WAEC 2010 Physics · Question 35

Which of the following factors will increase the sensitivity of moving coil meter?

Options

A.strong spring
B.low numbers of turns
C.small area of coil
D.soft iron core

Answer: D. soft iron core

Explanation written with AI and checked against the answer key

Analyze the question

Sensitivity of a moving coil galvanometer is the deflection produced per unit current:

θ/I = NBA / k

where

N = number of turns of the coil
B = magnetic flux density of the field
A = area of the coil
k = torsional constant (stiffness) of the hairsprings

Check each option

A. Strong spring: A large k reduces θ/I, so sensitivity decreases. ✗
B. Low number of turns: A small N reduces NBA/k, so sensitivity decreases. ✗
C. Small area of coil: A small A reduces NBA/k, so sensitivity decreases. ✗
D. Soft iron core: The soft iron cylinder sits inside the coil. It concentrates the magnetic field lines and makes the field radial and stronger (larger B) across the gap. A larger B increases NBA/k, so sensitivity increases. ✓

Correct answer: D, soft iron core

Exam tip: To increase sensitivity, make the top of the fraction big (N, B, A) and the bottom small (a weak spring). Remember "NBA over k". Any option that lowers N, B or A, or raises k, is wrong.

AI can make mistakes. Report anything that looks wrong.