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Physics 21
Fall, 2010
Solution to Hour Exam #1
The graders for the problems were:
1 Jones, 2 Faust, 3 Malenda, 4 and 5 Beels
For questions about the grading, see the grader by Oct. 27.
Problem 1.
Consider the following circuit:
2
Ω
I
1
11 V
21 V
7
Ω
3
Ω
I
2
I
3
ab
cd
loop 1
loop 2
loop 3
(a) (13 pts.) Write Kirchho±’s loop and junction (or
node) equations needed to determine the currents
I
1
,
I
2
,and
I
3
.
Use the currents and their di
rections speci²ed by the arrows in the diagram.
You must write these equations as defned
in Physics 21.
You should have three equations
for three unknowns.
Draw clearly and label
on the diagram the loop used to determine
each loop equation.
(b) (3 pts.)
Determine the currents
I
1
,
I
2
I
3
(including the correct sign) by explicit solution of
the equations you determined in part (a).
You
must show your work.
(c) (2 pts.) Use the currents you calculated in part
(b) to ²nd the potential di±erence
V
d

V
a
be
tween the corner points labelled
d
and
a
on the
diagram, using the path through corner point
b
.
Show your work.
(d) (2 pts.)
Repeat part (c), but take the path
through corner point
c
. Show your work, includ
ing the potential change across the resistor and
battery separately. You should get the same value
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 Spring '08
 Hickman
 Current

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