GATE 2020 BM – Question 62
A PMMC instrument has magnetic flux density 1.5 T, torsional spring constant 2×10⁻⁶ N m/degree, coil area 2.5 cm² and 500 turns. Find angular deflection for coil current 100 μA, in degrees to one decimal.
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Correct answer: 9.0 to 9.6
Explanation
Electromagnetic torque is NBAI. Convert coil area to 2.5×10⁻⁴ m² and current to 10⁻⁴ A. Torque=500×1.5×2.5×10⁻⁴×10⁻⁴=1.875×10⁻⁵ N m. Equating to spring torque kθ gives θ=1.875×10⁻⁵/(2×10⁻⁶)=9.375°, rounded 9.4°. The spring constant is already per degree, so no radian conversion is needed.