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: Understanding the Question:
A compass needle is placed near a current-carrying copper wire and shows some deflection due to the magnetic field. The question asks how this deflection changes as the compass is moved further away from the wire.

Step 2: Key Formula or Approach:
The magnetic field (\(B\)) produced by a long, straight, current-carrying wire at a perpendicular distance (\(r\)) is given by:
\[ B = \frac{\mu_0 I}{2 \pi r} \]
This shows that:
\[ B \propto \frac{1}{r} \]

Step 3: Detailed Explanation:

• An electric current in a wire produces a magnetic field around it, which exerts a magnetic force on the compass needle, causing it to deflect.

• According to the relationship \(B \propto \frac{1}{r}\), the strength of this magnetic field decreases as the distance (\(r\)) from the current-carrying wire increases.

• At larger distances, the weaker magnetic field exerts a smaller torque on the compass needle.

• Consequently, the deflection of the compass needle decreases as it is moved away from the wire.

• At very large distances, the magnetic field of the wire becomes negligible, and the needle aligns purely with the Earth's magnetic field.


Step 4: Final Answer:
Therefore, the deflection of the compass needle decreases.