17), the magnetic energy is
2
21
UB
LI
=
=




=
(
)
1
2
2
L
B
n
nI
µ
µ
0
0
sinceB
 
for a solenoid
,
 EXAMPLE 6 9
Rationalised 2023-24
Physics
170
 EXAMPLE 6 9
=




21
0
2
0
2
(
)
µ
µ
n Al
B
n
        [from Eq (6