6.1 Types of magnets
A magnet is a body that produces a magnetic field, attracting iron, nickel, cobalt and some of their alloys, and setting itself in a north to south direction when freely suspended. Magnets are grouped by how long they keep their magnetism (permanent or temporary), by their origin (natural or artificial), by how the magnetism is produced (by a material or by an electric current) and by their shape. Magnetism comes from the tiny magnetic effects of spinning electrons, which in magnetic materials line up in small regions called domains. Example: a scrap metal yard in Kegogi uses a large crane electromagnet to lift steel scrap and a permanent magnet bar to sort out non-magnetic aluminium and brass.
Magnetic and non-magnetic materials
- Ferromagnetic materials: Strongly attracted by magnets and able to be magnetised: iron, nickel, cobalt, steel and alloys such as Alnico and neodymium-iron-boron.
- Paramagnetic materials: Very weakly attracted: aluminium, platinum, oxygen gas.
- Diamagnetic materials: Very weakly repelled: copper, water, bismuth, most organic compounds.
- Non-magnetic in everyday terms: Wood, plastic, glass, brass, copper and aluminium, which a magnet does not noticeably attract.
Natural magnets
The natural magnet is lodestone, a form of the iron ore magnetite (Fe₃O₄), which was magnetised in the ground. It was used in the first compasses. Natural magnets are weak and irregular in shape, and are now of historical interest only. The Earth itself behaves like a giant natural magnet.
Artificial magnets
Artificial magnets are made by magnetising a ferromagnetic material, and they are of two main kinds.
- Permanent magnets: Made of magnetically hard materials, which are hard to magnetise but keep their magnetism for a long time. Examples are hard steel, Alnico (aluminium, nickel, cobalt and iron), ferrite (ceramic) magnets, samarium-cobalt and neodymium (NdFeB) magnets, the strongest available. Used in compasses, loudspeakers, motors, magnetic stirrer bars, door catches and sensors.
- Temporary magnets: Made of magnetically soft materials such as soft iron and silicon steel, which are easily magnetised but lose almost all their magnetism when the magnetising field is removed. Used as the cores of electromagnets, transformers, relays and motors, where the magnetism must switch on and off or reverse quickly.
Electromagnets
An electromagnet is a coil of insulated wire (a solenoid) wound round a soft iron core. When current flows, the core becomes a strong magnet; when the current is switched off, the magnetism almost disappears. Its strength increases with the current, the number of turns and the use of a soft iron core. Electromagnets are used in cranes, electric bells, relays, circuit breakers, MRI scanners and the magnets of mass spectrometers. (Their fields are studied in topic 6.7.)
Superconducting magnets
Coils of superconducting wire cooled with liquid helium (about 4 K) carry very large currents with no resistance, giving fields of several tesla. They are used in nuclear magnetic resonance (NMR) spectrometers, which identify the structure of organic compounds, and in hospital MRI scanners.
Shapes of magnets
| Shape |
Description |
Typical use |
| Bar magnet |
Rectangular bar with poles at the ends |
Laboratory experiments, field plotting |
| Horseshoe (U-shaped) magnet |
Bar bent so the poles are close together |
Strong field between poles for motor effect experiments |
| Ring (annular) magnet |
Flat ring magnetised through its thickness |
Loudspeakers, motors |
| Disc or button magnet |
Small flat cylinder |
Fridge magnets, sensors, earphones |
| Cylindrical rod magnet |
Rod, often PTFE coated |
Magnetic stirrer bars |
| Compass needle |
Light, pivoted magnetised needle |
Finding direction and plotting fields |
Methods of making magnets
- Single touch (stroking): One pole of a magnet is stroked repeatedly along a steel bar in one direction, lifting it clear at the end of each stroke.
- Double touch: Opposite poles of two magnets are stroked from the centre of the bar outwards to the ends.
- Electrical method: The steel bar is placed inside a solenoid carrying a large direct current, the most effective method, used commercially.
- Induction: A piece of iron near a strong magnet becomes a magnet itself while the magnet is close.
- Hammering in the Earth's field: A steel rod pointing north to south and hammered becomes weakly magnetised.
- Testing: Only repulsion proves that a body is a magnet; attraction may simply mean it is a magnetic material.
The rest of this chapter
🔒Create a free account to open more of this chapter.
Free: practical guides, quick cards, workplace scenarios and more.
Create a free account 🔒6.2 Properties of Magnets
Every magnet, whatever its type or shape, shows the same basic properties: it attracts magnetic materials, its magnetism is strongest at two regions called poles, it sets itself north to south when free to turn, and its poles obey the law of magnetic poles. Th…
🔒6.3 Lines of force
A line of force (magnetic field line) is an imaginary line drawn in a magnetic field such that the tangent to it at any point gives the direction of the field at that point. The direction of the field is defined as the direction in which the N pole of a small…
🔒6.4 Magnetic flux patterns
A magnetic flux pattern is the arrangement of lines of force produced by a magnet or a group of magnets, as shown by iron filings or plotted with a compass. Magnetic flux (symbol Φ, unit the weber, Wb) is the total "amount" of magnetic field passing through a…
🔒6.5 Flux density
Magnetic flux density (B) is the measure of the strength of a magnetic field at a point. It is defined as the magnetic flux passing at right angles through unit area, so where lines of force are packed closely the flux density is high and where they spread out…
🔒6.6 Properties of magnetic field lines
Magnetic field lines are imaginary, but they are drawn according to fixed rules so that any field diagram can be read correctly by anyone. These rules, the properties of field lines, follow from the nature of the magnetic field itself and allow the direction,…
🔒6.7 Magnetic field pattern
An electric current produces a magnetic field around it, a discovery made by Hans Christian Oersted in 1820 when he saw a compass needle swing beside a wire carrying current. The shape of this field depends on the shape of the conductor: a straight wire, a sin…
🔒6.8 Electromagnetism
Electromagnetism is the branch of physics that deals with the link between electricity and magnetism. An electric current produces a magnetic field (topic 6.7), a current-carrying conductor in a magnetic field feels a force (the motor effect), and a changing m…
🔒6.9 Storage of magnets
Permanent magnets slowly lose their strength if they are stored carelessly, a process called self-demagnetisation. Near the poles of a bar magnet, the free poles produce a field inside the magnet that runs from N to S, opposite to the magnetisation, and over t…
🔒6.10 Applications of electromagnetism
Electromagnetism is at work in almost every electrical machine. Devices that use a current to produce a magnetic field include electromagnets, bells and relays; those that use the force on a current in a field include motors, meters and loudspeakers; and those…
