(b) Magnetism
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
6.2Magnetic attraction and repulsionKnow that magnets repel and attract other magnets and attract magnetic substances6.3Hard and soft magnetic materialsDescribe the properties of magnetically hard and soft materials6.4Magnetic field linesUnderstand the term 'magnetic field line'6.5Induced magnetismKnow that magnetism is induced in some materials when they are placed in a magnetic field6.6Magnetic field practicalPractical: investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets6.7Uniform magnetic fieldsDescribe how to use two permanent magnets to produce a uniform magnetic field pattern
Predict magnetic attraction and repulsion
Every magnet has a north pole and a south pole. The force between two magnets depends on which poles face: like poles repel and unlike poles attract.
| Objects brought together | Result |
|---|---|
| north and north, or south and south | repel |
| north and south | attract |
| either pole and an unmagnetised magnetic substance | attract |
Magnetic substances include iron, steel, nickel and cobalt. A magnet attracts them, but many other materials—including plastic, copper and zinc—are not attracted. Being a metal does not automatically make a substance magnetic.
Repulsion is evidence that both objects are magnets, because an unmagnetised magnetic substance is attracted by either pole. Describe the interaction using magnetic poles, not positive and negative electric charge.
Compare magnetically hard and soft materials
Magnetically hard and magnetically soft describe how readily a material gains and loses magnetism—not how mechanically hard or soft it feels.
| Property | Magnetically hard material | Magnetically soft material |
|---|---|---|
| becoming magnetised | relatively difficult | relatively easy |
| being demagnetised | relatively difficult | relatively easy |
| after the magnetising field is removed | retains most of its magnetism | loses most of its magnetism |
| typical use | permanent magnet | temporary magnet or electromagnet core |
| common example | steel | iron |
Choose by function: a permanent bar magnet must keep its magnetism, so a hard magnetic material is suitable. A temporary magnet must switch its magnetism on and off readily, so a soft magnetic material is suitable.
A magnetically hard material is not necessarily easy to magnetise; it is valuable because it is difficult to demagnetise once magnetised. Calling iron a 'soft magnet' is imprecise—the material is magnetically soft and usually becomes only a temporary magnet.
Read magnetic field lines
A magnetic field line is a model showing the direction of the magnetic field at each point. Its arrow points in the direction that the north pole of a small test magnet would move; outside a magnet, this is from north to south.
Around a bar magnet, field lines leave the north pole, curve through the surrounding space and enter the south pole. Lines never cross: crossing would assign two different field directions to one point.
Line spacing represents field strength. Closely spaced lines show a stronger field; widely spaced lines show a weaker field. The field around a bar magnet is therefore strongest near its poles, where the lines are densest.
Field lines are a representation, not physical threads. A diagram with more drawn lines does not create a stronger magnet; strength is inferred from the relative spacing within the same field pattern. Every arrow outside the magnet must remain consistent from N to S.
Explain induced magnetism
Induced magnetism occurs when a magnetic material becomes magnetised because it is placed in a magnetic field.
The field induces magnetic poles in the material. The end nearest the inducing magnet becomes the opposite pole, so the two nearest poles attract. For example, beside a magnet's south pole, the nearest end of an iron object becomes a north pole and is pulled toward the magnet.
What remains after the field is removed depends on the material. Magnetically soft iron loses most of the induced magnetism, whereas a magnetically hard material retains much more and can become a permanent magnet.
The object does not need to be a permanent magnet before it is attracted: its poles are induced by the external field. Do not explain the attraction with electric charge, and do not assume that every material can be magnetised.
Investigate a magnetic field pattern
Investigate both shape and direction while keeping the permanent magnet or pair of magnets fixed beneath a sheet of card or paper.
To reveal shape with iron filings: 1. Place the card over the magnet arrangement. 2. Sprinkle a thin, even layer of filings. 3. Tap the card gently so the filings align with the field. The curved or connecting bands show the field pattern, but filings alone do not show its direction.
To map direction with a plotting compass: 1. Place the compass at a chosen starting point and mark the direction of its north-pointing end. 2. Move the compass so its tail is at the previous mark and mark again. 3. Join the marks with a smooth line and add an arrow. 4. Repeat from several starting points to build the pattern.
Repeat for one bar magnet and for two bar magnets with chosen poles facing. Keep magnet positions and orientation fixed, remove nearby magnetic materials, and use the same sampling spacing so patterns can be compared. Cover the magnets so iron filings do not stick directly to them, and avoid trapping fingers between strong magnets.
Produce and recognise a uniform magnetic field
A uniform magnetic field has the same strength and direction throughout the region being considered. It is represented by straight, parallel, equally spaced field lines with arrows all pointing the same way.
Place two permanent magnets with unlike poles facing each other across a small gap: north faces south, and the poles are close but not touching. In the central part of the gap, field lines run straight from N to S and are nearly equally spaced, producing an approximately uniform field.
Use three checks together: straight lines show an unchanging direction, parallel lines show that direction is shared across the region, and equal spacing shows constant field strength. Lines curve near the outer edges, so the most uniform region is between the central pole faces.
Like poles facing do not produce this connecting uniform pattern; their fields push apart. Bringing unlike poles into contact also removes the useful gap. 'Uniform' does not mean no field—it means the field does not change across the stated region.