The
field at the centre of the coil is B2 = m0I2 / 2r2 Since the other
co-axially placed coil has a very small radius, B2 may be considered
constant over its cross-sectional area Hence,
F1 = pr 2
1B2
     
2
0
1
2
22
r
I
µ πr
=
     = M12 I2
Thus,
2
0
1
12
2
2
r
M
r
µ π
=
From Eq (6