Section 1 of 4Structure of an Electromagnet
Solenoid with a core
- A coil of wire wound around a soft iron core, which becomes magnetised by the current and greatly increases the field.
Switchable magnetism
- The magnetism is switched on by closing the circuit and switched off by opening it, unlike a permanent magnet.
Soft iron core
- Soft iron is used because it loses its magnetism when the current stops, so the electromagnet is only temporary.
Polarity
- The poles of the electromagnet are reversed by reversing the direction of the current through the coil.
Section 2 of 4Factors Affecting Strength
Factors Affecting Strength
Current
- Increasing the current through the coil increases the strength of the magnetic field and the force on attracted objects.
Number of turns
- More turns of wire on the coil give a stronger magnetic field for the same current.
Core material
- A soft iron core greatly increases the field compared with an air core, because iron is easily magnetised.
Distance
- The field weakens with distance from the poles, so objects must be close to be attracted strongly.
Section 3 of 4Everyday Applications
1
In an electric bell, the current pulls a hammer against a gong, and the circuit then breaks so the hammer springs back.
2
In a relay, a small current in the coil closes a switch in a separate circuit that carries a much larger current.
3
In a scrapyard crane, a large current is switched on to lift iron and steel objects and switched off to release them.
4
In a circuit breaker, an excess current pulls a switch open, stopping the current in the circuit.
Section 4 of 4Exam Checklist & Tips
5.2.6 – Key Points for the Exam
- Electromagnet A solenoid with a soft iron core, whose magnetism can be switched on and off with the current.
- Soft iron core A core of easily magnetised and demagnetised iron placed inside a coil.
- Solenoid A coil of wire carrying a current to produce a magnetic field.
- Relay A switch operated by an electromagnet, used to control a separate circuit.
- Polarity The pattern of north and south poles of a magnet or electromagnet.
Common Mistakes
- Using a steel core in an electromagnet that must switch off, because steel stays magnetised.
- Saying that adding more turns of wire reduces the field strength.
- Forgetting that reversing the current reverses the poles of the electromagnet.
- Describing an electromagnet as permanently magnetised.