Question

What happens to the deflection of the compass needle when it is moved away from a copper wire carrying a constant current?

• A. The deflection increases.
• B. The deflection remains the same.
• C. The deflection decreases.
• D. The needle becomes magnetic.

Hint:

The strength of the magnetic field is inversely proportional to the distance from the wire:
\(B \propto \frac{1}{r}\).
As distance \(r\) increases, field \(B\) decreases, so the deflection of the needle must decrease.

Answer 1

The Correct Option is C

Step 1: Recall how field strength falls with distance.
The magnetic field of a straight current carrying wire follows \[ B = \frac{\mu_0 I}{2\pi r} \] so \(B\) is inversely proportional to the distance \(r\) from the wire.
Step 2: Connect field strength to needle deflection.
A stronger local field pulls the compass needle away from north by a larger angle, and a weaker field pulls it away by a smaller angle.
Step 3: Move the compass farther out.
As \(r\) grows, \(B\) shrinks, so the torque on the needle shrinks too, and the needle swings back closer to its original north south position, meaning the deflection gets smaller.
\[ \boxed{\text{The deflection decreases}} \]