Rationalised 2023-24
Physics
120
 EXAMPLE 4 8
Solution (a) Consider the case r > a The Amperian loop, labelled 2,
is a circle concentric with the cross-section For this loop,
L  = 2 p r
Ie = Current enclosed by the loop = I
The result is the familiar expression for a long straight wire
B (2p r) = m0I
π 
0
2
I
B
r
=µ
[4