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192
CHAPTER 20
20.47
()
2
23
11
70.0
10
H
2.00 A
0.140 J
22
L
PE
LI
−
== ×
=
20.48
(a) The inductance of a solenoid is given by
2
0
LN
A
µ
=
l
, where
N
is the number of
turns on the solenoid,
A
is its cross-sectional area, and
is its length. For the given
solenoid,
l
( )
( )
( )
2
2
7
2
0
3
4
10
T m A
300
5.00
1
m
4.44
10
H
0.200 m
Nr
L
µπ
π
−
×⋅
==
=
×
l
0
−−
×
(b) When the solenoid described above carries a current of
0.500 A
I
=
, the stored
energy is
2
4.44
10
H
0.500 A
5.55
10
L
PE
LI
×
=×
4
J
20.49
The current in the circuit at time
t
is
(
1
t
Ie
R
)
τ
ε
−
=−
, and the energy stored in the
inductor is
2
1
2
L
LI
=
PE
(a) As
t
,
→∞
max
24 V
3.0 A
8.0
II
R
→=
=
=
Ω
, and
2
2
max

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