Magnetic Fields of a Solenoid

When an electric current flows through a wire it creates a magnetic field around the wire. In this lesson you will learn about the magnetic field patterns around a straight wire and a solenoid, and how to increase the strength of an electromagnet. This is part of AQA GCSE Physics specification 4.7.

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Magnetic Field Around a Straight Current-Carrying Wire

When a current flows through a straight wire, a magnetic field is produced around the wire. The field lines form concentric circles centred on the wire.

Key Features

• The field lines are circular, centred on the wire.
• The field is strongest close to the wire (the circles are closest together).
• The field gets weaker further from the wire (the circles spread out).
• The direction of the field depends on the direction of the current.

graph TD
    subgraph "Field Around a Straight Wire (view from above)"
        W["Wire (current flowing UP out of page)"] --> C1["Circular field line (anticlockwise)"]
        C1 --> C2["Larger circular field line"]
        C2 --> C3["Even larger circular field line"]
    end

The Right-Hand Grip Rule (for a wire)

To find the direction of the magnetic field around a current-carrying wire:

1. Grip the wire with your right hand.
2. Point your thumb in the direction of the current (conventional current, from + to -).
3. Your fingers curl in the direction of the magnetic field.

Exam Tip: The right-hand grip rule for a wire tells you the field direction. Thumb = current direction, curled fingers = field direction. Do not confuse this with Fleming's left-hand rule (which is about forces, covered later).

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What Is a Solenoid?

A solenoid is a long coil of wire. When current flows through a solenoid, it produces a magnetic field pattern that is very similar to that of a bar magnet.

Key Features of a Solenoid's Magnetic Field

• Inside the solenoid, the field lines are parallel and evenly spaced — this means the field is strong and uniform inside the coil.
• Outside the solenoid, the field pattern is identical to that of a bar magnet.
• One end of the solenoid acts as a north pole and the other as a south pole.

Location	Field Strength	Field Pattern
Inside the solenoid	Strong and uniform	Parallel, equally spaced lines
At the ends	Moderate	Lines begin to diverge
Outside the solenoid	Weaker, non-uniform	Curved lines from N to S (like a bar magnet)

graph LR
    subgraph "Solenoid Magnetic Field"
        S["S pole (left end)"] ---|"Parallel field lines INSIDE (uniform)"| N["N pole (right end)"]
        N -->|"Curved field lines OUTSIDE"| S
    end
    style N fill:#ff6666,stroke:#cc0000
    style S fill:#6666ff,stroke:#0000cc

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Determining the Poles of a Solenoid

You can work out which end of a solenoid is north and which is south using the right-hand grip rule for a solenoid:

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

Alternatively, look at one end of the solenoid:

• If the current flows anticlockwise, that end is a North pole (think: the letter N has anticlockwise strokes).
• If the current flows clockwise, that end is a South pole (think: the letter S has clockwise curves).

Exam Tip: Being able to identify the poles of a solenoid is frequently tested. Learn the right-hand grip rule for a solenoid: curl fingers = current direction, thumb = north pole.

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Electromagnets

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

Why Use an Iron Core?

Iron is magnetically soft — it magnetises and demagnetises easily. This means:

• When the current is switched on, the iron core magnetises and strengthens the field.
• When the current is switched off, the iron core demagnetises and the magnetism disappears.

This is the key advantage of an electromagnet — it can be switched on and off.

Feature	Permanent Magnet	Electromagnet
Can be switched on/off	No	Yes
Field strength adjustable	No	Yes (change current)
Uses	Fridge magnets, compasses	Scrapyard cranes, circuit breakers, MRI machines

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Increasing the Strength of an Electromagnet

There are several ways to make the magnetic field of a solenoid or electromagnet stronger:

Method	Explanation
Increase the current	More current = stronger magnetic field
Increase the number of turns (coils)	More turns = stronger and more concentrated field
Add a soft iron core	Iron core becomes an induced magnet, greatly strengthening the field
Decrease the length of the solenoid	Same number of turns in a shorter length = more concentrated field

Exam Tip: If asked how to increase the strength of an electromagnet, give at least two methods. The most common answers are: increase the current and increase the number of turns on the coil. Adding an iron core is the third key method.

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Applications of Electromagnets

Electromagnets have many practical uses because they can be switched on and off:

Scrapyard Crane

A large electromagnet on a crane is used to pick up iron and steel objects. The current is switched on to pick up the metal and switched off to release it at the desired location.

Electric Bell

An electromagnet is energised when the bell push is pressed, pulling an iron armature towards it. This breaks the circuit, the armature springs back, and the process repeats rapidly — creating the ringing sound.

Circuit Breaker (Relay)

An electromagnet is used to open or close a switch in a circuit. When the current exceeds a safe level, the electromagnet becomes strong enough to trip the switch and break the circuit.

MRI Scanners

Magnetic Resonance Imaging scanners use very powerful electromagnets (often superconducting) to create detailed images of the inside of the human body.

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Comparing a Solenoid and a Bar Magnet