Given-

Radius of the circular wire = a

Electric current passing through the loop = i

Magnetic field Perpendicular to the plane = B

(a) Magnetic force due to presence of current on a small differential length dl given by –

where,

B= magnetic field

I = current

dl =differential length of the wire

and θ = the angle between B and dl

The direction of magnetic force,using Fleming’s left-hand rule is towards the centre for any differential length dl of the wire.

Also, dl and B are perpendicular to each other

(b) Suppose a part of loop subtends a small angle 2θ at the centre of a circular loop as shown in fig.

Then, looking into the fig. we can say

We know length l of an arc –

where ,

r is radius of the circle and the angle subtended by the arc at the center

here, the arc is subtending an angle 2θ

Since is small, sinθ will become negligible