Physics 21
Fall, 2008
Hour Exam #1
1
2
3
4
5
Total
Name:
Recitation Time
Recitation Leader
October 3, 2008
This exam is closed notes and closed book. You must show enough work on each problem to convince the grader
you understand how to solve the problem. You may use a calculator, but
show every number that you use
to get numerical results. The penalty for arithmetic errors is small if the grader can tell what you
intended to do.
Give units for all final answers. There is an equation sheet on the last page. All problems count
20 points.
Problem 1.
Consider the following circuit:
2
Ω
I
2
32 V
40 V
4
Ω
5
Ω
I
1
I
3
6
Ω
a
b
c
d
(a) (12 pts.) Write the loop and junction equations
needed to determine the currents
I
1
,
I
2
, and
I
3
.
You should have three equations in three un
knowns.
Indicate clearly the loop used to
determine each loop equation.
(b) (4 pts.)
Determine the currents
I
1
,
I
2
, and
I
3
(including the correct sign) by explicit solution
of the equations.
You must show your work.
(c) (4 pts.)
Find the potential difference
V
a

V
d
between the points labelled
a
and
d
on the di
agram. Use the calculated values of the currents
and
show how you get your answer.
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Problem 2.
A point charge
q
1
=

4
.
00 nC is at the
point
x
= 0
.
20 m,
y
= 0
.
10 m, and a second point
charge
q
2
= +6
.
00 nC is at the point
x
= 0
.
70 m,
y
=
0
.
30 m.
The coordinates of the point
P
are
x
= 1
.
00 m,
y
= 0
.
70 m
x
y
q
1
q
2
P
= (1.00,0.7)
1.0
1.0
Distances are in meters.
(a) (4 pts.) Give the general formula for the electric
field
E
at any point
r
due to a charge
q
at the
point
r
.
(b) (4 pts.) Calculate the
x
and
y
components of the
electric field
E
1
at the point
P
due to charge
q
1
.
(c) (4 pts.) Calculate the
x
and
y
components of the
electric field
E
2
at the point
P
due to charge
q
2
.
(d) (4 pts.) Calculate the
x
and
y
components of the
net electric field
E
at the point
P
due to these
two point charges.
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 Spring '07
 Hickman
 Electron, Mass, Work, Trigraph, NC, Ri, EY

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