Electromagnetism

This lesson covers how electric currents produce magnetic fields and how electromagnets work — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand the magnetic field around a current-carrying wire, the right-hand grip rule, the field of a solenoid, and the factors that affect electromagnet strength.

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Magnetic Field Around a Current-Carrying Wire

When an electric current flows through a wire, it produces a magnetic field around the wire. This is one of the most important discoveries in physics — it links electricity and magnetism.

Key Facts

• The magnetic field around a straight wire forms concentric circles centred on the wire.
• The field is perpendicular to the wire (at right angles to the direction of current).
• The field lines are closer together near the wire (stronger field) and further apart as you move away (weaker field).
• Reversing the current reverses the direction of the magnetic field.
• Increasing the current makes the magnetic field stronger (field lines closer together).

Exam Tip: The magnetic field around a straight wire is circular, not straight lines. This is different from the field around a bar magnet. Make sure you can describe and draw both types of field pattern.

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The Right-Hand Grip Rule

The right-hand grip rule allows you to determine the direction of the magnetic field around a current-carrying wire.

How to Use It

1. Point the thumb of your right hand in the direction of the current (conventional current — from positive to negative).
2. Curl your fingers around the wire.
3. Your fingers show the direction of the magnetic field (the direction the field lines circle around the wire).

Applying the Rule

Thumb Points	Fingers Curl	Meaning
Upward (current going up)	Anticlockwise when viewed from above	Field circles anticlockwise
Downward (current going down)	Clockwise when viewed from above	Field circles clockwise

Exam Tip: The right-hand grip rule is for the field around a straight wire or for determining the poles of a solenoid. For a solenoid, wrap your right hand around it with your fingers pointing in the direction of current flow — your thumb points to the north pole. Do not confuse this with Fleming's left-hand rule (which is used for the motor effect, covered in a later lesson).

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The Magnetic Field of a Solenoid

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

Key Features of the Solenoid's Field

• Inside the solenoid: the field lines are parallel, straight and evenly spaced — this means the field is uniform (the same strength and direction everywhere inside).
• Outside the solenoid: the field lines curve around from one end to the other — exactly like a bar magnet.
• One end of the solenoid acts as a north pole and the other as a south pole.
• The poles can be determined using the right-hand grip rule (thumb = north pole when fingers follow current direction).

graph TD
    A["Solenoid Magnetic Field"] --> B["Inside: Uniform field<br/>Parallel, evenly spaced lines<br/>Strong and constant"]
    A --> C["Outside: Curved lines<br/>from N to S<br/>Like a bar magnet"]
    A --> D["One end = North pole<br/>Other end = South pole"]
    D --> E["Use right-hand grip rule<br/>Thumb = North pole"]

    style A fill:#2c3e50,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

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Electromagnets

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

Advantages Over Permanent Magnets

• Can be switched on and off using an electric current.
• The strength can be varied by changing the current.
• The polarity can be reversed by reversing the current.

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Increasing the Strength of an Electromagnet

There are three main ways to make an electromagnet stronger:

Method	Explanation
Increase the current	A larger current produces a stronger magnetic field around each turn of wire
Increase the number of turns (coils)	More turns means more wire contributing to the field — the individual fields add together
Add a soft iron core	Iron is easily magnetised and greatly concentrates the magnetic field; it becomes an induced magnet that adds to the overall field

Why a Soft Iron Core?

• Iron is a magnetically soft material — it magnetises and demagnetises easily.
• This means the electromagnet can be switched on and off quickly.
• Steel would not be suitable because it is magnetically hard — it would retain its magnetism after the current is switched off (it would become a permanent magnet).

Exam Tip: If asked to explain how to increase the strength of an electromagnet, always mention all three factors: more turns, larger current, and an iron core. For full marks, explain why each works — e.g. "increasing the current increases the magnetic field strength around each turn of wire, producing a stronger overall field."

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Applications of Electromagnets

Electromagnets are used in many real-world devices:

1. Scrapyard Crane

• A large electromagnet is attached to a crane.
• When switched on, it picks up iron and steel objects (cars, girders).
• When switched off over the sorting area, the objects drop — this would not work with a permanent magnet.

2. Electric Bell

• Pressing the bell push completes a circuit, energising an electromagnet.
• The electromagnet attracts an iron armature, which pulls the hammer against the bell (ding).
• The movement breaks the circuit at the contact point, de-energising the electromagnet.
• The armature springs back, remaking the circuit — the process repeats rapidly, producing a continuous ringing.

3. Circuit Breaker

• An electromagnet in the circuit becomes stronger as the current increases.
• If the current exceeds a safe value, the electromagnet is strong enough to pull open a switch, breaking the circuit and stopping current flow.
• This protects the circuit from damage due to excessive current.

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