4.1 Simple phenomena of magnetism
- Syllabus
- 0625–2026–2027
- Topic
- 4.1
- Level
- —
Learning objectives
4.1.1Forces between magnetic poles and• Describe the forces between magnetic poles and between magnets and magnetic materials, including the use of the terms north pole (N pole), south pole (S pole), attraction and repulsion, magnetised and unmagnetised4.1.2Induced magnetism• Describe induced magnetism4.1.3Differences between the properties of• State the differences between the properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel4.1.4Difference between magnetic and• State the difference between magnetic and non-magnetic materials4.1.5Magnetic field as a region in which a• Describe a magnetic field as a region in which a magnetic pole experiences a force4.1.6Pattern and direction of magnetic• Draw the pattern and direction of magnetic field lines around a bar magnet4.1.7Direction of a magnetic field at a• State: the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point4.1.8Plotting of magnetic field lines with• Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field4.1.9Uses of permanent magnets and• Describe the uses of permanent magnets and electromagnets4.1.10Magnetic forces are due to• Explain that magnetic forces are due to interactions between magnetic fields4.1.11Relative strength of a magnetic field• Know: the relative strength of a magnetic field is represented by the spacing of the magnetic field lines
Describe forces between magnets and magnetic materials
Every magnet has a north pole (N pole) and a south pole (S pole). A magnetised object has its own magnetic poles; an unmagnetised magnetic material does not have a persistent pair of poles.
| Objects brought close | Force |
|---|---|
| N pole and N pole | repel |
| S pole and S pole | repel |
| N pole and S pole | attract |
| magnet and unmagnetised magnetic material | attract |
Repulsion is the decisive test for two magnets: an unmagnetised magnetic material can be attracted by either pole, but it cannot repel a pole.
Attraction alone does not prove that both objects are magnets. It may be unlike magnetic poles attracting, or a magnet attracting an unmagnetised magnetic material.
Explain induced magnetism
Induced magnetism is the magnetisation of a magnetic material caused by a nearby magnetic field.
| Nearby pole of the magnet | Pole induced at the nearest end | Pole induced at the far end |
|---|---|---|
| N | S | N |
| S | N | S |
The nearest induced pole is opposite to the approaching magnet pole, so the magnetic material is attracted. Soft iron usually loses most of this induced magnetism when the magnet is removed.
The unmagnetised object does not need to start with a labelled pole. The external field creates the temporary pole arrangement; it does not repel the object before magnetising it.
Compare temporary and permanent magnets
A temporary magnet is made from soft iron; a permanent magnet is made from steel in this syllabus comparison.
| Property | Temporary magnet: soft iron | Permanent magnet: steel |
|---|---|---|
| becoming magnetised | easy | harder |
| losing magnetism | easy when the magnetising field is removed | difficult |
| retained magnetism | little | substantial |
Choose soft iron when magnetism should appear and disappear readily. Choose steel when the object must remain magnetised after the magnetising field is removed.
Both soft iron and steel are magnetic materials. The difference is not magnetic versus non-magnetic; it is how readily they become magnetised and how well they retain magnetism.
Distinguish magnetic and non-magnetic materials
A magnetic material is attracted by a magnet and can be magnetised. A non-magnetic material is not attracted by a magnet and cannot be magnetised in this context.
| Magnetic materials | Non-magnetic materials |
|---|---|
| iron, steel, nickel, cobalt | copper, aluminium, glass, plastic |
Bring a known magnet close without touching. Attraction shows that the sample is magnetic; no magnetic attraction shows that it is non-magnetic under the test conditions.
Magnetic does not mean already magnetised. An unmagnetised piece of iron is still a magnetic material, and not every metal is magnetic.
Define a magnetic field
A magnetic field is a region in which a magnetic pole experiences a force.
| Location | Effect on a test magnetic pole |
|---|---|
| inside a magnetic field | a magnetic force acts |
| where the field is negligible | no detectable magnetic force acts |
A permanent magnet and an electromagnet both produce magnetic fields around them. The field describes how they can exert forces without direct contact.
A magnetic field is not the same as a magnetic material. It is the surrounding region in which a test magnetic pole would experience force.
