Reversing the current
The second part of activity 13.2 reverses the current and moves the compass to the other side of the wire.
Activity 13.2 — part 2
- Swap the battery terminals so the current flows through the wire the opposite way, in the direction BA. Put the compass below the wire. The needle now turns the opposite way to when it was below the wire before.
- Now put the compass above the wire and send the current in the direction BA. The needle turns the opposite way again.
| Current | Compass | Needle turns |
|---|---|---|
| A to B | under the wire | one way (left) |
| A to B | over the wire | the other way |
| B to A | under the wire | the other way |
| B to A | over the wire | one way again |
What the activity shows
The compass deflects only when a magnetic influence acts on it — that is, when a magnetic field is set up. So a current flowing in a conductor sets up a magnetic field. The activity also shows that the direction of the field around a conductor carrying a current depends on the direction in which the current flows.
Why above and below disagree
If the wire's field simply pointed one way, like the Earth's, the compass would swing the same way wherever it sat. It does not: above and below the wire give opposite swings. The only shape that fits is a field that goes round the wire in circles. Picture a circle round the wire: at the top of the circle it points one way, at the bottom the other way, just as the rim of a turning wheel moves forward at the top and backward at the bottom. Reversing the current reverses the direction round the circle, which flips both results. The next two pages give rules for which way round it goes.
The field goes round the wire
Reversing the current, or moving the compass to the other side of the wire, reverses the deflection. The field circles the wire, and its direction depends on the current's direction.