Properties of Magnets
This topic covers the fundamental properties of magnets, including the forces between them, the concept of magnetic fields, and how different materials respond to being magnetised.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Like poles repel (North-North, South-South) and unlike poles attract (North-South).
- Magnetic fields are regions where magnetic forces are exerted. Field lines are used to represent them, with their direction defined as the path a free North pole would take (i.e., from North to South).
- The density of magnetic field lines indicates the strength of the field; closer lines mean a stronger field.
- Soft magnetic materials (like iron) are easily magnetised and demagnetised. They are used in temporary magnets, like electromagnets.
- Hard magnetic materials (like steel) are difficult to magnetise but retain their magnetism well. They are used for permanent magnets, such as compass needles.
- The only definitive test to prove an object is a permanent magnet is to show it can repel another permanent magnet. Attraction can occur with any unmagnetised magnetic material due to induction.
Diagram
› Why does this happen?
Why are some materials 'soft' and others 'hard'?
Imagine a magnetic material like iron is full of tiny 'magnetic domains', which are like miniature magnets. In an unmagnetised material, these domains point in random directions, cancelling each other out. When a strong magnet is brought nearby, it forces these domains to line up.
- A 'soft' magnetic material (like iron) is easy to magnetise, but its domains quickly go back to being random once the magnet is removed. This makes it ideal for temporary magnets, like electromagnets.
- A 'hard' magnetic material (like steel) is more difficult to magnetise, but its domains stay aligned after the magnet is removed. This is how permanent magnets are made.
Why is repulsion the only definite test for a magnet?
Attraction isn't a sure test because a magnet attracts any magnetic material, not just another magnet. This is due to 'induced magnetism', where a magnet makes a nearby magnetic object (like an iron nail) temporarily magnetic, always causing attraction. So, attraction only proves an object is made of a magnetic material, not that it's a permanent magnet.
Repulsion, on the other hand, can only happen between two like poles (e.g., a North pole pushing away another North pole). An unmagnetised object is never repelled. Therefore, if an object repels your magnet, it must be a magnet itself.
Definitions
- Magnetic Field
- A region in space around a magnet or a current-carrying wire where a magnetic force can be detected.
- Induced Magnetism
- The process where a material becomes a magnet when it is placed within a magnetic field. This effect is temporary for soft magnetic materials but more lasting for hard ones.
- Magnetic Poles
- The points on a magnet where the magnetic field is strongest, labelled North and South.
Worked example
An unmagnetised nail made of soft iron is held near the North pole of a strong bar magnet. A small, unmagnetised paperclip made of steel is then brought close to the tip of the nail. The bar magnet is then removed. Describe and explain the forces involved and what is observed at each stage.
- 1
Stage 1:
The strong magnet induces magnetism in the soft iron nail.
The end of the nail nearest the magnet's North pole becomes a South pole, and the other end becomes a North pole.
This results in a force of attraction between the magnet and the nail.
- 2
Stage 2:
The now-magnetised nail induces magnetism in the steel paperclip.
The end of the paperclip nearest the nail's tip (its North pole) becomes a South pole.
This creates a force of attraction, allowing the nail to pick up the paperclip.
- 3
Stage 3:
When the original bar magnet is removed, the inducing field disappears.
Because the nail is made of soft iron, it quickly loses most of its magnetism.
- 4
Conclusion:
The force of attraction from the nail becomes too weak to hold the paperclip, so the paperclip falls.
However, as steel is a hard magnetic material, the paperclip will retain some of its induced magnetism and become a weak permanent magnet.
Answer: Initially, the nail is attracted to the magnet and the paperclip is attracted to the nail. When the magnet is removed, the nail demagnetises and drops the paperclip, which remains weakly magnetised.
Common mistakes
- ×Assuming attraction proves an object is a permanent magnet. Unmagnetised magnetic materials are always attracted to magnets through induction. Repulsion is the only definitive test.
- ×Confusing the properties of 'soft' and 'hard' magnetic materials with their physical hardness. The terms refer strictly to how easily they are magnetised and demagnetised.
- ×Drawing magnetic field lines incorrectly, for example, having them cross each other or pointing from South to North.
No-calculator tips
- ✓To remember material properties, associate them with a use: 'soft' iron is for electromagnets which must switch off, while 'hard' steel is for compass needles which must stay magnetised.
- ✓Visualise field lines to understand forces. Lines that connect from one magnet to another show attraction. Lines that bend away from each other in the middle show repulsion.
- ✓Remember the Earth's geographic North Pole is a magnetic south pole, which is why the north-seeking pole of a compass points towards it.