Question

A compass needle is placed near a current-carrying wire. State your observation for the following cases, and give reason for the same in each case. 

(a) Magnitude of electric current in the wire is increased. 

(b) The compass needle is displaced away from the wire.

Answer 1

(a) Observation: The deflection of the needle increases. 

Reason: Magnetic field strength due to current-carrying wire increases as current in the wire increases, B ∝ l.

(b) Observation: The deflection in the compass needle decreases as its displacement from the current-carrying wire increases. 

Reason: The strength of magnetic field reduces with the increase in distance from the wire as B ∝ l/r.