PHY2049
Spring 2005
1
Prof. Darin Acosta
Prof. Paul Avery
April 4, 2005
PHY2049, Spring 2005
Exam 3 Solutions
1. Which of the following statements is true about the
LR
circuit shown?
It is (1):
Before the switch is closed, there is no current flowing through the inductor. When the
switch is closed, a current will try to start flowing through the inductor, but by Faraday&s
law and Lenz&s law, the inductor will set up an emf to resist the increasing Bfield, thus
trying to keep the current zero.
Another way to see this is that immediately after the switch is closed, the voltage across
R
2
is the same as that of the battery. So the current in the branch containing the inductor
is just that for an LR circuit containing only R
1
and L.
The expression for that is:
1
1
1
10
w
h
e
n
0
R
t
L
ie
t
R
ε
−
=−
=
=
2. Suppose that the battery in the previous problem is replaced with an AC source with
emf given by
()
40sin 120
t
επ
=
. Which one of the following equations can possibly
describe the current in the inductor?
Again, the current in the inductor is given by that of a driven LR circuit containing only
R
1
and L, since the voltage across R
2
is given by the timevarying emf
40sin 120
t
=
.
The current in an inductor for a mostly inductive load (reactance from any capacitance is
(1) Just after the switch is closed R1 carries
zero current.
(2) Just after the switch is closed the two
resistors carry identical current.
(3) Just after the switch is closed the
current in R1 is greater than that in R2.
(4) A long time after the switch is closed
R1 carries zero current.
(5) If the switch is opened after being
closed for a long time the current in both
resistors immediately drops to zero.
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Spring 2005
2
negligible) always lags
the voltage:
1
tan
L
X
R
φ
=
, so the only choice among the
possibilities that shows this lag and is driven at the same frequency
is:
()
10sin 120
0.4
it
π
=−
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 Spring '08
 Avery
 Physics, Current, Magnetic Field, LR circuit, Prof. Darin Acosta Prof. Paul Avery

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