PHYS2424 – FALL 2000
EXAM #1 – PART 1
Electric Force, Fields, Potential, Capacitance, and Circuits
NAME ______________________________________________________
I.
Basic Equation and Definitions
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(30 points)
II.
Multiple Choice
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(80 points)
III.
Short Problems
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(15 points)
IV.
Units
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(10 points)
TOTAL PART 1
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(135 points)
TOTAL PART 2
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TOTAL POINTS
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GRADE
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I.
Basic Equations and Definitions – ( 5 marks each)
1.
Write Coulomb’s Law :
2.
Write Gauss’s Law for Electric Fields
3.
Mathematical formula used to find the electric field if one knows the electric
potential function.
4.
Lorentz Force Law For Electric Fields
5.
The definition of circuit elements in parallel
6.
Explain the difference between resistivity and resistance.
II.
Multiple Choice ( 5 marks each)
1.
A metallic sphere carrying a charge +Q is surrounded by an insulating material.
R
R
+Q
+Q
Compared to what it was without the insulator, the electric field at point R inside
the insulation is now
2.
A.
larger.
B.
unchanged.
C.
smaller.
D.
depends on the dielectric constant of the material
Consider a conducting sphere carrying a charge Q. If we replace the sphere by a
sphere of twice the radius, but also carrying a charge Q, which of the following
change(s)? (choose all that apply)
P
Q
P
Q
A.
the electric field at point P
B. the potential at point P
C.
the potential of the conductor
D. the capacitance of the conductor
3.
A negatively charged sphere is brought near an uncharged metal object. Positive
charges accumulate on the side of the uncharged metal object nearest to the
charged sphere, negative charges on the opposite side. On the uncharged metal
object, the potential is
V?
V?
-
4.
5.
V?
- -
+ +
- -
+ +
+
-
-
-
A.
largest on the positive side.
B.
largest in the middle.
C.
largest on the negative side.
D.
the same everywhere
Consider inserting a slab of dielectric material between the plates of a charged
capacitor. The electric field in the dielectric is
A.
largest near the positive plate.
B.
the same everywhere.
C.
largest near the negative plate.
Two experiments are carried out with a parallel plate capacitor (a) The capacitor
is charged to a potential difference of 100 V, disconnected from the source, and
then a slab of dielectric is inserted between the plates. (b) The capacitor is
connected to a battery which maintains a potential difference of 100 V between
the capacitor plates and then a slab of dielectric is inserted between the plates. In
which experiment does the final configuration have the largest capacitance?
A.
experiment (a)
B.
experiment (b)
C.
both the same
D.
need more information
6.
Consider the four charged particles illustrated below. Imagine a Gaussian surface
enclosing two of the four charge particles (q1 and q2).
q3
q1
q2
q4
Which charged particles contribute to the electric flux through the Gaussian
surface?
7.
A.
only q1 and q2.
B.
All four contribute.
C.
Only q3 and q4.
D.
Some other combination.
E.
Depends on the shape of the Gaussian surface.
Consider a rod carrying a uniformly distributed positive charge.
3
4
2
1
P
5
6
Which vector most closely represents the direction of the electric field at point P?
A.
Vector 1
B.
Vector 2
C.
Vector 3
D.
Vector 4
E.
Vector 5
F.
Vector 6
8.
A small positive charge is placed at point P in the electric field shown below
(represented by equipotential lines). Which way should the charge be moved if no
work is to be done on it as it moves?
3V
2V
1V
P
0V
-1 V
-2 V
9.
A.
Along the 1 V equipotential.
B.
Perpendicular to the equipotential lines.
C.
You can't avoid doing work, unless you move the charge along the 0 V
equipotential line.
D.
None of the above.
Consider the pair of charged metal spheres connected by a conducting wire shown
below. The radius of sphere A is larger than that of sphere B. Which of the
following quantities must be the same for both spheres?
A
B
A. potential at the surface
B. charge on the surface
C. field at the surface
D. surface charge density
E. more than one of the above
10.
A dipole is placed as illustrated.
y
P
x
-q
+q
The electric field at point P is
11.
A.
along +x.
B.
along -x.
C.
D.
along -y.
E.
in another direction
F.
The electric field at P is zero.
along +y
As more identical resistors R are added to the parallel circuit shown here, the total
resistance between points P and Q
R
Q
P
A.
increases.
B.
remains the same
C.
decreases.
12.
You apply a potential difference V to a resistor, causing a current to flow through
the resistor. Next, the resistor is removed from the circuit and cut in half
crosswise. One of the halves is placed back into the circuit, with the same
potential difference V applied to it. Compared to the original current, the current
through the new resistor is
A.
13.
larger
B.
smaller
C.
Same
The light bulbs in the circuit are identical. When the switch is closed,
12 V
A.
A
12 V
B
12 V
both bulbs go out.
B. the intensity of bulb A increases.
C.
the intensity of bulb A decreases.
D.
the intensity of bulb B increases.
E.
the intensity of bulb B decreases.
F.
some combination of the events described in A-E happens
G.
nothing changes.
14.
A simple circuit consists of a resistor R, a capacitor C charged to a potential Vo,
and a switch that is initially open but then thrown closed. Immediately after the
switch is thrown closed, the current in the circuit is
R
C
A.
15.
Vo / R
B.
zero
C.
need more information
Consider two identical resistors wired in series. If there is an electric current
through the combination, the current in the second resistor is
A.
equal to
B.
half
C.
smaller than, but not necessarily half
D.
larger than
the current through the first resistor.
16.
Charge flows through a light bulb. Suppose a wire is connected across the bulb as
shown. When the wire is connected,
I
A.
all the charge continues to flow through the bulb.
B.
half the charge flows through the wire, the other half continues through
the bulb.
C.
all the charge flows through the wire.
D.
none of the above.
III.
Short Problems ( 15 marks each)
1.
Draw and label the electric field lines and equipotential surfaces for the following
charge distribution.
+2 Q
-4Q
IV.
Units (1 points each)
1.
Power
2.
Permitivity ( )
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3.
Resitivity ()
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4.
Electrical Potential
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5.
Charge
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6.
Capacitance
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7.
Time Constant
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8.
Electric Field
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9.
Electric Flux
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10.
Resistance
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