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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.
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Solution
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"`
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