Download Exam I, vers

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Exam I, vers. 0001 - Physics 1120 - Spr, 2001
NAME_______________________________________________________
Signature_____________________________________________________
Student ID #__________________________________________________
Circle your TA’s Name: Mike Dima, Jenn Gannon, Shawn Campbell,
Eric Hudson, Charles Baily
What time does your Tues recitation begin (8am, noon, 1pm, etc.)?
Please do not open the exam until you are told to.
Your exam should have 8 pages, numbered 1 thru 8. This exam consists of 25 questions, worth 4
points each for a total of 100 points. Fill in the bubble sheet with a #2 pencil.
You must fill out your bubble sheet according to the following instructions or you
will automatically lose 10 points. Check each box as you complete the instructions.
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Print and bubble in your Exam version, 0001 or 0002, in the upper left of your bubble
sheet in the area marked 1234.
Erase mistakes as thoroughly as possible. Ask for a fresh bubble sheet if you fear you
cannot thoroughly erase mistakes.
As you take the exam, circle the correct answers on your exam and mark them on
the bubble sheet. That way we can grade the exam, if your bubble sheet is lost.
At the end of the exam, check that you have filled in the first 25 questions on the bubble
sheet, with only one bubble filled in for each question.
I have read and followed the instructions above.
Signature______________________________________________
Vers.0001
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Possibly useful information:
e  16
.  1019 C ,  o  8.85  1012 (SI units) , k = 1/(4o) = 9.0 109 (SI units)
The following two questions refer to this situation: Two positively charged particles,
labeled 1 and 2, are placed a distance R apart in empty space and are released from rest.
Particle 1 has mass m and charge Q; particle 2 has mass 2m and charge +2Q. Each
particle feels only the coulomb force due to the other particle. (The is no gravity or other
forces in this problem.)
particle 1
m, +Q
particle 2
2m, +2Q
R
1. The initial acceleration of particle 2, immediately after release, is
kQ 2
a)
mR 2
2kQ 2
b)
mR 2
4kQ 2
c)
mR 2
2kQ 2
d)
R2
e) None of these.
2. As the particles continue to move apart after their release, the speed of each particle...
a) increases
b) decreases
c) stays the same.
3. Two particles, one with positive charge +2Q and the other with negative charge -Q
are located on the x-axis as shown below. Consider the region (marked I below) on
the x-axis some finite distance to the left of the two charges (anywhere but infinity),
I
-Q
+2Q
In this region, the electric field ..
a) is everywhere non-zero and pointing right
b) is everywhere non-zero and pointing left
c) can be zero at some location.
4. A hydrogen atom consists of one electron in orbit about a nucleus consisting of one
proton. The distance between the electron and the nucleus in a hydrogen atom is
a=0.053 nm = 0.53  10 10 m . What is the magnitude of the electric field at the
position of the electron, due to the proton?
e
e
e2
e2
a)
b)
c)
d)
e) none of these.
2
4  o a 2
4 o a
4 o a
4 o a
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The next three questions refer to this situation: Three
charges +Q, –Q, and –2Q are each located a distance r
from the origin as shown.
5. At the origin the direction of
(a)
(b)
electric field is along which
one (if any) of these 45o
diagonals ..
(d)
(c)
y
+Q
r
-2Q
r
r
(e) None of these: either the field
is zero, or it points in some other direction.
6. The magnitude of the electric field at the origin is..
a) zero
b) 2
kQ
r2
c)
kQ
r2
d)
2
kQ
r2
e) None of these.
7. What is the voltage at the origin? (As usual, set the zero of voltage at infinite
distance from the origin.)
kQ
a) zero
b) 
r
c) 
2kQ
r
d) 
4kQ
r
e) 
4kQ
r
8. A hollow spherical shell of positive charge has radius R
and a total charge +2Q spread uniformly over its
surface. The shell surrounds a negative point charge
with charge -Q at its center. What is the magnitude of
the electric field at a point a distance R/2 from the center
point charge?
2 kQ
4 kQ
kQ
12 kQ
a)
b)
c) 2
d)
2
2
R
R
R
R2
e) None of these.
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+2Q
R/2
R
-Q
-Q
x
4
The next two questions refer to the following
y
situation: A box of mass m carries an
unknown net charge q. The box is placed on a
E
a
x
rough floor with a non-zero coefficient of
m, q

