How do magnets behave, and what does a magnetic field look like?
Describe the properties of magnets, magnetic materials, and the field around a bar magnet
Describe magnetic poles, attraction and repulsion, magnetic and non-magnetic materials, and the shape of the magnetic field around a bar magnet at N(A)-Level.
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What this dot point is asking
SEAB wants you to describe the properties of magnets, to tell magnetic materials from non-magnetic ones, and to describe the shape and direction of the magnetic field around a bar magnet. The big idea is that a magnet has two poles, that like poles repel and unlike poles attract, and that the space around a magnet contains an invisible field that we can map.
The answer
Magnetic poles
Every magnet has two poles, a north pole and a south pole. The poles are where the magnetism is strongest. If a magnet is free to turn, its north pole points towards the Earth's geographic north, which is how a compass works.
Attraction and repulsion
The basic rule of magnetism is:
- Like poles repel (north and north, or south and south push apart).
- Unlike poles attract (north and south pull together).
A magnet will also attract an unmagnetised piece of a magnetic material, such as an iron nail, because the magnet makes the nail slightly magnetic.
Magnetic and non-magnetic materials
Only a few materials are magnetic (attracted to a magnet): iron, steel, nickel and cobalt. Most materials, including copper, aluminium, plastic, wood and glass, are non-magnetic and are not attracted at all.
This lets us separate magnetic materials from a mixture, for example using a magnet to pull steel cans out of other rubbish.
The magnetic field
The space around a magnet where it can affect a magnetic material or another magnet is called its magnetic field. We picture the field using field lines:
- Field lines come out of the north pole and go into the south pole (outside the magnet).
- The arrow on a field line shows the direction a free north pole would be pushed.
- Where the lines are close together the field is strong; where they are spread out it is weak. The field is strongest at the poles.
Plotting the field
You can map the field with a small plotting compass: place it near a pole, mark the ends of the needle, move it along step by step, and join the dots to trace a field line. Iron filings sprinkled around a magnet also line up along the field lines and show the pattern quickly.
Examples in context
Example 1. The compass. A compass needle is a tiny magnet free to turn. It lines up with the Earth's magnetic field, so its north pole points roughly towards geographic north. Sailors and hikers have used this for centuries to find direction, because the Earth itself behaves like a giant bar magnet.
Example 2. Recycling steel. At a scrapyard, a large electromagnet on a crane lifts steel and iron objects out of mixed scrap, leaving aluminium, copper and plastic behind. This works because only the magnetic materials are attracted, allowing them to be sorted quickly and cheaply.
Try this
Cue. State what happens when the south pole of one magnet is brought near the south pole of another. [1 mark] They repel, because like poles repel.
Cue. Name two magnetic and two non-magnetic materials. [2 marks] Magnetic: iron and steel (or nickel, cobalt); non-magnetic: copper and plastic (or aluminium, wood, glass).
Cue. State the direction of the magnetic field lines outside a bar magnet and where the field is strongest. [2 marks] From the north pole to the south pole, strongest at the poles where the lines are closest together.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksTwo bar magnets are brought close together. (a) State what happens when two north poles are brought together. (b) State what happens when a north pole and a south pole are brought together. (c) Name two materials that are attracted to a magnet.Show worked answer →
(a) Two north poles repel each other (like poles repel).
(b) A north pole and a south pole attract each other (unlike poles attract).
(c) Any two magnetic materials, for example iron and steel (nickel and cobalt are also acceptable).
What markers reward: like poles repelling, unlike poles attracting, and two correct magnetic materials.
Original4 marks(a) Describe how to plot the magnetic field around a bar magnet using a small compass. (b) State the direction in which a field line points. (c) State where the field is strongest.Show worked answer →
(a) Place the magnet on paper. Put a small plotting compass near one pole, mark dots at each end of the needle, move the compass so it lines up with the last dot, and repeat. Join the dots to draw a field line. Repeat from different starting points.
(b) A field line points in the direction a north pole would be pushed, that is from the north pole of the magnet round to its south pole (outside the magnet).
(c) The field is strongest at the poles, where the field lines are closest together.
What markers reward: the compass plotting method, field lines running from north to south outside the magnet, and the field being strongest at the poles.
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