In figure, AB is a non-conducting rod. Equal charges of magnitude q are fixed at various points on the rod as shown. The rod is rotated uniformly about an axis passing through O and perpendicular to its length such that linear speed of the end A or B of the rod is 3 mis. Magnetic field at O is

Since the rod is a rigid body, every point has same angular velocity ω=1  rad/s.For points at a distance of 1 m, v1 = 1 m/sFor points at a distance of 2 m, v2 = 2 m/s and for points at a distance of 3m , v3=3msIf a charge rotating in a circle constitutes a current I,B= μ0i2r,  where i  qt = qv2πrSince field due to rotation of all charges are in same directionB= ∑μ0qv4πr2=μ0q4π   1+1+12 +12 +13 +13⇒B=  11μ0q12π