Download Show work on problems 7-12. A correct answer with

Name________________________
Physics 181 – Electricity and Magnetism: Exam 1
Important physical constants (may or may not be needed)
G = 6.6710 Nm /kg
g= 9.8 m/s2 ke = 1/4o = 8.99109 Nm2/C2
o = 8.8510-12 C2/ Nm2
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e = 1.6010 C
me = 9.1110 kg
mp = 1.6710 kg

G=giga=109, M=mega=106, k=kilo=103; c=centi=10-2, m=milli=10-3, =micro=10-6, n=nano=10-9, p=pico=10-12
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1. (12 Points) A positively charged insulating rod is brought close to an object that is suspended by a string. If
the object is attracted toward the rod we can conclude:
a. the object initially had an excess of positive charge
b. the object initially had an excess of negative charge
c. the object was initially uncharged
d. either (b) or (c) may be true
e. that none of the above are true
2. (12 Points) As used in the definition of electric field intensity, a “test charge”:
a. has zero charge
b. has charge of magnitude 1 C
c. has charge of magnitude 1.6010-19 C
d. must have a negative charge
e. none of the above
3. (12 Points) Electric field lines:
a. are vectors in the direction of the electric field
b. form closed loops
c. are trajectories of a test charge
d. cross each other in the region between two point charges
e. are none of the above
4. (12 Points) Choose the correct statement:
a. The potential of a negatively charged conductor must be negative
b. A proton tends to go from a region of high potential to a region of low potential
c. If E=0 at a point P then V must be zero at P
d. If V=0 at a point P then E must be zero at P
e. None of the above are correct
5. (12 Points) Experimenter A uses a test charge q0 and experimenter B uses a test charge 2q0 to measure an
electric field produced by stationary charges. A finds a field that is:
a. the same as the field found by B
b. greater than the field found by B
c. less than the field found by B
d. either greater or less than the field found by B, depending on the masses of the test charges
e. either greater or less than the field found by B, depending on the accelerations of the test charges
6. (12 Points) A hollow metal sphere is charged to a potential V. The potential at its center is:
a. V
b. 0
c. –V
d. 2V
e. need more information
Show work on problems 7-12. A correct answer with no work is worth 0 points.
7. (15 points)A 5.0 C point charge is placed at the center of a cube. The electric flux in Nm2/C through the
cube is:
a. 0
4
b. 7.110
4
c. 9.410
5
d. 1.410
5
e. 5.610
8. (18 points) A 200 N/C electric field is in the positive x direction. The acceleration on an electron (mass=me,
charge=-e) in this field is:
a. 3.51013 m/s2, in the positive x direction
b. 3.51013 m/s2, in the negative x direction
c. 3.210-17 m/s2, in the positive x direction
d. 3.210-17 m/s2, in the negative x direction
e. 0
9. (20 points) A 55 nC charge is fixed at the origin. A –23 nC charge is moved from x=3.5 cm on the x axis to
y=4.3 cm on the y axis. The change in potential energy is:
a. 3.1 mJ
b. –3.1 mJ
c. 60 J
d. -60 J
e. 0
10. (25 points) A proton (mass=mp, charge=+e) moves from point A to point B under the influence of
electrostatic force only. At point A the proton moves with a speed of 50 km/s. At point B the speed of the
proton is 80 km/s. Determine the potential difference V=Vb-Va
a. +20 V
b. –20 V
c. -27 V
d. +27 V
e. –40 V
11. (30 points) If Q=80 nC, a=3.0 m, and b=4.0 m in the figure below, what is the magnitude of the electric
field at point P?
a. 45 N/C
b. 70 N/C
c. 24 N/C
d. 47 N/C
e. 92 N/C
12. (30 points) A uniform linear charge density of 7.0 mC/m is distributed along the y-axis from y=2 m to y=5
m. Which of the following integrals is correct for the magnitude of the force in N on a 1 C charge placed
at the origin (with y in meters)?
5
63dy
a.  2
2 y
5
b.

2
5
c.
21dy
y2
63dy
 5  y 
2
2
2
d.
21dy
 5  y 
2
0
5
e.
21dy
 5  y 
2
2