Far from the wire, weaker field

The idea

The magnetic field around a current-carrying straight wire is represented by concentric circles that grow larger away from it; the field produced by a given current decreases as the distance from the wire increases.

Moving the compass

A steady current flows in a straight wire. A compass near the wire is deflected a lot. You slowly move the compass away from the wire.

What happens to the deflection?

• It gets smaller
• It gets larger
• It stays the same

Move away, move near

Slide the compass away from the wire and back. Watch the circles, the needle and the bar.

Circles and distance

Around a straight wire the magnetic field lines are circles with the wire at the centre. Far from the wire the circles are larger. The current stays the same, but the field is weaker the farther the compass is from the wire.

The magnetic field around a current-carrying straight wire is represented by concentric circles that grow larger away from it; the field produced by a given current decreases as the distance from the wire increases.

Notes

Around a straight wire the field lines are concentric circles, larger farther out, and the field decreases as the distance from the wire increases.

Check yourself

The compass is moved farther from a wire carrying a steady current. The magnetic field at the compass…

The magnetic field lines around a straight current-carrying wire are…

The deflection of a compass needle decreases when the compass is moved away from the wire, with the same current. What does this show?

• increases. The field of a given current decreases as the distance from the wire increases.
• decreases — correct. Yes!
• stays the same. The deflection of the needle gets smaller when the compass is moved away, so the field changes. It decreases.
• straight lines pointing away from the wire. The field lines about a straight wire are concentric circles.
• circles that become smaller away from the wire. The concentric circles become larger and larger as we move away from the wire.
• concentric circles that become larger away from the wire — correct. Yes!
• The field decreases as the distance from the wire increases — correct. Yes!
• The current in the wire has decreased. The current stays the same when only the compass is moved. The field of a given current gets weaker with distance.
• The field is the same at every distance. If the field were the same at every distance, the deflection would not change. The field decreases as the distance increases.