January 15, 2009
PHY 2054 Discussion Spring ‘09
Conceptual Review: Chapter 15
A . Coulomb Force
1.
Two Coulomb forces are exerted on a charged object and they are in different directions. How do you
calculate the net force on the object?
2.
I expect the net force in the previous question to be in the 2 nd quadrant. But my calculator shows it is
in the 4th quadrant. What should I do?
B. Electric Field
1. Does electric field change direction if you change the sign of a test charge?
2. Two positive charges are aligned on a line. Can the electric field be zero between the two charges?
Explain.
3. Two charges are aligned on a line. One is positive and the other one is negative. Can the electric field
be zero between the two charges? Explain.
C. Electric Flux
1. Electric flux for a uniform electric field is Ф = EAcosθ. θ is the the angle of the field with respect to:
2. For a closed surface, is the normal taken to be outside or inside of the surface?
D. Gauss’s Law
1. Gaussian surface should be chosen so that:
2. Point charge q is located at the center of an uncharged conducting spherical shell. Find the charge on
the inner surface and on the outer surface.
3. Consider an infinite non-conducting plane with uniform charge distribution. How do you choose a
Gaussian surface?
4. The flux through a Gaussian surface is measured to be zero. Can you conclude the electric field on the
surface is zero?
January 15, 2009
PHY 2054 Discussion – Spring ‘08
Practice Exam Problems (Chapter 15)
1.
A repelling force must occur between two charged objects under which conditions? (Properties of
Electronic Charges)
equal magnitude.
2.
a. Charges are of unlike signs.
b. Charges are of like signs.
c. Charges are of
d. Charges are of unequal magnitude.
A 6.0 μC charge is placed at the origin and a second charge is placed on the x-axis at x = 0.30 m. If
the resulting force on the second charge is 5.4 N in the positive x-direction, what is the value of its
charge? (Coulomb’s Law)
3.
a. 9.0 μC
b.9.0 nC
c. -9.0 μC
d. -9.0 nC
Four charges are at the corners of a square, with B and C on opposite corners. Charge A and D, on
the other two corners, have equal charge, while both B and C have a charge of +1.0 C. What is the charge
on A so that the force on B is zero? (Coulomb’s Law-2D)
a.-1.0 C
b. -.0.50 C
c.-0.35 C
d.
-0.71 C
4.
Charges of 4.0 μC and -6.0 μC are placed at two corners of an equilateral triangle with sides of 0.10
m. At the third corner, what is the electric field magnitude created by these two charges? (Electric Field)
a. 4.5×106 N/C
5.
b. 3.1×106 N/C
c. 1.6×106 N/C
d. 4.8×106 N/C
An initially uncharged hollow metallic sphere with radius of 5 cm has a small object with a charge
of +10 μC carefully placed at the center of the sphere. With the charge in place, what charge is now
present on the outside surface of the sphere? (Conductors in Equilibrium)
a. zero
b. -10μC
c. +4 000
μC
d. +10 μC
6.
A spherical volume of space has an electric field of intensity 100 N/C directed radially outward
from its surface of radius 0.600 m. What is the net charge enclosed within this surface? (Gauss’s Law)
a. 6.8 nC
7.
b. 4.0 nC
c. -6.8 nC
d. -4.0 nC
A closed surface contains the following point charges: 6C, 4C, -2C, -4C. The electric flux coming
out of the surface is: (Electric Flux)
a. 16 C/ε0.
Answers: 1-b
6-b
2-a
3-c
4-d
5-d
7-c
b. -16 C/ε0.
c. 4 C/ε0.
d. -4 C/ε0.