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Magnets (Cambridge O Level Physics)
Revision Note
Magnets
Magnetic and Non-magnetic Materials
Magnetic materials are attracted to a magnet; non-magnetic materials are not
- Very few metals in the Periodic Table are magnetic. These include:
- Iron
- Cobalt
- Nickel
- Steel is an alloy which contains iron, so it is also magnetic
- Magnetic materials (which are not magnets) will always be attracted to the magnet, regardless of which pole is held close to it
Magnetism in Magnetic Materials
Magnetic materials are attracted to both poles of a magnet
- To test whether a material is a magnet it should be brought close to a known magnet
- If it can be repelled by the known magnet then the material itself is a magnet
- If it can only be attracted and not repelled then it is a magnetic material
Types of Magnets
- There are two types of magnets
-
- Permanent magnets
- Induced magnets
Permanent Magnets
- Permanent magnets are made out of permanent magnetic materials, for example steel
- A permanent magnet will produce its own magnetic field
- It will not lose its magnetism
Temporary (Induced) Magnets
- When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet
- This is called induced magnetism
-
- Some objects such as paperclips or needles (which are made from steel) can be magnetised and will remain magnetic for a while
- Other objects, such as electromagnets or transformers (which are made from iron) will be demagnetised as soon as the cause of the induced magnetism is removed
- When magnetism is induced on a material:
- One end of the material will become a north pole
- The other end will become a south pole
- Magnetic materials will always be attracted to a permanent magnet
- This means that the end of the material closest to the magnet will have the opposite pole to magnets pole closest to the material
Magnetic Induction in Magnetic Materials
Magnetic poles are induced in a magnetic material; the induced pole is always the opposite pole to the pole of the permanent magnet
- When the magnetic material is removed from the magnetic field it will lose most/all of its magnetism quickly
Worked example
The diagram below shows a magnet held close to a piece of metal that is suspended by a light cotton thread. The piece of metal is attracted towards the magnet.
Which of the following rows in the table gives the correct type of pole at X and the correct material of the suspended piece of metal?
| Type of pole at X | Material of suspended piece of metal | |
| A | North | Nickel |
| B | South | Nickel |
| C | North | Aluminium |
| D | South | Aluminium |
Answer: A
- X must be a north pole
- The piece of metal is being attracted towards the magnet
- The law of magnetism states that opposite poles attract
- The material of the suspended piece of metal is nickel
- Nickel is a magnetic material (It will experience a force when it is placed in a magnetic field, in this case it is attracted towards the magnet)
- B is incorrect because X cannot also be a south pole (and hence is a north pole)
- If the pole at X was a south pole then the piece of metal would be repelled from the magnet because the law of magnetism states that like poles repel
- C and D are incorrect because aluminium is not a magnetic material
- A non-magnetic material would be unaffected by the magnetic field produced by the magnet.
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