Magnetism & Electromagnetism
Magnetism & Electromagnetism
Poles and the law of magnetism
- Every magnet has two poles: a north pole and a south pole
- Bringing two magnets close together produces a force between them. The direction of the force follows the law of magnetism:
- Like poles repel: N–N pushes apart; S–S pushes apart
- Opposite poles attract: N–S pulls together
- The force between two magnets is a non-contact force, so the magnets feel each other across an air gap without any physical link
Magnetic and non-magnetic materials
- A magnetic material is one that is attracted to a magnet (whether or not it is itself a magnet). The standard list is:
- iron
- cobalt
- nickel
- steel (an alloy of iron, so it counts)
- Almost every other metal is non-magnetic: copper, aluminium, gold, silver, lead, brass
- Non-metallic materials (plastic, wood, glass, rubber) are non-magnetic
- A magnetic material is attracted to either pole of a permanent magnet; only a magnet itself can be repelled by another magnet. So a quick test for whether an unknown bar of iron-coloured metal is a magnet or just a magnetic material: bring a known magnet near both ends in turn. If one end repels, you've found a magnet; if both ends only attract, it is a magnetic material
Hard and soft magnetic materials
- materials (e.g. steel) are difficult to magnetise, but once magnetised they hold their magnetism for a long time. They are used for
- materials (e.g. pure iron) are easy to magnetise, but lose their magnetism almost immediately when the magnetising field is removed. They are used for the cores of because you want the magnetism to switch off when the current does
Common exam question
Why steel makes a permanent magnet and iron does not
Question: Explain why steel suits a bar magnet, describe what magnetically hard means, name a permanent-magnet material, or explain why iron magnetised in a field is not a permanent magnet (1–2 marks).
Asked in 5 of the 24 papers. For steel the two marks are that it is magnetic and that it therefore stays magnetised for a long time; one scheme gives both marks for "steel is a hard magnetic material" on its own, so call it magnetically hard, and any mention of charge is rejected. Defining magnetically hard needs two of: difficult to magnetise, difficult to demagnetise, retains its magnetism. For a named material give steel, nickel, cobalt or neodymium; iron is only condoned.
For iron the pair is reversed: it is magnetically soft, so it loses its magnetism as soon as it leaves the field. Calling it "a soft magnet" or "a temporary magnet" earns nothing.