Draw the field around a bar magnet
Magnetic field lines form continuous loops. Outside a bar magnet they leave the N pole, curve through the space around the magnet and enter the S pole.
| Feature to draw | Correct representation |
|---|---|
| symmetry | similar curved loops above and below the bar |
| connection | lines meet the magnet at both poles |
| external arrows | N → S |
| intersections | field lines never cross |
A complete sketch uses several smooth loops rather than isolated straight arrows. Between close unlike poles, central field lines run approximately straight from N to S.
Do not reverse the arrows outside the magnet: external field direction is from N to S. A field line does not stop in empty space or cross another field line.
Use the definition of magnetic-field direction
The direction of a magnetic field at a point is the direction of the force on an N pole placed at that point.
| Test object at the point | Relation to field direction |
|---|---|
| N test pole | force is along the field direction |
| S test pole | force is opposite to the field direction |
| plotting compass | its N-seeking end points along the field direction |
Outside a bar magnet, follow the arrow from N towards S. At any point on a curved line, the field direction is along the tangent to that line.
The reference object is an N magnetic pole, not a positive electric charge. A compass shows direction with its N-seeking end, not with whichever end happens to be closest to the magnet.
Plot magnetic field lines
Iron filings reveal the field pattern, while a plotting compass determines the direction of the field.
| Step | Plotting-compass method |
|---|---|
| 1 | place the compass near the magnet and mark the position of both needle ends |
| 2 | move the compass so its S end is at the previous N-end mark |
| 3 | repeat to trace a sequence of points, then join them with a smooth line |
| 4 | add an arrow in the direction indicated by the compass N end; repeat from other starting positions |
For the pattern method, place paper over the magnet, sprinkle iron filings evenly and tap gently. The filings align with the local field and show the curved line pattern, but they do not by themselves show arrow direction.
A single compass position gives only one local direction. Move it through many positions to map a line, and use its N end—not the iron filings—to assign direction.
Choose permanent magnets and electromagnets for uses
A permanent magnet provides a field without electrical power. An electromagnet produces a field when current flows, so it can be switched and its strength can be controlled.
| Device or task | Suitable magnet | Why |
|---|---|---|
| compass or magnetic door catch | permanent magnet | field is needed continuously without a power supply |
| scrapyard lifting crane | electromagnet | can lift magnetic metal, then release it when switched off |
| relay or electric bell | electromagnet | current switches a magnetic force that moves an iron part |
| separating magnetic from non-magnetic material | either, depending on the system | magnetic material is attracted while non-magnetic material is not |
For an electromagnet, switching current on creates the useful magnetic field and switching it off removes most of the field when a soft-iron core is used.
An electromagnet is not always magnetised and its core should not be steel when rapid release is required. A permanent magnet cannot be switched off simply by opening a circuit.
Explain magnetic force as interacting fields
A magnetic force occurs when magnetic fields overlap and interact. Each magnet responds to the combined field in the region around it.
| Facing poles | Field interaction and motion |
|---|---|
| unlike poles | the interaction produces attraction; magnets move together if free |
| like poles | the interaction produces repulsion; magnets move apart if free |
| magnet in Earth's field | interaction turns the magnet until it aligns with the surrounding field |
The interaction gives forces on both objects in opposite directions. If one magnet is held fixed, the force on the free magnet is still caused by the interaction of their fields.
Magnetic force is not caused by field lines physically pulling like strings. Field lines are a representation of the interacting magnetic field and its direction.
Read magnetic-field strength from line spacing
The relative strength of a magnetic field is represented by the spacing of its field lines: closer lines indicate a stronger field, while wider spacing indicates a weaker field.
| Field-line pattern at a point | Relative field strength |
|---|---|
| lines very close together | strong |
| lines farther apart | weak |
| lines converge towards a region | field becomes stronger towards that region |
Around a bar magnet, lines are usually closest near the poles, so the field is strongest there. To rank labelled points, compare local spacing at each point.
Do not judge strength from arrow direction or from the length of one drawn line. Compare the separation or density of neighbouring field lines in the same diagram.