Electromagnetism — Current-Carrying Conductors

When an electric current flows through a conductor, it produces a magnetic field around it. This discovery — that electricity and magnetism are linked — is called electromagnetism. This lesson covers the magnetic field around a straight wire and around a flat circular coil. It maps to AQA GCSE Combined Science Trilogy (8464) specification section 6.7.2 — The magnetic effect of a current.

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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.

Key Features

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

graph TD
    subgraph "Field Around a Straight Wire — Top View"
        W["Wire (current flowing OUT of page ⊙)"] --> C1["Inner circle — strong field"]
        C1 --> C2["Middle circle"]
        C2 --> C3["Outer circle — weaker field"]
    end

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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 conventional current (from + to −).
3. Your fingers curl in the direction of the magnetic field.

Thumb Direction (Current)	Fingers Curl (Field Direction)
Upward (out of page ⊙)	Anticlockwise when viewed from above
Downward (into page ⊗)	Clockwise when viewed from above

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

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Magnetic Field Around a Flat Circular Coil

When the wire is bent into a flat circular coil (a single loop), the magnetic field has a different pattern:

• Each small section of the coil produces concentric circular field lines (as for a straight wire).
• These combine to produce a field that passes through the centre of the loop perpendicular to the plane of the coil.
• The field at the centre of the loop is relatively strong and uniform.

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Factors Affecting the Strength of the Magnetic Field

Factor	Effect
Increase the current	Stronger magnetic field
Decrease the current	Weaker magnetic field
Move closer to the wire	Stronger field (field lines closer together)
Move further from the wire	Weaker field (field lines further apart)

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Demonstrating the Magnetic Field of a Wire

Method

1. Pass a straight wire vertically through a horizontal piece of card.
2. Connect the wire to a power supply and switch on the current.
3. Sprinkle iron filings on the card and tap gently.
4. The filings arrange themselves in concentric circles around the wire.
5. Place a plotting compass on the card to determine the direction of the field.

Exam Tip: If asked to describe how you would show the field around a wire, mention both iron filings (for the shape) and a plotting compass (for the direction).

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Worked Example

Q: A student sets up a straight wire carrying a current vertically through a piece of card. She sprinkles iron filings on the card. Describe and explain the pattern she observes.

A: The iron filings form concentric circles centred on the wire. This is because the current flowing through the wire produces a magnetic field around it. The field lines are circular, and the iron filings, which are small magnetic materials, align with the field to show its shape. The circles are closer together near the wire because the field is stronger close to the wire.

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Common Mistakes

Mistake	Correction
"The field lines around a wire are straight"	They are concentric circles
Using the wrong hand for the grip rule	It is the RIGHT-hand grip rule
Confusing the grip rule with Fleming's left-hand rule	Grip rule = field direction around a wire; Fleming's LHR = force on a wire in an external field
"The field is the same strength everywhere"	The field is strongest near the wire

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Summary

• A current-carrying wire produces a magnetic field of concentric circles around it.
• The right-hand grip rule gives the direction: thumb = current, fingers = field direction.
• The field is strongest close to the wire and weaker further away.
• Increasing the current makes the field stronger.
• A flat circular coil produces a field that passes through the centre of the loop, perpendicular to the coil.

Exam Tip (AQA 8464): Be ready to draw the concentric circle field pattern around a straight wire and to use the right-hand grip rule to determine the direction. AQA may show a cross-section view with ⊙ (current out of page) or ⊗ (current into page).

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Historical Context: Oersted's Discovery

In 1820, Danish scientist Hans Christian Ørsted noticed that a compass needle moved when placed near a current-carrying wire. This was the first experimental evidence that electricity and magnetism are linked — a unification later formalised by Maxwell and at the heart of all electromagnetic technology. For GCSE you do not need dates, but understanding that a moving electric charge produces a magnetic field is essential for explaining electromagnets, motors and transformers later in the topic.

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Reading the Symbols ⊙ and ⊗

AQA exam diagrams commonly show current flowing through wires viewed in cross-section:

Symbol	Meaning	Think of it as…
⊙ (dot in circle)	Current flowing out of the page	The tip of an arrow flying towards you
⊗ (cross in circle)	Current flowing into the page	The feathers of an arrow flying away from you