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Topic 4 · 4.1

Simple magnetism

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the forces between magnetic poles and between magnets and magnetic materials; describe induced magnetism.
  • State the differences between temporary (soft iron) and permanent (steel) magnets, and between magnetic and non-magnetic materials.
  • Describe a magnetic field; draw the field around a bar magnet and state the direction of the field; plot field lines with a compass or iron filings.
  • Describe uses of permanent magnets and electromagnets; explain magnetic forces as interactions between fields, and field strength from line spacing EXTENDED.

📚The physics

Poles and forces

Every magnet has a north (N) pole and a south (S) pole. Like poles repel; unlike poles attract. A magnet also attracts magnetic materials (iron, steel, nickel, cobalt) that are unmagnetised. Non-magnetic materials (copper, aluminium, plastic, wood) are not attracted. Repulsion is the only sure test that an object is itself a magnet.

A bar magnet with its N pole next to an iron bar: the iron bar becomes a magnet with an S pole next to the magnet's N pole, and it holds a chain of pins. A box compares soft iron, easy to magnetise and losing its magnetism when the field is removed, used in electromagnets, relays and transformers, with steel, harder to magnetise but keeping it, used for permanent magnets.
Induced magnetism: a magnetic material placed in a magnetic field becomes a magnet itself.

Magnetic fields

A magnetic field is a region in which a magnetic pole experiences a force. The direction of the field at a point is the direction of the force on the N pole of a magnet at that point — so field lines run from N to S outside a magnet.

The magnetic field around a bar magnet, computed from a two-pole model. Field lines leave the N pole, curve round and enter the S pole; they are closest together near the poles. Small compasses placed in the field point along the lines.
Lines never cross. EXTENDED Closer lines mean a stronger field: strongest at the poles.

To plot the field: place a small plotting compass near one pole, mark where its needle points, move it on so its tail is at the last mark, and repeat, joining the dots into a line from N to S; repeat from other starting points. Iron filings sprinkled on paper over the magnet show the pattern quickly but not the direction.

Left: two bar magnets with N facing S; field lines run straight across the gap from one to the other, and the magnets attract. Right: two magnets with N facing N; the field lines from each pole bend away from each other, leaving a neutral point midway where there is no field, and the magnets repel.
EXTENDED Magnetic forces are due to interactions between magnetic fields.

Uses

permanent magnets: compasses, loudspeakers, fridge door seals, electric motors and generators, magnetic catches
electromagnets (a coil with a soft-iron core, switched on and off by the current): scrap-metal cranes, relays, electric bells, MRI scanners, magnetic door locks

✏️Worked example

A student has three identical-looking bars: one is a magnet, one is soft iron and one is aluminium. (a) Describe how she can identify all three using only the bars. [3] (b) State why the core of an electromagnet is made of soft iron and not steel. [2] (c) Explain why a crane in a scrap yard uses an electromagnet rather than a permanent magnet. [1]

(a) Bring the bars together in pairs. The aluminium bar is attracted by neither of the others. The other two attract each other. To tell them apart, touch the end of one bar to the middle of the other: if the bar you are holding is the magnet, its pole attracts the iron’s middle strongly; if you are holding the iron, there is almost no attraction, because the field at the middle of a magnet is very weak.

(b) Soft iron is easily magnetised and loses its magnetism as soon as the current is switched off; steel would stay magnetised.

(c) It can be switched off to drop the load.

Check it. Attraction only proves that at least one object is magnetic; only repulsion (or pointing N–S when free) proves it is a magnet.
“All metals are magnetic.” Only iron, steel, nickel and cobalt (and their alloys) are; copper, aluminium and brass are not.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Why does a scrap-yard crane use an electromagnet? A: Electromagnets are always stronger than permanent magnets. B: Electromagnets attract all metals. C: Electromagnets can be switched on and off. D: Electromagnets repel non-metals.
C.
2. (Theory.) State the direction of a magnetic field at a point. [1]
The direction of the force on the N pole of a magnet placed at that point.
3. (Theory.) Draw (describe) the field pattern of a bar magnet, including direction. [3]
Closed curved lines from the N pole round to the S pole on both sides, symmetrical, closer together near the poles, never crossing, with arrows from N to S.
4. (Theory.) Describe how to plot the magnetic field around a bar magnet using a plotting compass. [3]
Place the compass near the N pole; mark the position of the needle’s tip with a dot; move the compass so its tail is at that dot and mark the new tip; repeat until reaching the S pole; join the dots and add arrows N to S; repeat from other starting points.
5. (Theory.) Give two differences between soft iron and steel as magnetic materials. [2]
Soft iron is easily magnetised, steel is harder to magnetise; soft iron loses its magnetism easily (temporary), steel keeps it (permanent).
6. (Theory.) EXTENDED How does a field-line diagram show where the field is strongest? [1]
The field is strongest where the lines are closest together.
7. (Theory.) Which of these are attracted to a magnet: aluminium can, steel paperclip, copper coin, nickel coin, plastic ruler? [2]
Steel paperclip and nickel coin.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Magnets and Electromagnets” — field lines and compasses
  • Institute of Physics — plotting magnetic fields