For this, we consider a plane circular loop of
radius r whose plane is perpendicular to the direction
of the current-carrying wire, and which is centred
symmetrically with respect to the wire [Fig 8 1(a)] From
symmetry, the magnetic field is directed along the
circumference of the circular loop and is the same in
magnitude at all points on the loop so that if B is the
magnitude of the field, the left side of Eq