An electromagnet is a magnet made by passing an electric current through a coil of wire (solenoid), usually wound around a soft iron core.
How an electromagnet works
When current flows through the coil, a magnetic field is produced. Adding a soft iron core concentrates the field and makes it much stronger because iron is easily magnetised. When the current is switched off, the soft iron loses its magnetism quickly (soft magnetic material), so the electromagnet can be turned on and off.
Factors affecting electromagnet strength
| Factor | Effect |
|---|---|
| Increase current | Stronger magnetic field |
| Increase number of turns of wire | Stronger magnetic field |
| Add a soft iron core | Much stronger field (core becomes temporarily magnetised) |
Electromagnets vs permanent magnets
| Feature | Electromagnet | Permanent magnet |
|---|---|---|
| Can be switched on/off | Yes | No |
| Strength can be varied | Yes (change current) | No |
| Needs electricity | Yes | No |
| Uses | Scrapyard cranes, circuit breakers, relays, bells | Compass needles, fridge magnets |
Why soft iron, not steel?
Soft iron magnetises and demagnetises quickly. Steel retains its magnetism, so it would stay magnetised after the current is switched off, defeating the purpose of a switchable electromagnet.
Applications
- Scrapyard crane: the electromagnet picks up iron and steel when the current is on and drops them when the current is switched off.
- Electric bell: the electromagnet attracts an iron armature, which strikes the bell. The circuit breaks, the armature springs back, and the cycle repeats.
- Relay: a small current in the electromagnet coil switches a separate circuit that may carry a large current.
Common errors and how to correct them
Using steel as the core material. Steel would remain magnetised after the current stops. The core should be soft iron for an electromagnet that can be switched off.
Saying the electromagnet “creates electricity.” The electromagnet creates a magnetic field when current flows through it. It does not generate electricity (that is electromagnetic induction, a different topic).
How to apply this in an exam
State the three factors that increase strength. When describing an application, explain how the ability to switch the magnet on and off is essential to its function.
Need help with this concept?
A 0625 specialist can work through the student's current question and help identify whether the difficulty is the concept, the calculation or the exam technique.