Simple phenomena of magnetism
Magnetic materials, magnetic fields and field lines, and induced magnetism.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 4.1.1Describe the forces between magnetic poles and identify magnetic materials.
- 4.1.2Draw the magnetic field pattern around a bar magnet, including direction.
- 4.1.3Distinguish between the properties of permanent magnets and electromagnets.
6 minute read
Magnetism
Poles and forces
Every magnet has a north pole and a south pole. Like poles repel; unlike poles attract. These forces act without the poles needing to touch.
Magnetic materials
Only a small set of elements are strongly attracted to a magnet: iron, steel, cobalt and nickel. These are called magnetic (or ferromagnetic) materials. Most other materials, including most metals such as copper and aluminium, are not magnetic.
Magnetic fields
A magnetic field is the region around a magnet where a magnetic material or another magnet experiences a force. Field lines are drawn from north to south outside the magnet, and the closer together the lines, the stronger the field. Around a bar magnet, the field is strongest near the poles, where the lines are most concentrated.
Permanent magnets vs electromagnets
A permanent magnet keeps its magnetism all the time, with no need for a current.
An electromagnet is a coil of wire, usually wound around a soft-iron core, that becomes magnetic only when a current flows through it, and loses its magnetism almost completely when the current is switched off. This makes electromagnets useful wherever magnetism needs to be turned on and off, such as in cranes for lifting scrap metal, or in relays and electric bells.
Induced magnetism
A magnetic material placed near a magnet can itself become a temporary magnet — this is induced magnetism — and it loses most of this magnetism once removed from the field.
Think of it like this
An electromagnet is like a magnet with an "on/off switch" — a permanent magnet is always doing its job, but an electromagnet only works while electricity is flowing through it, which is exactly why a scrapyard crane can drop the metal it is carrying just by cutting the current.
Common misconceptions
- Thinking all metals are magnetic. Only iron, steel, cobalt and nickel are strongly attracted to a magnet — copper, aluminium, gold and most other metals are not.
- Believing magnetic field lines can cross each other. They never do; where lines are close together the field is strong, and where they spread out the field is weaker.
- Assuming an electromagnet stays magnetised after the current is switched off. Its magnetism disappears almost completely without the current.
In the exam
- When drawing field lines around a bar magnet, always add arrowheads pointing from north to south, and draw them denser near the poles.
- A common exam question asks for one advantage of an electromagnet over a permanent magnet — the answer is that its magnetism can be switched on and off, and often its strength can be varied by changing the current.