Exam I Review.
RI-1. How many V (microvolts) in one kV (kilovolt)?
A) 1000
B) 105
C) 106
D) 109
E) None of these.
Answer: 109
RI-2. How many ohms in a coulomb?
A) one gazillion
D) None of these
B) one coullion
C) 17
Answer: None of these. The question makes no sense. It's like asking "How many
pounds in February?"
RI-3. A calculator question:
105 = ?
A) 1E5
C) something else.
B) 10E5
1E5 means 1105
RI-4. A box of mass m and net charge Q is on a frictionless
plane inclined at angle . What is the magnitude of the
electric force, F = QE, necessary to keep the box from sliding
down the incline?
m, +Q
E

A) Not enough info given to answer the question.
B) mg
C) mg /cos
D) mg sin
E) mg/ sin
N
QE
Answer: mg sin

RI-5. Does the magnitude of the normal force N depend on
the magnitude of the E-field?
A) Yes
mg
B) No
Answer: No, the normal force is N = mg cos
RI-6. Two pith balls, each of mass m, are suspended by light strings from the same pivot
point. The two balls are each positively charged, so they repel. The left ball has a larger
charge of +3Q, while the right ball has a smaller charge of +Q. Which figure most
accurately shows the equilibrium positions of the charged pith balls?
A
uncharged
+3Q
B
+Q
C
+3Q
+Q
+Q
+3Q
Answer: A By Newton's 3rd Law, each ball feels the same magnitude force. Since each
has the same mass, the angle will be the same for both.
RI-7. Three equal mass charges are released from rest at the positions shown on the xaxis. Which mass has the largest initial acceleration?
+Q
+
A
A) A
B) B
+Q
+
-Q
-
B
C
x
C) C
D) 2 of the masses have the same acceleration.
E) Some other answer.
Answer: B feels the largest size force, so it has the largest acceleration.
RI-8.
Four charges are arranged as shown, all a distance r from the origin.
What is the direction of the E-field AT THE ORIGIN?
y
-Q
r
+Q
x
-4Q
A
C
-Q
D
A) Up
B) down
E) None of these
B
C) upper right D) lower left
Answer: None of these. The E-field at the origin points right.
9. What is the magnitude of the E-field at the origin, in units if kQ/r2 ?
A) 2
B) 3
C) 4
D) None of these.
Answer: None of these. the magnitude of the E-field is 5kQ/r2.
RII-10.
A point charge +q is brought from infinity to a point b near 3 other charges +Q, -Q, and
+2Q. The charge q is brought along 3 different paths in turn, path 1, path 2, and path 3,
as shown. Along which path is the most work done by the external agent carrying the
charge +q?
+2Q
+Q
Path 1
b
Path 3
A) 1
B) 2
Path 2
-Q
C) 3
D) Same work on all three paths
Answer: Same work on all paths. W = PE = q V=q(Vb – V) = qVb.
11. Only the three charges +Q, -Q, and +2Q are present (q is absent) and the point b is
midway between +Q and -Q. The voltage at point b is...
A) positive
B) negative
C) zero
Answer: Positive.
RI-12.
TRUE (A) or FALSE (B) ?
If the voltage throughout a region of space is constant, the E-field throughout that region
must be zero?
Answer: True. |V| = E|| d. If V=0 between any two points throughout a volume of
space, the E must be zero.
RI-13. TRUE (A) or FALSE (B) ?
If the E-field in a region of space is constant, the voltage throughout that region must be
zero.
Answer: False
RII-14.
Two equal and opposite charges +q and -q are a distance r apart or a distance 2 r apart.
Which configuration (r or 2r) is the higher energy configuration?
A) separation r is higher energy
r
B) separation 2r is higher energy
+q
+q
-q
2r
C) the energy is independent of separation/don't
know
-q
Answer: separation 2r is higher (more positive) energy. For separation r, the PE = –kq2/r,
for separation 2r, PE = -kq2/(2r). Although separation r has the higher magnitude PE, it
is more negative.
15.A capacitor is attached to a battery which maintains a constant voltage V across the
capacitor plates. While the battery is attached, the plates are moved further apart. The
energy stored in the capacitor..
A) increased.
B) decreased.
V
C) remained constant.
Hint: U = (1/2)QV =
(1/2)Q2/C = (1/2) CV2
Answer: U decreases! V is fixed! (by the battery). C decreases. So U = (1/2) CV2
decreases. This is a different from the case where Q is fixed.
16. A parallel-plate capacitor has square plates of edge length L, separated by a distance
d. A second capacitor is made with L doubled and d decreased by a factor of 2.
L
L
d
By what factor is the capacitance of the new capacitor increased?
A) 1
B) 2
C) 4
D) 16
E) None of these/don't know.
Answer: C = A/d = L2/d. If L is doubled and d is halved, then C increases by 8.
17.Two capacitors, labeled 1 and 2 are identical is size and shape, except capacitor 2 has
a dielectric between the plates, while 1 has air. The same charge Q is placed on both
capacitors. Which has the higher voltage V?
A) the air-core capacitor
B) the dielectric capacitor
C=Q/V. The one with the higher V will have the smaller C (since both have same Q).
The air core capacitor has the smaller C, and the higher V.