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General Physics – PH 213
Midterm I (Ch 25 - 27)
October 24, 2008
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Name:____________
Exam is closed book, closed notes. Use only your note card.
Write all work and answers in the color papers provided.
Show all your work and explain your reasoning (except for true/false or multiple choices
ones)
Partial credit will be given. No credit will be given if no work is shown (except for
true/false or multiple choices ones)
Part I – True or False: For questions 1 – 8, state whether each statement is true or false.
1. The electric field of an infinite sheet of charge is the same everywhere above the sheet.
2. Any charge q can be expressed as q   ne where n is an integer and e is the charge of an
electron.
3. An electron moving in +x direction is slowed down when it enters the field E  E0 iˆ .
4. If the net charge on a conductor is zero, its surface charge density must be uniform.
5. If the net flux through a closed surface is zero, then there can be no charge within that
surface.
6. Units of electric fields are newtons per coulomb, N/C.
7. Inside a conductor the electric field is always zero.
8. A charged insulator and uncharged metal always attract each other electrostatically when
brought near each other.
Part II – Multiple choice questions: For questions 9 – 16 select the one alternative that best
completes the statement or answers the question.
9. The units of electric dipole moments are
A) N/m
B) C2m
C) N/C
D) C2/m
E) mC
10. A positive point charge is placed at the center of a spherical conducting shell with radii
Ri and Ro as shown. Which of the following statements is true for the electric field
vector?
Ro
A) E  0 for r < Ri and E = 0 for r > Ro .
B) E  0 for r > Ro and E = 0 for r < Ri .
C) E  0 for r < Ri and for r > Ro .
+q
D) E  0 for Ri < r < Ro .
E) The E is zero everywhere.
Ro . Ri
.
11. When a negatively charged conductor touches a neutral conductor, the neutral conductor will
A) gain protons.
D) lose electrons.
B) lose protons.
E) gain electrons.
C) stay neutral
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12. Particle A of charge +2q is located at the origin, and particle B of charge +q is located on
the positive x-axis. The electric force on the particle B due to particle A is represented by
vector F. Which vector represents the force on particle A due to particle B?
+2q
+q
F
A
B
A)
B)
C)
D)
F
-F
2F
-2F
E) 1/2 F
F) -1/2 F
G) The force is zero.
13. A charged particle of mass m is suspended between the horizontal plates of a charged
capacitor. Which of the following statements is not true?
A) The magnitude of the electric force on the
particle is equal to its weight.
––––––––––––––––
B) The electric field between the plates
point downward.
C) The plates are at different potentials.
D) The particle is charged positively.
+++++++++++++++++++++
14. A uniform spherical shell of charge of radius R surrounds a point charge at its center. The
point charge has value Q and the shell has total charge -Q. The electric field at a distance
R/2 from the center
A) is zero
B) does not depend on the charge of the spherical shell
C) is half of what it would be if only the point charge were present
D) is directed inward if Q>0.
15. A conducting rectangular plate is connected to ground through a switch. The switch is
initially closed. A negative charge Q is brought near the plate as shown, but the charge is
not brought in contact with the plate. While the charge is near the plate, the switch is
opened and then the charge is removed. What is now the charge state of the plate?
A) It is negatively charged
B) It is positively charged.
C) It is uncharged.
D) It can be any of the above depending
–Q
on the initial charge on the plate before
the charge +q was brought nearby.
16. A charge Q is uniformly distributed throughout a nonconducting sphere of radius R. The
charge density in the sphere is
Q
4R
3Q
B)
4R 3
A)
Ph 213 Midterm1 LBCC F08
Q
4R 2
4Q
D)
4R 2
C)
E) Not given
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Part III – Questions and Problems
Please explain or show your work clearly for each of the following questions or problem.
17. The figure shows four spheres, each
with charge Q uniformly distributed
through its volume.
A) Rank the spheres according to
their volume charge density,
greatest first. The figure also
shows a point P for each sphere,
all at the same distance from the center of the sphere.
B) Rank the spheres according to the magnitude of the electric field they produce at
point P, greatest first
18. A positively charged rod is held near, but not touching a neutral
metal sphere.
A) Add plusses and minuses to the figure to show the charge
distribution on the sphere.
B) Does the sphere experience a net force? If so, in which
direction. Explain
19. A point charge +Q is placed at the center of the conductors.
Find the induced charges for all surfaces (i.e inside1, I1,
outside1, O1, and inside2, I2 and outside2, O2).
20. An 8.85 x 10-12 C point charge sits at the origin of a
coordinate system. What is the flux of electric field
through a 10 cm tall right circular cylinder of diameter 6.0
cm centered on the origin?
21. Five +1.0 nC point charges are equally spaced around a
semicircular arc as shown. An electron sits at the center
of the semicircle.
A) What is the electric field at the center of the
semicircle?
B) What force does the electron experience?
2a
has a volume
b
charge density  (r )  ar  br 2 , where a and b are
constants.
What is the total charge contained in the volume of sphere?
What is the electric field at a distance of r from the center of the sphere.
What is the electric field at a distance of 2R from the center of the sphere.
At what radius does the electric field vanish?
22. An insulating sphere of radius R 
A)
B)
C)
D)
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23. Two curved plastic rods with uniform linear
charge densities form a semicircle as shown.
One rod carries a total charge +Q and the
other –Q uniformly distributed over their lengths.
Find the magnitude and direction of the electric
field at point P, the center of the semicircle.
–Q
R
P
x
+Q
24. (Bonus Problem) An electron is initially traveling at 3.0 x 106 m/s upward when it enters
a uniform electric field of 200 N/C directed to the left. What is the velocity of the
electron after it has traveled to a point that has a vertical coordinate that is 10.0 cm
upward of where the electron entered the field?
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