Simple Phenomena of MagnetismCambridge IGCSE Physics: Revision notes
Section 1
What are magnetic poles and how do they interact?
Every magnet has two poles: a north pole and a south pole. Magnetic poles exert forces on each other according to a simple rule:
- Unlike poles attract (north attracts south, south attracts north)
- Like poles repel (north repels north, south repels south)
These forces occur because magnetic poles create regions of influence called magnetic fields. The force between poles is stronger when the poles are closer together.
Magnetised materials are those that have been made magnetic and have distinct north and south poles. Unmagnetised materials show no magnetic properties and have no distinct poles.
Examiners expect you to describe forces using the specific terms 'attraction' and 'repulsion' rather than just 'push' or 'pull'. Always state which poles are involved and the direction of the force.
Think of magnetic poles like opposite charges in electricity: like charges repel, opposite charges attract. The same pattern applies to magnetic poles.
Section 2
What is induced magnetism and how does it differ from permanent magnetisation?
Induced magnetism occurs when a magnetic material is placed in a magnetic field and becomes temporarily magnetised. The material aligns with the external field and develops a north and south pole.
Key points about induced magnetism:
- The induced magnet always attracts the original magnet (unlike the permanent magnet, which can attract or repel)
- Induced magnetism disappears when the material is removed from the magnetic field
- Soft iron is easily magnetised and loses its magnetism quickly when removed from the field
- Steel retains its magnetism much longer and is used for permanent magnets
| Property | Soft Iron (Temporary Magnet) | Steel (Permanent Magnet) |
|---|---|---|
| Ease of magnetisation | Easy to magnetise | Harder to magnetise |
| Retention of magnetism | Loses magnetism quickly | Retains magnetism for a long time |
| Uses | Electromagnets, relays | Compass needles, bar magnets |
| Field strength | Weaker when magnetised | Stronger and more stable |
Students often confuse induced magnetism with the magnet 'turning on and off'. Induced magnetism is still real magnetism—it's just temporary because soft materials cannot sustain the aligned atomic structure without the external field.
If you place a paperclip near a permanent magnet, the paperclip becomes temporarily magnetised by induction and sticks to the magnet. When you remove it from the magnetic field, the paperclip loses its magnetism and falls off.
Section 3
What is a magnetic field and how can it be visualised?
A magnetic field is a region in space where a magnetic pole experiences a force. It exists around all magnets and magnetic materials.
Direction of the magnetic field:
- The direction of the magnetic field at any point is defined as the direction of the force on a north pole at that point
- Field lines always point from the north pole towards the south pole (outside the magnet)
- Inside the magnet, field lines point from south to north
Visualising magnetic fields:
Magnetic fields can be plotted using two methods:
- Using a compass: A compass needle aligns with the magnetic field. By moving the compass around a magnet, you can trace the direction at each point.
- Using iron filings: Iron filings sprinkled around a magnet align with the field, creating a visible pattern of the field lines.
Field line patterns around a bar magnet:
- Lines emerge from the north pole and enter the south pole
- Lines are closest together at the poles (strongest field)
- Lines spread out further away from the magnet (weaker field)
- Lines never cross each other
Remember: field lines show the direction a north pole would be pushed. Always draw them pointing FROM north TO south (outside the magnet). Examiners check that your field lines never cross and are closer together where the field is stronger.
Magnetic field lines are like invisible roads around a magnet. A north pole travels along these roads—the lines show where it would be pulled or pushed.
Section 4
How does magnetic field strength relate to field line spacing?
The relative strength of a magnetic field is represented by the spacing of the field lines:
- Closer field lines = stronger magnetic field
- Wider spacing of field lines = weaker magnetic field
This relationship is crucial for understanding magnetic field patterns:
Around a bar magnet:
- Field lines are closest together at the north and south poles → field is strongest here
- Field lines spread out as you move away from the magnet → field becomes weaker
- Field lines are furthest apart far from the magnet → field is weakest
Between two magnets:
- If poles are attracting: field lines are dense between the poles → strong field
- If poles are repelling: field lines push away from each other → weaker field in the middle
The spacing of field lines provides a quick visual way to estimate the relative strength of a field at different locations without making measurements.
Examiners often ask you to compare field strengths at different locations around a magnet. Always refer to the field line spacing: 'The field is stronger here because the lines are closer together.'
Section 5
What are the practical uses of permanent and temporary magnets?
Permanent magnets and electromagnets (temporary magnets) have different applications based on their properties:
| Use | Type of Magnet | Reason |
|---|---|---|
| Compass needles | Permanent (steel) | Must retain magnetism continuously |
| Bar magnets (laboratory) | Permanent (steel) | Reliable, long-lasting, no power needed |
| Electric motors | Electromagnet | Can be switched on/off, strength can be varied |
| Doorbells and buzzers | Electromagnet | Can be controlled electronically |
| Loudspeakers | Electromagnet and permanent | Creates variable fields for sound production |
| Lifting heavy iron objects | Electromagnet | Can be switched off to release the load |
| Relays and switches | Electromagnet | Controlled by electric current |
| Magnetic locks | Electromagnet | Can be electronically controlled |
Key distinction:
- Permanent magnets are useful when you need a constant, reliable magnetic field without power supply
- Electromagnets (soft iron coils) are useful when you need to control the magnetic field strength or switch it on and off
A scrapyard uses an electromagnet to pick up iron cars. When the current is switched on, the magnet attracts the cars. When switched off, they fall. A permanent magnet cannot do this—it would be stuck holding the cars permanently.
Section 6
What is the underlying cause of all magnetic forces?
A fundamental principle of magnetism states:
Magnetic forces are due to interactions between magnetic fields.
This means:
- Every magnet has a magnetic field around it
- When two magnets are brought near each other, their fields interact
- The force between magnets (attraction or repulsion) results from this field interaction, not from direct contact
- A magnet can only exert a force on another magnet or magnetic material because of the field it creates
Practical implications:
- Magnetic forces can work through non-magnetic materials (wood, plastic, paper)
- Magnetic fields extend infinitely but become weaker with distance
- The force between two magnets depends on the strength of each magnet's field and the distance between them
- Induced magnetism occurs because an external field causes the atomic structure of a material to align
Understanding that forces arise from field interactions helps explain why magnets don't need to touch to exert forces and why field strength (shown by line spacing) determines the magnitude of forces.
This concept links all magnetism together. When answering questions about why magnets attract or repel, refer to 'the interaction of their magnetic fields' to show understanding of the underlying physics.
Must Know
- Magnetic poles: North and south poles; unlike poles attract, like poles repel. Forces exist due to magnetic field interactions.
- Magnetised vs. unmagnetised: Magnetised materials have distinct poles and magnetic properties; unmagnetised materials do not.
- Induced magnetism: When a magnetic material is placed in a magnetic field, it becomes temporarily magnetised. Soft iron loses magnetism quickly; steel retains it (permanent magnets).
- Magnetic field: A region where a magnetic pole experiences a force. Direction is defined as the direction of force on a north pole. Visualised using compasses (needle alignment) or iron filings (visual pattern).
- Field lines and strength: Closer field lines indicate stronger magnetic fields. Lines emerge from north poles and enter south poles. Lines are never cross.
- Practical uses: Permanent magnets for compasses and fixed applications; electromagnets for switchable/controllable applications (motors, doorbells, lifts).
- Fundamental principle: All magnetic forces result from interactions between magnetic fields, not direct contact between objects.
That's the notes covered.
Carry on to the next subtopic.