Simple phenomena of magnetism
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Magnetic and Non-Magnetic Materials
A material can be described as magnetised or unmagnetised.
- Magnetic materials can be magnetised by a magnet, and in their unmagnetised state are attracted by a magnet - e.g. iron, steel, nickel, cobalt
- Non-magnetic materials cannot be magnetised and are not attracted by a magnet - e.g. aluminium, wood, plastic, copper
Forces Between Magnetic Poles
The poles of a magnet are the places to which magnetic materials are most strongly attracted. They are located near the two ends of a magnet and occur in pairs of equal strength: a north pole (N pole) and a south pole (S pole).
- Like poles repel each other (N-N or S-S)
- Unlike poles attract each other (N-S)
- A magnet always attracts an unmagnetised magnetic material, whichever pole is presented to it
Like poles repel, unlike poles attract
Explaining Magnetic Forces
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Magnetic forces arise from the interaction between the magnetic fields of the two magnets (or magnet and magnetic material). Where the fields overlap, the combined field pattern determines whether the poles are pushed together or pulled apart.
Induced Magnetism
When an unmagnetised magnetic material touches, or is brought close to, a permanent magnet, it becomes a magnet itself. This is called induced magnetism.
- The end of the material nearest the permanent magnet's pole becomes the opposite pole
- This induced opposite pole is why an unmagnetised magnetic material is always attracted to a magnet, never repelled
- A temporary magnet (e.g. soft iron) loses its induced magnetism almost immediately once it is removed from the permanent magnet's influence
The end of the iron bar nearest the magnet's N pole is induced to become a S pole
Temporary and Permanent Magnets
Magnetic materials differ in how easily they are magnetised and how well they retain their magnetism.
| Property | Temporary Magnets (Soft Iron) | Permanent Magnets (Steel) |
|---|---|---|
| Ease of magnetisation | Magnetised easily | Magnetised with more difficulty |
| Retaining magnetism | Loses magnetism easily | Retains magnetism well |
| Description | "Soft" magnetic material | "Hard" magnetic material |
Magnetic Fields
A magnetic field is a region in which a magnetic pole experiences a force.
- Magnetic field lines show the pattern and direction of a magnetic field, and always point from the N pole to the S pole
- The direction of a magnetic field at a point is defined as the direction of the force it would exert on a N pole placed at that point
The magnetic field pattern around a bar magnet
Field Strength
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The relative strength of a magnetic field is represented by the spacing of the field lines: field lines drawn closer together represent a stronger field, and lines drawn further apart represent a weaker field. This is why the field lines around a bar magnet are closest together near the poles.
Plotting Magnetic Field Lines
The pattern of a magnetic field can be plotted using either a plotting compass or iron filings.
Method 1: Plotting Compass
- Place the magnet on a sheet of paper and draw around it
- Place a plotting compass near one pole of the magnet and mark the position of each end of the needle with a dot
- Move the compass so its tail sits over the previous dot, and mark the new needle position
- repeat this until the other pole of the magnet is reached
- Join the dots with a smooth line and add an arrow showing the direction from N to S
- this direction is given by the N pole of the compass needle at each point
Marking the compass needle's position at each step traces out a field line
Method 2: Iron Filings
- Place a sheet of paper over the magnet
- Sprinkle iron filings evenly over the paper
- Tap the paper gently
- the filings settle along the field lines, but unlike a compass, cannot show the field's direction
Iron filings align with the field lines, but give no indication of direction
Uses of Permanent Magnets and Electromagnets
Permanent magnets and electromagnets are each suited to different uses.
| Permanent Magnets | Electromagnets |
|---|---|
| Compasses | Cranes for lifting scrap iron |
| Electric motors | Electric bells |
| Generators | Relays |
| Loudspeakers | Circuit breakers |
- Permanent magnets need no current and their field cannot be varied
- Electromagnets require a current, and their strength, and even their polarity, can be varied or switched off entirely - this makes them useful wherever a magnet needs to be turned on and off
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