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Physics 212
Exam I
NAME:_________________________
Useful Constants:
k  9.0  109 N  m 2 C2
0  8.85  1012 C2 N  m 2
e  1.6  1019 C
Part I: Multiple Choice(4 points. ea.)
Read each question carefully, choose the best answer
"none of the above" may can be a valid answer
Fall 2001
MC
P
me  9.1  10 31 kg
m p  1.67  10 27 kg
1eV  1.6  1019 J
___ 1 . A rod with an unknown charge attracts a pith ball (as in the attempted lecture demonstration). Which of
the following could describe the situation?
(A) The rod has negative charge and the pith ball has positive charge.
(B) The rod has positive charge and the pith ball has negative charge.
(C) They both have positive charge.
(D) They both have negative charge.
(E) Both A and B above.
___ 2. Total electric charge in a closed system is conserved
(A) always.
(B) never.
(C) only in conductors.
(D) except within conductors.
(E) electric charge conservation was never discussed in this class.
___ 3. An electron has negative charge
(A) means that the electric force on the electron and the electric field are in the same directions.
(B) as a consequence of the conventions set by Thomas Jefferson.
(C) results in an attractive force between the electrons and the positively charged protons in an atom's
nucleus.
(D) all of the above.
(E) none of the above.
___ 4. A perfectly spherical gaussian surface has a an electric dipole (two charges (Q ) of
equal size but opposite signs separated by a distance l ) at its geometric center. The
total Electric flux through the gaussian surface is
(A) Q/o.
(B)
Q/2o.
(C)
Q/6o.
(D)
0.
(E)
much more information is necessary to answer.
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Physics 212
Exam I
Fall 2001
___5. The electric field lines at a certain location
(A) are to perpendicular lines (surfaces) of constant electric potential at that location.
(B) are parallel to the electric force on a test charge placed at that location.
(C) never cross.
(D) start on positive charges and end on negative charges.
(E) all of the above.
___ 6. The electric field magnitude near (but outside of) an irregularly shaped conductor
(A) will be greatest where the surface has the sharpest curvature.
(B) will be greatest where the surface has the flattest curvature.
(C) does not depend upon the shape of the conductor.
(D) will necessarily be zero.
(E) trick question; since the electric field is a scalar quantity, the term “electric field magnitude” is
meaningless.
___ 7. The electric potential is
(A) is simply electrical energy.
(B) is simply electrical charge.
(C) is potential energy per unit charge.
(D) is electrical force per unit charge.
(E) always zero within conductors.
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Physics 212
Exam I
Fall 2001
Part II Short Questions (6 points each)
1) What are the qualitative similarities and differences between Coulomb's Law and Newton's Law of
Universal Gravitation. (be brief and specific)
2) Draw the electric field lines for the two conductors below (the charges on each sphere are equal, and both
positive)
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Physics 212
Exam I
Part II Problems
(20 points each).
Show all work. No work = no credit!
1) Three source charges are placed as shown. Q1 is located
at (0,3m) and has a charge of +120C. Q2 is located at (0,0)
and has a charge of -80C. Q3 is located at (4m,0) and has a
charge of 40C.
Q1
a) Calculate the Electric Field (x and y components) at the
point (4m,3m).
b) Calculate the electric potential at the point (4m,3m).
Fall 2001
A test charge of 30 C is placed at the point (4m,3m).
c) What is the force on this charge?
d) What is its (electrostatic) potential energy?
Q2
Q3
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Physics 212
Exam I
Fall 2001
2) A solid conducting sphere of radius a carries a charge q. It is inside a
concentric hollow conducting sphere of inner radius b and outer radius c. The
outer sphere carries a net charge of –q.
(A) Give an expression for the electric field in terms of the distance r from the
center of the spheres for the following regions:
a
b
c
ra
arb
brc
E(r)
cr
(B) Draw a graph of the electric field magnitude for 0  r c
r
a
b
c
(C) What is the charge on the inner surface of the hollow field?
(D) What is the charge on the outer surface of the hollow field?
(E) Using your expression for E(r) for the region a  r  b , determine Vab by evaluating the final term in
b 
 b
Vab   E  dl   E (r )dr
a
a
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Physics 212
Exam I
Fall 2001
3) An alpha particle (q = +2e, m = 6.68x10-27kg) is accelerated from rest across a potential difference of 20kV.
a) What is the kinetic energy (in electron volts) of the alpha particle as it leaves the accelerator?
b) What is this energy in Joules?
c) What is the speed of the alpha particle ?
The alpha particle now reaches a region with a uniform electric field produced by parallel conducting
plates separated by 2cm with a potential difference of 2000V between them, producing a upward directed
electric field which extends 4 cm in the horizontal direction.
d) What is the electric field magnitude in the region between the plates?
e) How long is the alpha particle in this region?
f) What is the acceleration of the alpha particle while in this region?
g) What are the x and y components of velocity of the alpha particle as it leaves the region?
4 cm
"alpha" gun

E
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