Q. A coil in the shape of an equilateral triangle of side l is suspended between the pole pieces of a permanent magnet such that B⃗ is in plane of the coil. If due to a current i in the triangle a torque τ acts on it, the side l of the triangle is :
A) (2/√3) (τ/Bi)^(1/2)
B) (2/√3) (τ/Bi)
C) 2 (τ/(√3 Bi))^(1/2)
D) (1/√3) (τ/Bi)
Explanation
[Solution from source] With B in the plane of the coil the torque is maximum: τ = iAB with A = (√3/4)l². So l² = 4τ/(√3 Bi) and l = 2(τ/(√3 Bi))^(1/2). The trap (2/√3)(τ/Bi)^(1/2) wrongly keeps √3 outside the square root. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.