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By the end of these notes, you should be able to:
Whenever an electric current flows through a wire, it creates a magnetic field (an invisible region of force) in the space around it. This is one of the most important ideas in electromagnetism — electricity and magnetism are connected.
The shape and direction of the magnetic field depend on the shape of the wire and the direction of the current flowing through it.
There are three important shapes you need to know:
Before looking at each shape, you need to know one key tool: the Right-Hand Grip Rule. This rule tells you the direction of the magnetic field around a current-carrying wire.
How to use it:
Imagine gripping the wire with your right hand. Point your thumb in the direction of the current (conventional current — from positive to negative). Your curled fingers show the direction the magnetic field lines circle around the wire.
Important notation:
| Current Direction | Field Direction (viewed from above or front) |
|---|---|
| Upwards (out of page — dot) | Anti-clockwise |
| Downwards (into page — cross) | Clockwise |
Think of it this way: the dot is the tip of an arrow flying towards you; the cross is the tail feathers of an arrow flying away from you.
When current flows through a long, straight wire, the magnetic field lines form concentric circles — perfect rings centred on the wire.
Key features to draw and remember:
How to sketch it:
Draw a dot or cross in the centre (representing the wire in cross-section). Then draw at least four concentric circles around it, with the inner circles closer together and the outer circles more spread out. Add arrows in the correct rotational direction.
📝 Exam Tip: Always draw arrows on every field line. Examiners will deduct marks if arrows are missing. Also make sure the spacing between circles increases as you go outward — this shows the field weakening with distance.
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