PHY2054 Spring 2008
1
Prof. P. Kumar
Prof. P. Avery
April 3, 2008
Exam 3 Solutions
1. (
Ch. 20
) In the circuit shown,
L
= 56 mH,
R
= 4.6
Ω
and
V
= 12.0 V. The switch
S
has been
open for a long time then is suddenly closed at
t
= 0. At what value of
t
(in msec) will the current
in the inductor reach 1.1 A?
(1) 6.67
(2) 10.5
(3) 2.88
(4) 19.0
(5) None of these
The current in an RL circuit, starting at 0, is
(
)
/
max
1
t
i
i
e
τ
−
=
−
, where
/
L
R
τ
=
and
max
/
i
V
R
=
and V is the voltage. Here i
max
= 12.0 / 4.6 = 2.609 and
τ
= 0.056 / 4.6 = 12.17
msec.
The solution is
(
)
max
ln 1
/
6.67
t
i i
τ
= −
−
=
msec.
2. Refer to the previous problem. What is the total energy stored in the inductor a long time after
the switch is closed?
(1) 0.19 J
(2) 0.048 J
(3) 0.76 J
(4) 0.034 J
(5) None of these
After a long time, the inductor can be ignored and the current in the inductor is i
∞
= 12.0/4.6 =
2.61 A.
The energy in the inductor is
2
1
2
0.19
L
U
Li
∞
=
=
J.

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