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, ${\displaystyle I=20\times {10}^{-}3A}$

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 ${\displaystyle \left(\tau \right)}$ is given by 

${\displaystyle \tau =nI(\overrightarrow{A}\times \overrightarrow{B})}$

⇒ ${\displaystyle \tau =nIAB {\sin 90}^{\circ }}$

⇒ ${\displaystyle \tau =50\times 20\times {10}^{-3}\times 4\times {10}^{-4}\times 0.5}$

⇒ ${\displaystyle \tau =2\times {10}^{-4} \text{N - m}}$