Magnetic Fields Around Wires and Solenoids

This lesson explains how an electric current produces a magnetic field around a straight wire and inside a solenoid (coil of wire), as required by the Edexcel GCSE Combined Science specification (1SC0). You will learn to predict the field direction using the right-hand rule and understand how to increase the strength of the field.

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Magnetic Field Around a Straight Current-Carrying Wire

When a current flows through a wire, a magnetic field is produced around the wire. The field has the following properties:

Property	Detail
Shape	Concentric circles centred on the wire
Direction	Determined by the right-hand rule
Strength near the wire	Stronger (circles closer together)
Strength far from the wire	Weaker (circles further apart)

The Right-Hand Rule (Grip Rule)

To find the direction of the field around a straight wire:

1. Point your right thumb in the direction of the current (conventional current: positive to negative).
2. Your fingers curl in the direction of the magnetic field lines.

Exam Tip: The right-hand rule works for conventional current (positive to negative). If a question gives electron flow, reverse the direction first before applying the rule.

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Demonstrating the Field

The field around a wire can be shown using iron filings or plotting compasses placed on a card through which the wire passes:

• Iron filings form concentric circles around the wire.
• Plotting compasses point in the direction of the field (tangent to the circles).

Reversing the current reverses the direction of the field.

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Magnetic Field of a Solenoid

A solenoid is a long coil of wire. When current flows through a solenoid, it produces a magnetic field:

• Inside the solenoid: the field is strong and approximately uniform (parallel, equally spaced lines).
• Outside the solenoid: the field pattern is identical to that of a bar magnet.

graph LR
    subgraph "Solenoid Field"
    direction LR
    A["N (left end)"] -->|"Uniform field lines inside"| B["S (right end)"]
    end
    style A fill:#4a90d9,color:#fff
    style B fill:#d94a4a,color:#fff

Identifying the Poles of a Solenoid

Use the right-hand rule for a solenoid:

1. Curl the fingers of your right hand in the direction the current flows 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.
• If the current flows clockwise → that end is a South pole.

Exam Tip: The clock trick is very handy: anti-clockwise = N, clockwise = S. Remember the letters in aNticlockwise and ClockwiSe.

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Increasing the Magnetic Field Strength

You may need to explain how to make the magnetic field of a solenoid stronger. There are three main methods:

Method	Why It Works
Increase the current	More current → stronger magnetic effect from each turn of wire
Increase the number of turns (coils)	More turns → more wire contributing to the field in the same length
Add a soft iron core	Iron is easily magnetised and concentrates the field lines (the solenoid becomes an electromagnet)

Effect of Current Direction

Reversing the direction of the current through a solenoid reverses the poles (north becomes south and vice versa). The field strength remains the same if the current magnitude is unchanged.

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Comparing a Solenoid to a Bar Magnet

Feature	Solenoid	Bar Magnet
External field shape	Identical to a bar magnet	Identical to a solenoid
Internal field	Strong, uniform	Not easily accessible
Can reverse poles?	Yes — reverse the current	No
Can switch off?	Yes — turn off the current	No
Strength adjustable?	Yes — change current, turns or core	No (fixed)

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Worked Example

A solenoid has 200 turns of wire and carries a current of 2 A. State two ways to increase the strength of the magnetic field produced.

1. Increase the current through the solenoid (e.g. to 4 A).
2. Insert a soft iron core into the centre of the solenoid.

(Increasing the number of turns of wire would also be acceptable.)

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Common Misconceptions

Misconception	Correction
A single straight wire produces no magnetic field	Any current-carrying wire produces a magnetic field (concentric circles)
The field inside a solenoid is circular	The field inside a solenoid is approximately uniform (straight, parallel lines)
Adding more turns always increases the field	Adding more turns in the same length increases the field; stretching the coil out may weaken it

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Summary

• A current-carrying wire produces a magnetic field of concentric circles around the wire.
• The right-hand rule (thumb = current, fingers = field direction) gives the field direction.
• A solenoid produces a field that is uniform inside and like a bar magnet outside.
• The solenoid's poles are found using the right-hand rule for a coil (thumb = north).
• The field is made stronger by increasing the current, increasing the number of turns or adding a soft iron core.
• Reversing the current reverses the poles of the solenoid.

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Deeper Dive: Why a Coil Produces a Stronger Field Than a Straight Wire

A single straight wire produces circular field lines that, far from the wire, rapidly weaken. But if you bend the wire into a loop, the field lines inside the loop all point the same direction (they add together), while the lines outside partially cancel. Stacking many loops together — a solenoid — concentrates this effect along the central axis.

graph LR
    subgraph "Why Solenoids Are Strong"
    A["Single straight wire: circular field"] --> B["Single loop: field reinforced inside, weakened outside"]
    B --> C["Solenoid (many loops): strong, uniform internal field"]
    end

Inside an ideal solenoid, the magnetic field strength is given by:

$B = \mu_0 n I$B=μ0​nI

where $n$n is the number of turns per unit length and $\mu_0$μ0​ is the permeability of free space. You do not need this equation for Combined Science, but the underlying relationships — field is proportional to current and to turns per unit length — must be understood.

Worked Example: Designing a Stronger Electromagnet

A student wants to build an electromagnet that can lift a 50 g steel paperclip. Their first attempt uses 20 turns of wire and a 1.5 V battery, but the electromagnet barely attracts the paperclip. Suggest three modifications.