An electric current i enters and leaves a uniform circular wire of radius a through diametrically opposite points. A charged particle q moving along the axis of the circular wire passes through its centre at speed v. The magnetic force acting on the particle when it passes through the centre has a magnitude
(a) qv(μ0i/2a)
(b) qv(μ0i/2πa)
(c) qv(μ0i/a)
(d) zero.
