Question

A moving-coil galvanometer has a 50-turn coil of size 2 cm × 2 cm. It is suspended between the magnetic poles producing a magnetic field of 0.5 T. Find the torque on the coil due to the magnetic field when a current of 20 mA passes through it.

Answer 1

Given : 

Number of turns in the coil, n = 50
Area of the cross section of the coil, A = 2 cm × 2 cm = 2 × 2 × 10⁻⁴ m²

Current flowing through the coil, `I = 20 × 10^−3 A`

Magnetic field strength due to the presence of the poles, B = 0.5 T

The torque experienced by the coil placed in an external magnetic field `(tau)` is given by 

`τ = nI (vec A xx vec B)`

⇒ `τ = nIAB  sin 90^circ`

⇒ `τ = 50 xx 20 xx 10^-3 xx 4 xx 10^-4 xx 0.5`

⇒ `τ = 2 xx 10^-4  "N - m"`