Magnets, magnetic fields and electromagnets

Physics · Magnetism and electromagnetism. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A magnet has two poles, north and south, where its magnetic forces are strongest. Like poles repel and unlike poles attract. These are non-contact forces. A permanent magnet produces its own magnetic field. An induced magnet is a magnetic material that becomes a magnet only while it is in a magnetic field, and induced magnetism always causes attraction.

A magnetic field is the region where a magnet or a magnetic material feels a force. Field lines show its direction and strength.

A current in a wire also produces a magnetic field. Winding the wire into a solenoid makes the field stronger, and adding an iron core makes an electromagnet.

Rules to remember

Magnetic materials: iron, steel, cobalt and nickel.
Field lines point from the north pole to the south pole outside the magnet; closer lines mean a stronger field.
A compass needle is a small bar magnet; it points along the Earth's field, which is evidence that the Earth's core is magnetic.
Straight wire: field lines are concentric circles around the wire; stronger with more current, weaker further away.
Solenoid: strong, uniform field inside; outside it has the same shape as a bar magnet's field.
Stronger electromagnet: more current, more turns, an iron core.
An electromagnet can be switched on and off; reversing the current reverses its poles.

Common mistake

Students think an unmagnetised piece of iron can be repelled by a magnet. Induced magnetism always attracts; only two magnets can repel each other.

Worked example

An electromagnet with an iron core holds some iron nails. The current is switched off and the nails fall. Why would a steel core be a poor choice here?

The current in the coil produces a magnetic field, which makes the iron core an induced magnet.
With no current there is no field from the coil, and iron loses its magnetism quickly, so the nails fall.
Steel keeps its magnetism after the current stops, so a steel core would go on holding the nails.

Answer: Steel stays magnetised, so the electromagnet could not be switched off.

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