Magnetic Fields of Coils and Solenoids

This lesson provides a deeper look at the magnetic fields produced by coils and solenoids, including practical techniques for investigating these fields — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to be able to describe field patterns, explain how to strengthen the field, and relate these concepts to real-world applications.

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The Magnetic Field of a Solenoid — In Detail

As covered earlier, a solenoid is a coil of wire that produces a magnetic field when current flows through it. This lesson examines the field in more detail.

Inside the Solenoid

• The field lines are straight, parallel and evenly spaced.
• This means the field is uniform — it has the same strength and the same direction at every point inside.
• The uniform field inside a solenoid is very useful for scientific and engineering applications (e.g. MRI scanners, particle accelerators).

Outside the Solenoid

• The field lines curve around from the north pole to the south pole.
• The pattern is identical to the field around a bar magnet.
• The field is non-uniform outside — it is stronger near the poles and weaker further away.

Identifying the Poles

Using the right-hand grip rule for a solenoid:

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

graph LR
    A["Current flows<br/>through coil turns"] --> B["INSIDE solenoid:<br/>field is UNIFORM<br/>straight, parallel,<br/>evenly spaced lines"]
    A --> C["OUTSIDE solenoid:<br/>curved lines from<br/>N pole to S pole<br/>(like a bar magnet)"]
    B --> D["Right-hand grip rule:<br/>fingers curl with current<br/>thumb = NORTH pole"]
    C --> D
    D --> E["Strengthen field by:<br/>+ current<br/>+ turns per unit length<br/>+ soft iron core"]

    style A fill:#e74c3c,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#8e44ad,color:#fff

Exam Tip: The Edexcel specification requires you to be able to describe the field pattern of a solenoid and identify the poles. Remember: inside = uniform (parallel lines), outside = like a bar magnet (curved lines from N to S). Use the right-hand grip rule to find which end is north.

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Increasing the Magnetic Field Strength of a Solenoid

There are three main ways to increase the strength of the field inside a solenoid:

Method	Explanation
Increase the current	A larger current produces a stronger magnetic field around each turn of wire
Increase the number of turns per unit length	More closely packed turns mean more wire per metre contributing to the field — the individual fields from each turn add together more effectively
Add a soft iron core	The iron core becomes an induced magnet, greatly concentrating and strengthening the magnetic field

Turns Per Unit Length vs Total Turns

It is important to note that what matters is the number of turns per unit length (turns per metre), not just the total number of turns. Spreading the same number of turns over a longer solenoid makes the field weaker because the turns are further apart.

Configuration	Field Strength
100 turns over 10 cm (1000 turns/m)	Stronger
100 turns over 50 cm (200 turns/m)	Weaker
500 turns over 50 cm (1000 turns/m)	Same as first

Exam Tip: If asked how to increase the field of a solenoid, say "increase the number of turns per unit length" rather than just "add more turns." Adding more turns only helps if the solenoid does not get longer. This level of precision can earn you extra marks.

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The Magnetic Field of a Single Flat Coil

A single flat circular coil (one loop of wire) also produces a magnetic field when current flows through it:

• The field through the centre of the coil is roughly perpendicular to the plane of the coil.
• At the centre of the coil, the field lines are approximately straight and closely spaced (relatively strong).
• Outside the coil, the field lines curve around — similar to a very short solenoid (or a squashed bar magnet field).
• The field of a single coil is weaker than that of a solenoid with many turns.

Comparing Field Patterns

Arrangement	Field Inside/Through Centre	Field Outside
Bar magnet	Lines from S to N (inside the magnet)	Curved from N to S
Single flat coil	Approximately straight through centre	Curved, like a short bar magnet
Solenoid (many turns)	Uniform — straight, parallel, evenly spaced	Curved from N to S (like a bar magnet)

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Investigating Magnetic Field Patterns — Core Practical

The Edexcel specification includes a core practical on investigating magnetic field patterns. There are two main methods:

Method 1: Using Iron Filings

1. Place the magnet (or solenoid connected to a power supply) on a flat surface.
2. Place a piece of stiff white card or paper on top of (or around) the magnet.
3. Sprinkle iron filings evenly over the card.
4. Gently tap the card to allow the filings to align with the field.
5. Observe the pattern — the filings line up along the magnetic field lines.
6. Record the pattern (photograph or sketch).

Advantages: Shows the overall pattern quickly. Limitations: Does not show the direction of the field — only the shape of the lines.

Method 2: Using a Plotting Compass

1. Place the magnet or solenoid on paper.
2. Place a small plotting compass near one pole.
3. Mark the position of both ends (N and S) of the compass needle with dots.
4. Move the compass so that the S end is where the N end was.
5. Mark the new N end position.
6. Repeat until you reach the other pole or the edge of the paper.
7. Join the dots to create a smooth field line.
8. Add an arrow showing the direction (N to S).
9. Repeat from different starting positions to build up the full pattern.

Advantages: Shows both the shape and direction of field lines. Limitations: Slower than iron filings; only shows one line at a time.

Exam Tip: The core practical on magnetic field patterns is frequently examined. Be prepared to describe both methods, state their advantages and limitations, and draw the expected field patterns. The most common question asks you to describe how to use a plotting compass to map a magnetic field — include all the steps above.

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Magnetic Fields in MRI Scanners

Magnetic Resonance Imaging (MRI) machines use very powerful solenoid electromagnets to create medical images: