66.
(a) The magnitude of the magnetic field at the center of the loop, using Eq. 30-11, is
B =
µ
0
i
2R
=
4π
× 10
−7
H/m
(100 A)
2(50
× 10
−3
m)
= 1.3
× 10
−3
T .
(b) The energy per unit volume in the immediate vicinity of the center of the loop is
u
B
=
B
2
2µ
0
=
(1.3
× 10
−3
T)
2
2(4π
× 10
−7
H/m)
= 0.63 J/m
3
.