The invisible shape around a wire
Show on a big screenA compass needle turns when it is near a wire carrying current.
Clockwise follows the hands of a clock. Anticlockwise is the opposite way.
- No current. No field around the wire.
- Current flows toward you. Rings appear, turning anticlockwise.
- Current flows away from you. The rings turn clockwise instead.
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The rule
A current-carrying wire has a magnetic field around it. The field forms rings round the wire. Reverse the current and the rings turn the other way. A bigger current makes a stronger field. The field is weaker farther from the wire. A compass needle is a small magnet. That is why the field turns it.
A magnetic field is the space where a magnet feels a push. A compass needle shows its direction.
The right-hand thumb rule gives the direction of the field. Imagine you hold the wire in your right hand. Point your thumb along the current. Your curled fingers show the way the field goes.
Look down at the end of the wire. Toward you means coming up at you.
| Current, seen from above | The rings turn |
|---|---|
| Toward you | Anticlockwise |
| Away from you | Clockwise |
Worked example
A compass sits near a wire. It turns by 30°. Three things change its turn.
- Move the compass farther away. The field is weaker there, so the turn gets smaller.
- Raise the current. The field is stronger, so the turn gets bigger.
- Reverse the battery. The rings turn the other way, so the needle does too.
Try the lab to finish this lesson.