Electromagnets

This lesson covers how electromagnets are constructed, their advantages over permanent magnets and their applications, as required by the Edexcel GCSE Combined Science specification (1SC0). Electromagnets are used in a huge number of everyday devices because they can be switched on and off.

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What Is an Electromagnet?

An electromagnet is a solenoid (coil of wire) with a soft iron core inside it. When current flows through the coil, the iron core becomes strongly magnetised. When the current is turned off, the iron core loses its magnetism almost immediately.

Component	Role
Coil of insulated wire	Carries the current that generates the magnetic field
Soft iron core	Becomes a strong temporary magnet; concentrates the field
Power supply	Provides the current to switch the electromagnet on/off

Why Soft Iron?

Property	Soft Iron	Steel
Easily magnetised?	Yes	Yes
Easily demagnetised?	Yes — loses magnetism when current stops	No — retains magnetism (hard magnetic material)
Suitable for electromagnets?	Yes	No (would stay magnetised)

Soft iron is said to be magnetically soft — it magnetises and demagnetises easily. Steel is magnetically hard — once magnetised it retains its magnetism (making it suitable for permanent magnets but not electromagnets).

Exam Tip: If a question asks why soft iron is used for the core of an electromagnet, always mention that it magnetises and demagnetises easily, so the electromagnet can be switched off.

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Controlling the Strength of an Electromagnet

Factor	Effect of Increasing It
Current through the coil	Stronger magnetic field
Number of turns of wire	Stronger magnetic field (more turns in the same length)
Inserting a soft iron core	Much stronger field (compared to an air core)

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Advantages of Electromagnets over Permanent Magnets

Advantage	Explanation
Can be switched on and off	No current → no magnetic field
Strength can be varied	Adjust the current to change field strength
Polarity can be reversed	Reverse the current direction to swap N and S

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Applications of Electromagnets

1. Scrapyard Crane

A powerful electromagnet is used to pick up scrap iron and steel:

1. The crane is positioned over the scrap metal.
2. The current is switched on — the electromagnet attracts and lifts the metal.
3. The crane moves the metal to the desired location.
4. The current is switched off — the metal drops.

A permanent magnet could not release the metal on demand.

2. Circuit Breaker (Electromagnetic Type)

A circuit breaker protects a circuit from excessive current:

1. Normal current flows through the coil — the electromagnet is too weak to attract the iron catch.
2. If the current becomes too large, the electromagnet becomes strong enough to pull the iron catch.
3. The catch releases, the contacts separate and the circuit is broken (current stops).
4. Once the fault is fixed, the circuit breaker can be reset (unlike a fuse, which must be replaced).

3. Electric Bell

graph TD
    A["Push button pressed"] --> B["Current flows through coil"]
    B --> C["Soft iron core magnetised — attracts armature"]
    C --> D["Hammer strikes gong"]
    D --> E["Armature movement breaks contact"]
    E --> F["Current stops — electromagnet demagnetises"]
    F --> G["Spring pulls armature back — contact remade"]
    G --> B

The make-and-break mechanism causes the hammer to strike repeatedly, producing a continuous ringing sound while the button is held down.

4. Relay

A relay uses a small current in one circuit to switch on a larger current in a separate circuit:

1. A small current flows through the electromagnet coil.
2. The electromagnet attracts an iron armature, which closes a switch in a second (high-current) circuit.
3. When the small current is turned off, the armature springs back and the high-current circuit is broken.

This allows sensitive components (e.g. a microprocessor) to safely control powerful devices (e.g. a motor).

Exam Tip: In relay questions, make sure you mention two separate circuits — the control (low-current) circuit and the main (high-current) circuit.

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Worked Example

Explain why a scrapyard crane uses an electromagnet rather than a permanent magnet to move scrap metal.

An electromagnet can be switched off by turning off the current. This means the scrap metal can be released when needed. A permanent magnet cannot be switched off, so the metal could not be dropped at the desired location.

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Common Misconceptions

Misconception	Correction
All magnets can be switched off	Only electromagnets can be switched on/off; permanent magnets always produce a field
Steel is used for electromagnet cores	Soft iron is used because it demagnetises easily; steel stays magnetised
A relay uses the same circuit for the control and the device	A relay uses two separate circuits — a low-current control circuit and a high-current device circuit

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Summary

• An electromagnet is a solenoid with a soft iron core.
• Soft iron is used because it magnetises and demagnetises easily (magnetically soft).
• Electromagnets can be switched on/off, have variable strength and can have their polarity reversed.
• Key applications: scrapyard crane, circuit breaker, electric bell, relay.
• Strength is increased by increasing the current, number of turns or using a soft iron core.

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Deeper Dive: Soft Iron vs Steel at the Atomic Level

Magnetically soft and magnetically hard are technical terms with specific meanings. They refer to how readily a material's magnetic domains can be rearranged by an external field.