Solenoids and Electromagnets

A solenoid is a coil of wire wound into a cylindrical shape. When a current flows through a solenoid it produces a magnetic field pattern similar to that of a bar magnet. This lesson covers solenoids, electromagnets and how to control their strength. It maps to AQA GCSE Combined Science Trilogy (8464) specification section 6.7.2 — The magnetic effect of a current.

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What Is a Solenoid?

A solenoid is a long coil of wire, usually wound into a tight helix (cylinder shape). When current flows through it, the magnetic fields from each loop of wire combine to produce a strong, uniform magnetic field inside the solenoid.

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Magnetic Field Pattern of a Solenoid

The magnetic field of a solenoid has two key features:

Inside the Solenoid

• The field is strong and approximately uniform (parallel, equally spaced field lines).
• The field lines run along the axis of the solenoid from one end to the other.

Outside the Solenoid

• The field pattern is identical to that of a bar magnet.
• Field lines emerge from the north pole end and curve around to enter the south pole end.

graph LR
    subgraph "Solenoid Magnetic Field"
        S_pole["S pole end"] -->|"Uniform field INSIDE solenoid"| N_pole["N pole end"]
        N_pole -->|"Field curves outside like a bar magnet"| S_pole
    end
    style N_pole fill:#ff6666,stroke:#cc0000
    style S_pole fill:#6666ff,stroke:#0000cc

Exam Tip: The field inside a solenoid is uniform (parallel, equally spaced lines). The field outside looks like a bar magnet's field. AQA often asks you to compare the solenoid field to a bar magnet field — state they are similar outside but the solenoid field is uniform inside.

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Determining the Poles of a Solenoid

Use the right-hand grip rule for a solenoid:

1. Curl the fingers of your right hand in the direction of the conventional current flowing through the coils.
2. Your thumb points towards the north pole of the solenoid.

Alternatively, if you look at one end of the solenoid:

• If the current flows anticlockwise → that end is a North pole.
• If the current flows clockwise → that end is a South pole.

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What Is an Electromagnet?

An electromagnet is a solenoid with a soft iron core placed inside it. The iron core becomes an induced magnet when the current flows, greatly increasing the strength of the magnetic field.

Key Advantage of Electromagnets

Unlike permanent magnets, an electromagnet can be switched on and off by controlling the current. This makes electromagnets extremely useful in applications where you need to control when the magnetic field is present.

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Factors Affecting the Strength of an Electromagnet

Factor	Effect on Field Strength
Increase the current	Stronger field
Increase the number of turns (coils)	Stronger field
Add a soft iron core	Much stronger field
Decrease the current	Weaker field
Use fewer turns	Weaker field
Remove the iron core	Weaker field

Exam Tip: If asked how to make an electromagnet stronger, give three factors: increase the current, increase the number of turns, and add (or use a larger) soft iron core. Giving all three shows thorough understanding and earns full marks.

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Why Use a Soft Iron Core?

• Soft iron is a magnetically soft material — it is easy to magnetise and easy to demagnetise.
• When current flows, the iron core magnetises and adds to the solenoid's field, making it much stronger.
• When the current is switched off, the iron core demagnetises quickly, so the electromagnet can be turned off.
• Steel is not suitable because it is magnetically hard — it would stay magnetised after the current is switched off.

Core Material	Magnetises Easily?	Demagnetises Easily?	Suitable for Electromagnet?
Soft iron	Yes	Yes	Yes — can be switched on/off
Steel	No (harder to magnetise)	No (retains magnetism)	No — stays magnetised

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Worked Example

Q: A student wraps 50 turns of insulated copper wire around a soft iron nail and connects it to a battery. She uses the electromagnet to pick up paper clips. Suggest three changes she could make to pick up more paper clips.

A:

1. Increase the current — use a higher voltage battery or reduce the resistance in the circuit. A larger current produces a stronger magnetic field.
2. Increase the number of turns — wrap more coils of wire around the nail. More turns means a stronger combined magnetic field.
3. Use a larger soft iron core — a bigger core provides more magnetic material to magnetise, increasing the field strength.

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Common Mistakes

Mistake	Correction
"Use a steel core for an electromagnet"	Steel is magnetically hard — use soft iron so it can be switched off
"The field inside a solenoid is strongest at the ends"	The field inside a solenoid is uniform (same strength throughout the interior)
"Increasing wire thickness makes the field stronger"	Wire thickness does not directly affect field strength — current and number of turns do

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Summary

• A solenoid is a coil of wire that produces a magnetic field like a bar magnet when current flows.
• The field inside the solenoid is strong and uniform; the field outside is like a bar magnet.
• An electromagnet = solenoid + soft iron core. It can be switched on and off.
• Electromagnet strength increases with: more current, more turns, soft iron core.
• Soft iron is used because it is easy to magnetise and demagnetise; steel would retain magnetism.

Exam Tip (AQA 8464): Be able to draw and label the field pattern of a solenoid. Inside: parallel, equally spaced lines. Outside: curved lines from N to S, just like a bar magnet. Use the right-hand grip rule to determine which end is N.

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Why Does a Solenoid Behave Like a Bar Magnet?

A solenoid is simply many flat coils stacked together. Each loop produces a circular field; when the loops are lined up closely, their fields reinforce each other along the axis of the coil. The result:

• The fields inside each loop all point the same way along the axis, so they add up to produce a strong, uniform field inside.
• The fields outside each loop spread out and wrap around, returning to the other end — exactly the pattern of a bar magnet.

This is why a solenoid has a clearly defined N end and S end, with the axial field emerging from the N end and re-entering at the S end.

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The Right-Hand Grip Rule for a Solenoid — Step-by-Step