up along it. That last point is how the lines are found in the first place: put a small compass down, mark where its needle points, move it along and repeat.
Where is the field strongest?
On this bar magnet, tap the NORTH pole (the red end): one of the places where the field lines are packed closest and the field is strongest.
The biggest magnet of all
A compass needle is a small permanent magnet, free to turn. Left alone anywhere on Earth it settles pointing roughly north, and it has been doing so for navigators for a thousand years. It does that because the Earth itself has a magnetic field, and the needle lines up along it just as it would along the field of a bar magnet on a desk. The field is thought to come from the movement of molten iron in the Earth's core, so the compass is doing something rather remarkable: it is evidence, available to anyone, that the inside of the planet is magnetic and moving. There is a catch buried in it, though, and the next question is about that.
North seeks north?
The end of a compass needle that points towards the geographic North Pole is the needle's NORTH pole. Given that unlike poles attract, what must the Earth's magnetic pole up there actually be?
- A magnetic SOUTH pole, because it is attracting the needle's north pole
- A magnetic north pole, since it is at the geographic North Pole
- Neither: the Earth has no poles, only a general field
- Both, which is why the needle can settle in that direction at all
Why that material?
- Steel, chosen for a permanent bar magnet
- Iron, chosen for the core of an electromagnet
- Copper, chosen for the wire wound round that core
- Plastic, chosen for the casing around the whole thing
- It holds on to its magnetism once magnetised, which is exactly what a permanent magnet needs
- It magnetises strongly but lets go again as soon as the field is removed, so the device can be switched off
- It is not magnetic at all, but it is an excellent conductor
- It is neither magnetic nor conducting, which is precisely why it is safe to hold
Which are true?
Select ALL THREE statements that are TRUE.
- An induced magnet is always attracted and never repelled, because the pole induced nearest the magnet is always the opposite one
- A compass needle pointing north tells you that the Earth's magnetic pole in the far north is a magnetic south pole
- Field lines drawn closer together mean a stronger field, which is why they crowd at the poles
- Copper is attracted to a magnet, because it is a metal
- A permanent magnet loses its field once it is taken away from another magnet's field
- Outside a magnet, field lines run from south to north
The magnet rules
Like poles _____ and unlike poles attract, without anything touching. A permanent magnet produces its own field, but an _____ magnet is only magnetic while in a field, and is always attracted because the pole induced nearest is the _____ one. Magnetic field lines run from north to _____ outside the magnet, and crowd together where the field is strongest.
Explain the paperclip
Exam practice. In about 50 words, explain why a bar magnet attracts a steel paperclip whichever pole is held towards it, when two bar magnets can repel each other. Include:
- what happens inside the paperclip when the magnet is brought near
- which pole is induced in the end nearest the magnet, and why that is not a coincidence
- what happens to the paperclip once the magnet is taken away