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5.2.6Electricity and Magnetism

Electromagnets — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 17 of 18

📚 Oxford AQA International GCSE Combined Science - Physics Hub / Topical Revision
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Part 1 of 7What is an electromagnet?

An electromagnet is a solenoid with a soft iron core.

What is an electromagnet?
When current flows in the coil, the coil's field magnetises the iron, and the core's field adds to the coil's field,
  • giving a much stronger magnet than the coil alone.
When the current is switched off, the soft iron quickly loses its magnetism, so the magnet is 'off'.
This control is the main advantage over a permanent magnet.

Part 2 of 7Strength of an electromagnet

The strength can be increased by:

1
increasing the current (larger p.d. or lower resistance);
2
increasing the number of turns of wire;
3
using a soft iron core (rather than air, plastic or steel);
4
making the core thicker or closing the magnetic circuit by bringing the poles close to the object being lifted.

The strength can be tested by counting how many paper clips or how many grams of iron filings are lifted.

Part 3 of 7Advantages over permanent magnets

1
Can be switched on and off.
2
Strength can be varied by changing the current.
3
Much stronger magnets can be made.
4
The poles can be reversed by reversing the current.

Part 4 of 7The magnetic relay

A relay allows a small current in one circuit to switch a larger current in another.

The magnetic relay
When the low-current circuit switches on, the coil becomes an electromagnet, attracts an iron armature,
  • which pivots and closes the contacts of the second circuit.
    • (for a motor or a heater)
When the coil current stops, a spring pulls the armature back and the contacts open.
The relay also isolates a safe low voltage from a dangerous mains circuit.

Part 5 of 7Electric bell and crane

Electric bell

  • pressing the push closes the circuit and the electromagnet attracts an iron armature, which makes the hammer strike the gong.
  • Moving the armature breaks the circuit at the contact screw, the electromagnet switches off, a spring pulls the armature back, the circuit remakes and the process repeats, so the hammer vibrates rapidly.

Scrapyard crane a large electromagnet picks up iron and steel scrap when the current is on, and drops it when the current is switched off.

Part 6 of 7Why the iron core helps

Soft iron contains domains that line up very easily when the coil's field passes through them.

Why the iron core helps
When they do so, each domain adds its own tiny field to the coil's, so the total field may be hundreds of times larger than that from the coil alone.
When the current is switched off, the domains return to random directions and the core stops being a magnet.
Steel would hold its domains in line and stay magnetised, so it is not suitable where switching is needed.

Design considerations

The wire is covered with insulating varnish so current passes round the turns and not directly across between turns.

Why the iron core helps
Thicker wire has lower resistance and so allows a bigger current, but heats the coil less.
The coil also becomes warm when a large current flows, so very strong electromagnets need cooling.
When the current is switched off, the collapsing field can induce a large voltage across the coil,
  • which is why switches and relays often have a protecting diode in parallel with the coil.

Worked test of electromagnet strength

A student wound 20, 40 and 60 turns on the same iron nail and counted how many pins each picked up with a fixed current of 1.0 A: 6, 13 and 19 pins.

Why the iron core helps
The count rises roughly in proportion to the number of turns, showing that the strength of the magnet increases with the number of turns.
Repeating with 0.5 A gives about half as many pins, showing that the strength also depends on the current.
To make this a fair test the nail, the wire and the pins must be the same each time,
  • and the circuit should be switched on only briefly because the wire heats up when the current flows.

Part 7 of 7Worked examples

  1. A student wants to pick up more paper clips with an electromagnet. Suggest three changes.

  2. Explain why soft iron rather than steel is used in a relay core.

Key terms

TermMeaning
ElectromagnetSolenoid with a soft iron core; magnetic only while current flows.
RelaySwitch operated by an electromagnet.
ArmatureIron piece pulled by an electromagnet.
CoreSoft iron inside a coil.

Avoid these mistakes

1
Using steel for the core of a relay.
2
Assuming an electromagnet needs direct current: it also works on AC, but then its poles reverse every half cycle.
3
Forgetting the spring in the relay and the bell.
4
Saying the relay contains a permanent magnet.

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