WAEC Physics 2010 Paper

Concept:Sensitivity of a moving coil meter is the deflection produced per unit current. It increases when the deflecting torque per unit current increases.Explanation:The deflecting torque is given by $\tau = B I A N$.Here, $B$ is the magnetic flux density, $I$ is the current, $A$ is the area of the coil, and $N$ is the number of turns.To increase sensitivity, we need a larger deflection for a small current.This is achieved by using a stronger magnetic field, more turns of wire, a larger coil area, or a weaker spring.A soft iron core concentrates the magnetic field in the air gap and makes it stronger.A stronger magnetic field, $B$, increases the torque $\tau = B I A N$ for the same current $I$.Therefore, the soft iron core increases the sensitivity of the moving coil meter.A strong spring, low number of turns, and small coil area all reduce sensitivity.Answer:D. soft iron core