kinetic friction . There is a constant, uniform
electric field E  E x parallel to the floor
everywhere and the box is found to accelerate to the right with a constant acceleration of
magnitude a.
9. What is the correct expression for the magnitude of the charge q?
ma
b) m(a  g) E
E
e) None of these
a)
c)
b g
m a g
E
d)
b g
m  ga
E
10. If the floor is frictionless (that is, if =0), what is the magnitude of the acceleration
of the box?
m
a)
b) qE
qE
c) qEm
d)
qm
E
e)
qE
m
y
11.A uniformly charged line of total length L and with
charge/length =  stretches along the x-axis from -L/2 to
+L/2. A student wishes to compute the electric field at a
point on the y-axis a distance y from the line. The student
begins by drawing the diagram shown.
dE  dEy
y
 r
The correct expression for dEy is
a)
d)
k  y dx
cx  y h
2
2 3/ 2
k  x dx
cx  y h
2
2 3/ 2
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b)
k  x dx
x2  y2
c
h
e) None of these.
c)
k y dx
 x2  y2
x dQ
x
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The next two questions refer to the diagram to
the right which shows a negative point charge
with charge –Q near a bar of metal. The
electric field in the vicinity of the point charge
and the bar are shown by the field lines in the
figure.
+
-
12.From the figure, what can you say about the
net charge on the bar?
a) Qbar = 0
b) Qbar > 0 (the bar has a net positive charge)
c) Qbar < 0 (the bar has a net negative charge )
d) Not enough information in the figure to
answer the question.
13.From the figure, what can you say about the magnitude of the charge on the bar
Q bar , compared to the magnitude of the charge Q of the negative point charge.
a) Q bar  Q
b) Q bar  Q
c) Q bar  Q
Three parallel, infinite planes(shown edge-on
in the diagram are all charged with the same
magnitude charge per area . The two planes
on the left have a positive charge density +
and the plane on the right has a negative
charge density –. The three planes divide
space up into four regions labeled I, II, III, and
IV, as shown. Note: these are planes of
charge, not charged metal plates.
+
I
+
II
-
III

14. In region IV, the electric field E is
a)
b)
c)
d)
to the right 
to the left 
zero
Answer depends on exact location in
region IV.
15. How does the magnitude of the field in region I, EI , compare to the magnitude of
the field in region IV, EIV ?
a) EI > EIV
b) EI = EIV
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c) EI < EIV
IV
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16. Two very large(effectively infinite) parallel, metal plates are placed close together
and are each charged positively with the same net charge +Q. Because like charges
repel, the outside surfaces of the plates each acquires a charge density +, while the
inside surfaces have zero

charge density, as shown.
+++++++++++++++++++++++++++++++
At point A, just outside the
bottom plate, the magnitude
=0
of the electric field is

2
a)
b)
+++++++++++++++++++++++++++++++
o
o

A

c)
d) zero
2 o
The following two questions refer to the following
situation: A thick spherical metal shell in
electrostatic equilibrium has a net positive charge
of +3Q. A positive point charge of charge +Q is
placed at the center of the charged metal shell.
net charge
+3Q
+Q
+
17.What is the net charge residing on the outer
surface of the metal shell?
a) +Q
b) +2Q
c) +3Q
d) +4Q
e) None of these.
18. Outside the shell, at a distance r from the center of the sphere(r >shell radius), what
is the magnitude of the electric field?
kQ
2 kQ
3kQ
a) 2
b) 2
c) 2
r
r
r
d)
4 kQ
r2
e) None of these.
19. A solid metal cube with a negative net charge of -Q is in
electrostatic equilibrium. Consider a spherical surface S
entirely contained within the cube and off-center as
 
shown. What can you say about the flux   E  da
through the closed surface S?
a)  > 0
b)  = 0
c)  < 0
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z
S
7
20. Compare the following two situations: Situation
B
A
A: a spherical surface S surrounding a point
charge +q at its center and no other charges
present. Situation B: same as A except now there
is another charge -q nearby, just outside the
surface S. How does the flux through the surface
 
S,   E  da , in these two situations compare?
+
+
z
S
a) A> B
b) A< B
c) A= B
21.Two points, labeled 1 and 2, are in a
uniform electric field of magnitude
15 V/m =15 N/C, as shown below. Point 1
is at location (x=2m, y=0); point 2 is at
location (x=5m, y = 4m). The zero of
voltage has been chosen so that the voltage
at point 1 is zero. To two-place precision,
what is the voltage at point 2?
a) +60 V
b) +45 V
c) +75 V
lines surrounding a point charge q at the
origin. Note that the voltages are in kilovolts and the distances in meters. What is
the value of the charge q?
b) 5.0C
d) 9.0mC
E = 15 V/m
point 1 = (2,0)
d) -75 V
22.The diagram shows the equipotential
a) 5.0 mC
c) 6.0 nC
e) none of these
point 2 = (5, 4)
e) None of these.
y
4
3
9kV
15kV
2
30kV
1
1
2
3
4
2
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x(m)
-
8
a) a
b
c
-
a
d
when the electron is moved to position..
b) b c) c
d) d e) None of these; the work done is zero.
24. The voltage at the final position of the electron is greatest (most positive) when it is
moved to position
a) a
b) b c) c
d) d
e)None of these; there is no change in voltage when the electron is moved.
25. A cubical box of edge length L is placed so that its
z
edges are parallel to the coordinate axes, as shown.,
The space in and around the box is filled with a
constant, uniform electric field of magnitude E
E
E
given by E 
yˆ 
zˆ . What is the total electric
2
2
flux  E  da through this closed surface?
a) zero
b) E L2
e) None of these.
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c) 2 E L2
d) 6 E L2
L
y
x
------- ----------
23.The work done by the external agent is greatest (most positive)
++++++++++++
The next two questions refer to the diagram at right. An electron is
held in place by an external agent (with very fine tweezers) between
two charged capacitor plates as shown. The left plate has uniform
positive charge and the right plate has uniform negative charge. The
electron can be moved by the external agent to any of the new
locations a, b, c, d.