The field pushes back
Show on a big screenA wire with current hangs in a magnet's field. It swings sideways. The thrust is the push on the wire.
- A wire sits in a field. No current, no push.
- Current flows. Field, current and thrust stand at right angles.
- Reverse the current. The thrust flips to the other side.
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The rule
A magnetic field pushes on a wire that carries current. Reverse the current or the field and the push reverses. The push is biggest when the wire is square to the field. Square to the field means at 90° to it. Fleming's left-hand rule gives the direction.
Fleming's left-hand rule: hold three left fingers at right angles.
- Forefinger: the direction of the field.
- Middle finger: the direction of the current.
- Thumb: the direction of the push.
Use the thumb, the forefinger and the middle finger. Keep each at right angles to the other two.
| Reversed | The push |
|---|---|
| Current only | Flips |
| Field only | Flips |
| Both together | Back as it was |
Each reversal flips the push once. Two flips bring it back.
Worked example
The field runs left to right. The current runs into the page. Point the forefinger right, along the field. Point the middle finger into the page, along the current. The thumb then points down, the way of the push. The wire is pushed down.
Now reverse the current. The thumb points up. The wire is pushed up.
With no current there is no push, even with the field on.
Watch out!
Use your left hand for the push on a wire. Use your right hand for the field round a wire. Use your right hand for a generator too.
Try the lab to finish this lesson.