Physics 161 Exam 2b
PHYSICS 161 Spring 2011
Exam 2b (problems 1-15)
SIGNATURE:
.
INSTRUCTIONS:
This is a take-home exam that is due at the beginning of class, 10AM, Wednesday March 30. You may
use any source of information except another person. Please fill in your Banner ID and enter your
answers onto the Scantron form. Please sign this exam above, signifying you followed the UNM Honor
Code, and return this exam with the Scantron form Monday morning.
USEFUL: !o=8.854x10-12 C/(V m), 1/(!4!o) =8.99x109 V m/C. Surface area of sphere of radius
R: 4!R2. Volume of sphere of radius R: (4!R3)/3.
1 and 2) The capacitive network above is assembled with initially uncharged capacitors. A
potential difference, Vab = +100 V, is applied across the network.
1. The charge on the 15-"F capacitor, in C, is closest to:
a. 540 "C
b. 780 "C
c. 660 "C
d. 600 "C
e. 720 "C
2. The total energy stored in the seven capacitors, is closest to:
a. 48 mJ
b. 96 mJ
c.
120 mJ
d. 144 mJ
e. 72 mJ
3. A nonconducting sphere contains positive charge distributed uniformly throughout its volume.
Which statement about the potential due to this sphere are true? All potentials are measured
relative to infinity.
a.
The potential at the surface is higher than the potential at the center.
b.
The potential is highest at the center of the sphere.
c.
The potential at the center of the sphere is zero.
d.
The potential at the center is the same as the potential at infinity.
e.
The potential at the center of the sphere is the same as the potential at the surface.
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Physics 161 Exam 2b
4. Two large conducting parallel plates A and B are
separated by 2.4 m. A uniform field of 1500 V/m, in the
positive x-direction, is produced by charges on the plates.
The center plane at x = 0.0 m is an equipotential surface on
which V = 0. The x-coordinate, at which the electric
potential is V = +600 V, is closest to:
a.
+0.6 m
b.
–0.6 m
c.
-0.2 m
d.
–0.4 m
e.
+0.4 m
5. A conducting sphere of radius R carries an excess positive
charge and is very far from any other charges. Which one of the following graphs best illustrates
the potential (relative to infinity) produced by this sphere as a function of the distance r from the
center of the sphere?
a.
b.
d.
e.
c.
6. Two conductors are joined by a long copper wire. Thus
a.
the potential on the wire is the average of the potential of each conductor.
b.
the electric field at the surface of each conductor is the same.
c.
each carries the same free charge.
d.
each conductor must be at the same potential.
e.
no free charge can be present on either conductor.
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Physics 161 Exam 2b
7. Two identical large parallel metal sheets carry equal but opposite charges. When they are 2.00
cm apart, the potential difference between them is 20.0 V. If they are now moved closer until
they are 1.00 cm apart, the potential difference between them will be closest to:
a. 80.0 V
b. 40.0 V
c. 20.0 V
d. 10.0 V
e. 5.00 V
8. Three conducting spheres of radii 1.0 m, 2.0 m, and 3.0 m are connected by wires 50 m long,
as shown here. A charge of 4.0 x 10-4 C is initially placed
on the large sphere. Determine the charge on the large
sphere in equilibrium.
a.
1.33 x 10-4 C
b.
2.02 x 10-4 C
c.
0.67 x 10-4 C
d.
2.57 x 10-4 C
e.
zero
9. A point charge +Q is a distance d above an insulating sheet with fixed charge density ". What
is the Electric Field at point P?
a.
!
1 Q
î
4"#O d 2
b.
!
1 Q
$
î +
ĵ
2
4"#O d
2# O
c.
!
î
2" O
d.
0
e.
Cannot determine
P
y
x
d
+Q
d
+σ
10. A hollow conducting sphere has radii of 0.80 m and 1.20 m. The sphere originally carries a
charge of –500 nC. A point charge of +300 nC is present at the center. The surface charge density
on the inner spherical surface is closest to:
a.
Zero
b.
+6 x 10-8 C/m2
c.
+4 x 10-8 C/m2
d.
–6 x 10-8 C/m2
e.
-4 x 10-8 C/m2
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Physics 161 Exam 2b
11. Which of the following statements about Gauss's law are correct?
a.
If there is no charge inside of a Gaussian surface, the electric field must be zero at points of that
surface.
b.
Only charge enclosed within a Gaussian surface can produce an electric field at points on that
surface.
c.
The electric flux passing through a Gaussian surface depends only on the amount of charge inside
that surface, not on its size or shape.
d.
If zero electric flux passes through a Gaussian surface, the electric field is zero at all points on that
surface.
e.
Gauss's law is valid only for symmetric charge distributions, such as spheres and cylinders.
12. A particle of charge q and mass m is located at a position in space where the electric field
has a magnitude E. There are no other forces acting on this particle. What is the magnitude of the
acceleration on this particle at this position?
a. (qE)/m
b.
q2E
c. (qm)/E
d. qE
e. (q2E)/m
13. Which one of the following is equal to a volt?
a. coulomb
b.
(newton meter)/
coulomb
c. newton meter
d.
newton /
coulomb
e.
coulomb / newton
14. A negative point charge of magnitude –q is completely surrounded by a cardboard box of
dimensions 0.5 m x 0.5 m x 1.0 m. What is the electric flux through the cardboard box?
a.
–q/!o
b.
–q 2.5 m2 /!o
c. –q 0.25 m3/!o
d. –q 0.25 m3
e.
q2 2.5 m2
15. The indicated charge densities are placed in infinite sheets and arranged as shown in the
figure below. In which region(s) is the electric field zero?
z
a.
A
b.
B
c.
C
d.
D
e.
More than one
A
2D
B
D
C
0
D
4
σ=-30 nC/m-2
σ=-60 nC/m-2
σ=+30 nC/m-2