Electricity and magnetism · GCSE Physics
Electromagnets
Teacher-written GCSE Physics revision on electromagnets: the solenoid field, iron cores, how current and turns change strength, and uses in scrapyard cranes, relays and circuit breakers.
A coil of wire is a solenoid. Its field looks like a bar magnet while the current is on, and it vanishes when the current is off. Iron, more turns and more current make it stronger.
The important bits
What you need to know
- 1
A solenoid is a long coil of wire. While current flows, one end behaves as a north pole and the other as a south pole. Reverse the current and the poles swap.
- 2
The field inside a solenoid is strong and almost uniform, shown as parallel lines. Outside, the pattern matches a bar magnet.
- 3
Right-hand grip for a coil: fingers in the current direction around the turns, thumb points to north.
- 4
An iron core becomes an induced magnet and makes the electromagnet much stronger. Steel would stay magnetised; soft iron is used because it demagnetises when switched off.
- 5
Strength increases with larger current, more turns, and an iron core. Increasing the coil’s length without adding turns can weaken the field.
- 6
Uses need a magnet you can switch: scrapyard cranes, electric bells, relays, circuit breakers and magnetic door locks.
- 7
A relay uses a small current in an electromagnet to close a switch in a separate high-current circuit, so a dashboard switch can control a starter motor.
- 8
When the current is switched off, the field of a solenoid with a soft-iron core disappears (or almost disappears), which is why the crane can drop the load.
Quotations worth analysing
Short evidence. Real method.
“The magnetic field of a solenoid is like that of a bar magnet.”
Like a bar magnet — while the current is on. Unlike a bar magnet, you can switch it off and reverse it.
“An iron core increases the strength of an electromagnet.”
Name current, turns and core as the three practical ways to make it stronger. “Bigger magnet” is not a method.
Go deeper
Electromagnets are magnets you can switch
A coil of wire is a solenoid. Its field is like a bar magnet while the current is on, and it vanishes when the current is off. More turns, more current, and an iron core all increase the strength. That is why scrapyard cranes, relays and circuit breakers use electromagnets: they can drop the load. Draw the field through the centre of the solenoid as straight lines, looping round the outside, and mark north and south with the right-hand grip. If the field through a coil changes, you have left the simple electromagnet and entered induced potential: the coil behaves like a tiny generator. For Combined, switching and strength are the core. For Physics, that changing field is the next topic.
Go deeper
Relays and breakers are electromagnets doing a job
In a relay, a small current magnetises a coil, which pulls an iron armature, which closes (or opens) a second circuit that can carry a much larger current. The two circuits stay electrically isolated. In a circuit breaker, a large fault current magnetises a coil strongly enough to pull a catch and spring the switch open, interrupting the live supply faster than a fuse melts, and it can be reset. An electric bell uses the same pull to break its own circuit, so the armature chatters. In each case write: current → magnetic field → force on iron → mechanical switch. That chain is the explain answer. A labelled diagram with coil, core and armature beats a paragraph that only says “it uses magnetism”.
See the idea in action
A solenoid with 200 turns carries 0.50 A and just supports a 0.40 kg scrap plate. Weight W = mg = 0.40 × 9.8 = 3.92 N, so the magnetic force on the plate is 3.92 N upwards when the current is on. Switching the current off removes the field (soft-iron core), the force falls to zero, and the plate drops. Doubling the current, doubling the turns, or adding a thicker iron core would increase the holding force. A steel core might keep some magnetism and fail to drop the load cleanly — that is why soft iron is specified.
Exam technique
Turn knowledge into marks
Label the coil, iron core, current direction and N/S poles. To increase strength, name current, turns and core. For uses, explain the switch-off: no current, no field, load released or contacts opened.
Common mistakes
Do not give these marks away
- 01
Saying the field remains after the current is switched off (unless a steel core retains magnetism).
- 02
Mixing up the jobs of more turns and a longer coil, or using copper as a “magnetic core”.
- 03
Drawing a solenoid field as circles around one wire instead of a bar-magnet pattern.
What happens to the magnetic field of a solenoid when the current is switched off?
AIt reverses direction and stays on
BIt disappears
CIt becomes a permanent field like steel
DIt only remains if the coil is copper
Show the answer
It disappears. An electromagnet’s field is caused by the current. No current means no field, which is why electromagnets can be switched. A steel core might retain some magnetism; the coil itself does not.
Quick questions
If this is the bit you searched
How can you make an electromagnet stronger?
Increase the current, increase the number of turns, and add a soft-iron core. Those are the three standard methods.
Why is soft iron used rather than steel?
Soft iron is easily magnetised and demagnetised, so the electromagnet can be switched off. Steel tends to stay magnetised.
What is a relay?
A switch operated by an electromagnet. A small current in the coil closes contacts in a separate circuit that can carry a larger current.
How do you find the north pole of a solenoid?
Use the right-hand grip: fingers follow conventional current around the coil, thumb points to north. A plotting compass will confirm it.