Magnetism

This page covers the basics of magnetism for GCSE; for further reading and A-level material see Electromagnetism.

Overview
Magnetism is a phenomenon of action at a distance of which most people will be at least somewhat familiar from everyday magnets found around the house, in toys, cupboard doors and on fridges (as fridge magnets!). Certain rocks containing ferrous (i.e. containing iron) metals are naturally magnetic, meaning that this is also a phenomenon known to ancient people. There is evidence that the ancient Chinese used such "lode stones" as compasses, that the ancient Greeks and Mayans used them in religious statues, and I can imagine shamans (and charlatans) using magnets to do "magic" tricks!

At GCSE the phenomena that need to be understood are:

1) The properties of magnets 2) Distinguish between ferrous and non-ferrous materials 3) Be familiar with the pattern of fields around a bar magnet (and ways to "see" it), and around wires and solenoids (aka coils of wire) 4) Distinguish between the magnetic properties of iron and steel 5) Identify the advantages and uses of permanent- and electro- magnets. 6) A basic understanding of the forces on a current carrying conductor (or beam of charged particles) and the operation of a d.c. motor (or generator).
 * That certain metals can be magnetised (and how)
 * That magnets have 2 poles which attract magnetic materials
 * That we call the pole which is attracted towards the Earth's geographic north pole the "north pole" of the magnet
 * Like poles repel; unlike poles attract (aka: the law of magnetic poles).

Many of these ideas are examined in more detail below.

The properties of magnets:
a) Magnetic materials - certain metals are attracted strongly to magnets and can be magnetised (see below). Such metals include: iron, steel (an iron alloy, usually made with carbon), nickel, and cobalt. These metals are called ferromagnetics (although this does not mean they all contain iron!). Mumetal, an alloy of mainly iron and nickel (~75%), is often used for transformer cores as it is very "magnetic". Non-magnetic materials include: copper, brass, wood, glass.

Ferromagnetic materials can be magnetised by: - placing the object is a strong magnetic field for a long period (either from a bar magnet or a electro magnet, e,g, in the middle of a solenoid); - stroking the object with a bar magnet. Either of the above can be enhanced by initially heating the magnet and then letting it cool while the external magnetic field is applied.

Demagnetising an object can be done by dropping, or hitting the magnet, placing it in a solenoid which has a decreasing a/c current running through it, or by heating it.

b) Magentic poles - the poles of a magnet are the places to which magnetic materials are attracted. They occur in pairs of equal strength. A magnet produces a magnetic force in the space around it and we call this a magnetic field. The field is often represented by "field lines", where the density of the lines represents the strength of the field (like the density of contour lines on a map which indicate the steepness of the slope) and the direction of the force is shown from north to south (to show the direction that the north pole of a free magnet would point, or the direction of force of the north pole of a magnet). The pattern of field lines round a magnet can be discovered using (i) iron filings (on a sheet of paper over the magnet) or (ii) using a plotting compass, among other methods.

c) North and south poles - we call the pole which is attracted towards the Earth's geographic north pole the "north pole" of the magnet. Note that this means the imaginary bar magnet running through the centre of the Earth therefore has a magnetic south pole at (approximately) the geographic north pole! Note that early ideas about why a compass points to the geographic north included the existence of a large island at the north pole made of lode stone!

d) Law of magnetic poles - Like poles repel; unlike poles attract.

Magnetisation of iron and steel:
A bar magnet can be used to pick up a chain of iron nails and/or steel paper clips. If the magnet is removed carefully the chain of nails will fall apart, while the chain of paper clips will stay attached to each other for a short time. This shows that the magnetism induced by the bar magnet in the iron is temporary, while the magnetism induced in the steel is more permanent. The terms "soft" (de-magnetises quickly) and "hard" (the induced magentic field persists) are used to describe this property of ferromagnetic materials. Very hard ferromagnetic materials are used to make permanent magnets. Hard materials are more difficult to magnetise than soft ones.

Summary
 * Steel - "Hard" as the induced magnetic field persists; difficult to magnetise
 * Iron - "Soft" as it demagnetises quickly; easy to magnetise.

Uses of permanent magnets and electromagnets
Permanent magnets:
 * Medical instruments (e.g. to remove ferromagnetic materials from the throat).
 * Recording information (magnetic tapes)
 * Notice boards
 * Toys

Electromagnets
 * Electric motors
 * Electric bells/alarms
 * Relays (i.e. an electromagnetic switch - useful if one circuit is to control another).