36.

(a) Replacing i/πR

2

with J = 100 A/m

2

, in Eq. 30-22, we have



B

=



µ

0

J

2



r = 1.3

× 10

−7

T

where r = 0.0020 m.

(b) Similarly, writing i = J πR

2

in Eq. 30-19 yields



B

=

µ

0

J R

2

2r

= 1.4

× 10

−7

T

where r = 0.0040 m.