Consider a ring of radius r n thickness dr in d disc.
dA = 2

r.dr
charge in dA, dq = 2

r

dr
f=w/2pi
hence current, di = dq*f = 2

r

dr * w/2

= wr

dr
Magnetic field due to small ring at P is,
dB =

/2 * w

/d
2 * r
2dr
Hence, net magnetic field (assuming d<<<R),
B =
[ ]
[ dB
=

w

R
3/6d
2