SP212 Exam 1
Name:
1) Do not open this exam until instructed to do so.
2) This exam consists of 14 multiple choice questions. Clearly circle your answers. For some questions,
relevant information might be given in the included figure.
3) You have 50 minutes to complete this exam.
4) Do not interact with anyone except Dr. Rittenhouse during the exam.
5) Do not discuss this exam with any other students who have not taken the exam.
6) When you have completed the entire exam, write your final answers on the answer sheet provided. Be
sure to write your name at the top of the answer sheet and hand it in. These will be the answers you are
graded on. Take the rest of the exam packet with you for use in exam corrections.
1. Rank the pairs of conducting spheres according to the magnitude of charge that would be transferred
between them if the spheres were to be brought into contact, greates first.
A) (2) > (3) > (1)
B) (1) > (2) > (3)
C) (2) > (1) > (3)
D) (3) > (2) > (1)
E) (3) > (1) > (2)
2.Three charges are shown in an xy-coordiante system below. What is the net electrostatic force acting on
the charge at the origin (both magnitude and direction) by the other two charges? Assume Q > 0.
1 kQ2 ◦
A) √
45 up from the +x axis
2 a2
√ kQ2
B) 2 2 45◦ up from the +x axis
a
√ kQ2 ◦
C) 2 2 45 down from the −x axis
a
kQ2 ◦
D) 2 2 45 up from the +x axis
a
kQ2 ◦
E) 2 2 45 down from the −x axis
a
3. A charge q1 is placed on the x-axis. A second charge q2 is also placed on the x-axis a distance 4L to the
right (in the +x direction) of the first. A final third charge of size q3 is then placed at a point in between a
distance L to the left of q2 . After placing the third charge, it is found that the net force from the first two
charges acting on q3 is zero. What is the ratio of the first two charges, q1 /q2 ?
A) −9
B) −3
C) 1
D) 3
E) 9
4. A hydrogen atom consists of one electron that is approximately 5 × 10−9 cm away from a single proton.
What is the magnitude of the electric force on the electron due to the proton?
A) 9.2 × 10−8 N
B) 5.8 × 1011 N
C) 9.2 × 10−12 N
D) 4.6 × 10−20 N
E) None of these
5. A charge of size −8.0 nC is placed at the origin of an xy-coordinate system. Another charge of size 4.0 nC
is placed a distance 2.0 cm up on the positive y-axis. What is the net electric field due to these two charges
at a point 6.0 cm up on the y-axis?
A) 4.3 × 103 ĵ N/C
B) −4.3 × 103 ĵ N/C
C) 2.5 × 103 ĵ N/C
D) −2.5 × 103 ĵ N/C
E) 5 × 103 ĵ N/C
6. What is the direction of the electric field at the point P due to the two charges shown below. Assume
q > 0?
-q
d
d
+q
A) ↑
B) →
C) ↓
D) ←
~ =0
E) E
7 An electron is placed in uniform electric field of magnitude E = 1.71 × 106 N/C. If the electron starts from
rest, how long does it take to reach 1/11th of the speed of light c = 3.00×108 m/s. (choose the closest answer)
A) 9 × 10−11 s
B) 8 × 10−11 s
C) 7 × 10−11 s
D) 6 × 10−11 s
E) none of these
8. A spherical shell of radius 5.7 cm has −220 nC charge spread evenly over its surface. What is the surface
charge density σ of the shell?
A) −3.7 µC/m2
B) −4.2 µC/m2
C) −4.8 µC/m2
D) −5.1 µC/m2
E)−5.4 µC/m2
9. The figure below shows a line charge of length L which has been laid down on the y-axis. The line charge
has total charge Q evenly distributed along its entire length. Also shown a infinitesimally small piece of
~ to the
length dy a distance y up the y axis has been selected as shown. What is the small contribution dE
electric field at the point P indicated due to this infinitesimal piece?
dy
y
P
x
~ = kQ
A) dE
L
dy
(x2 + y 2 )3/2
xî +
dy
(x2 + y 2 )3/2
!
y ĵ
dy
dy
~ = kQ
B) dE
x
î
−
y ĵ
2 + y 2 )3/2
L (x2 + y 2 )3/2
(x
dy
Q dy
~
î + 2 ĵ
C) dE = k
L x2
y Q
dy
dy
~ =k
D) dE
î − 2 ĵ
L x2
y
Q
dy
dy
~
E) dE = k
xî − 2
y ĵ
L (x2 + y 2 )
(x + y 2 )
!
10. A dipole is made by placing a charge of size +3 µC is placed on the y-axis a distance 2 cm up from the
origin and a second charge of size −3 µC a distance 1 cm down from the origin on the y-axis. What this
dipole’s dipole moment?
A) P~
B) P~
C) P~
D) P~
E) P~
= −9 × 10−8 ĵ
= −3 × 10−8 ĵ
= 0 N/C
= +3 × 10−8 ĵ
= +9 × 10−8 ĵ
N/C
N/C
N/C
N/C
~ = 4000î N/C. What is
11. A dipole with dipole moment P~ = 5 × 10−30 î Cm is placed in an electric field E
◦
the change in the dipoles potential energy if it is rotated by 90 .
A) −4 × 10−26 J
B) −2 × 10−26 J
C) 0 J
D) 2 × 10−26 J
E) 4 × 10−26 J
12. An electron is to be released from rest on the central axis of a uniformly charged disk of radius R. The
surface charge density on the disk is +4.0 µC/m2 . What is the magnitude of the electron’s initial acceleration
if it is released
at a distance
2R above the disk. The electric field along the axis of a uniform disk is given by
z
σ
1− √
E=
.
20
R2 + z 2
A) 2.4 × 10−14 m/s2
B) 3.8 × 10−15 m/s2
C) 2.6 × 1015 m/s2
D) 4.2 × 1015 m/s2
E) 6.1 × 1015 m/s2
13. A square of side length 5 cm sits flat in the xy-plane. What is the magnitude of the flux of an electric
~ = (2.0î − 4.0ĵ + 1.0k̂) N/C through this square?
field E
A) 1.0 × 10−3 N m2 /C
B) 1.4 × 10−3 N m2 /C
C) 1.9 × 10−3 N m2 /C
D) 2.1 × 10−3 N m2 /C
E) 2.5 × 10−3 N m2 /C
14. What is the net electric flux through the Gaussian surface shown in the diagram below?
A) −420 Nm2 /C
B) −390 Nm2 /C
C) −340 Nm2 /C
D) −310 Nm2 /C
E) −260 Nm2 /C
Name:
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