Physics 121 Honors
Common Exam 2
Spring 2003
Name:
Section:
Closed book exam. Only one 8
.
5
00
×
11
00
formula sheet (front and back side) can be used. Calculators are
allowed. Use the scantron forms (pencil only!) for the multiple choice problems. Circle the answers on the
examination sheet as well, and return it together with the scantron form. Use the back of these pages, or
attach your own pages with solutions for problems that require calculations.
The eight multiplechoice problems are 6 points each. Partial credit (up to 4 points) will be given for
multiplechoice problems, if you provide a detailed solution on the examination sheets. The four workout
problems are 13 points each. The nominal number of points is 100.
Clearly print your ﬁrst and last name and indicate your section number on both the scantron form and the
examination sheet.
Good luck!
MultipleChoice Problems
Problem 1:
What is the electric potential at point
P
, located at the center of the square of point charges? The
distance is
d
=
15 cm, and the charges are
q
1
= +
21
μ
C,
q
2
=

56
C,
q
3
= +
47
C, and
q
4
= +
21
C.
A)

7
.
6
×
10
5
V
B)

5
.
4
×
10
5
V
C)
+
2
.
0
×
10
6
V
D)
+
2
.
8
×
10
6
V
E)
+
1
.
9
×
10
7
V
r
=
d
√
2
V
=
1
4
πε
0
r
(
q
1
+
q
2
+
q
3
+
q
4
)
=
√
2
4
0
d
(
q
1
+
q
2
+
q
3
+
q
4
)
=
2
.
80
×
10
6
V
Problem 2:
The Bohr model of the Hydrogen atom states that the single electron can exist only in certain
allowed orbits around the proton. The radius of each Bohr orbit is
r
=
r
0
n
2
, where
r
0
=
0
.
0529 nm and
n
=
1
,
2
,
3
,...
. Calculate the electric potential energy of a hydrogen atom when the electron is in the third
(
n
=
3) allowed orbit!
A)
3.0 eV
B)
6.8 eV
C)
9.1 eV
D)
27 eV
E)
6.3 GeV
U
=
eV
=
e
±
e
4
0
r
0
n
2
¶
=
3 eV
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View Full DocumentProblem 3:
A capacitor consists of two concentric shells of inner radius
a
=
2
.
0 cm and outer radius
b
=
5
.
0 cm. The capacitor has a charge of
q
=
24
.
6 nC. How much energy is stored in the electric ﬁeld of
this charged conductor?
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 Spring '08
 Opyrchal
 Physics, Capacitance, Electric charge, Parallel Plate Capacitor, NC, PWire

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