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 12)
2
0
1
12
21
2
2
r
M
M
r
µ π
=
=
Note that we calculated M12 from an approximate value of F1, assuming
the magnetic field B2 to be uniform over the area p r1
2 However, we
can accept this value because r1 << r2