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Concept Tests -- Final Review
CTF-1. Calculator question:
105 = ?
A) 1E5
B) 10E5
C) something else.
CTF-2. A cube is made larger and larger...
area
As the cube grows, the ratio volume ..
A) increases
B) decreases
C) remains constant.
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CTF-3. A box with mass M and charge +Q is pushed along
a rough surface at constant velocity by a uniform horizontal
electric field of magnitude E. The coefficient of kinetic
friction between the box and the floor is µ. What is the net
force on the box?
A) QE
B) µ Μ g
D) QE – Mg + FN – µ Mg .
C) QE – Mg
E) None of these
M, Q
E
v = constant
µ
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CTF-4. A row of positive charges is stationary on the
ground. A person with a gauss-meter (which measures the
magnetic field) is running to the right along the row of
charges, at the same height as the charges and in front of
them (in the diagram below).
Does the person measure a non-zero B-field?
A) Yes
B) No
v
What is the direction of the B-field which the moving
observer measures?
A) Up↑
B) Down↓
C) Forward direction →
D) backward direction ←
E) no direction, B-field zero
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CTF-5. A charge q is located at the center of an gaussian
cubical box as shown. A student is asked: What is the
electric flux ΦE though the top face of the cube?
Is the answer hard to calculate or easy to calculate?
A: Easy
B: Hard.
q
CTF-6. A capacitor with capacitance C is attached to a
G G
battery with voltage V. What is the flux E ⋅ da through the
cubical volume shown? (The end faces of the cube are
within the metal plates of the capacitor.
z
C
CV
A) ε
o
2CV
B) ε
o
C) zero
D) None of these.
V
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CTF-7.
Points A and B are distances r and 3r respectively from a
point charge q. What is the voltage difference between
points A and B?
r
q
2kq
A) 3r
2r
A
kq
B) 3r
B
8 kq
C) 9 r
3kq
D) 4r
E) None
CTF-8. Consider a point in empty space near several
charges, which might be positive, negative, or both.
Consider the following statements.
I. The E-field can be zero while the voltage is non-zero.
II. The voltage can be zero while the E-field is non-zero.
Which of these statements can be true?
A) both can be true
B) neither can be true
C) only I can be true
D) only II can by true
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CTF-9. A capacitor has a voltage V across its plates. An
electron, initially at rest, is released from at a point very
close to the negative plate of a capacitor and it accelerates
toward the positive plate. The electron has charge –e and
mass m. There is no gravity in this problem. What is the
final kinetic energy of the electron just before it collides
with the positive plate?
A) m ⋅ e ⋅ V
B) 2eV
C) (1/2)mV2
D) eV
E) None of these
V
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CTF-10. A resistor with resistance R is plugged into a
120VAC wall socket. The graph below is either resistance
R, voltage V across, current I thru, or power P dissipated in
the resistor vs. time.
?
time
What could the graph be?
A) V only
B) I only
C) V or I only
D) V, I, or P E) Some other combination
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CTF-11. Consider the circuit shown, with the switch
initially open. When the switch is closed, the current I1
through resistor R1
A) increases, B) decreases,
C) stays the same.
I bat
V
R1
I1
R2
When the switch is closed, the current from the battery Ibat
A) increases, B) decreases,
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C) stays the same.
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CTF-12. When a resistor is plugged
into a standard AC outlet, the fuse
blows and all the lights go out. If we
want to repeat this dangerous
experiment, and not have the fuse blow,
we need a resistor that is
A) larger
B) smaller
C) Same R, but larger power rating
CTF-13. A charge q is released from rest at point
in empty space were there may be E- and/or Bfields. There are no forces on the charge except for
the forces due to the E and/or B-fields (no gravity,
etc.). The charge is observed for a short while and
is seen to move along a curved path.
q
Consider the following possibilities
I. There is only an E-field present and no B-field.
II. There is only a B-field present and no E-field.
III. There is both an E-field and a B-field present.
Which possibilities could account for the observed motion?
A) all three B) I and III only C) II and III
D) III only
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CTF-14. At time to, a particle with charge q has
G
instantaneous velocity vo as shown and is moving in a
JG
uniform magnetic field B .
A) True or B) False ?
V
B
y
q
x
As time increases, the x-component
of the velocity vx remains constant.
A) True or
B) False:
The KE of the particle remains constant as time increases.
CTF-15. A long solenoid with many turns per length has a
uniform magnetic field B within its interior. Consider the
imaginary rectangular path of length c and width a with one
edge entirely within the
solenoid as shown.
c
G G
a
What is B ⋅ dl ?
z
B
A) Bc
B) 2B(c + a)
D) None of these
C) 2Bc
The solenoid has n turns per length and carries a current I.
G G
What is B ⋅ dl ?
B) µ o ncI
C) µ o n( a + c) I
A) µ o nI
D) None of these.
z
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CTF-16. A horizontal loop of wire is in a vertical magnetic
field. (caused by an external magnet). The magnetic field
varies smoothly from pointing upward to pointing
downward, as shown. (The B-field gradually gets smaller,
goes to zero, then grows in the other direction.)
At the moment when the external magnetic field in the loop
is zero, is there an induced current in the loop?
A) Yes
B) No
At the moment when the external B-field in the loop is
zero, what is the direction of the induced current?
A) Clockwise (as seen from above)
B) Counterclockwise
C) No direction because induced current is reversing
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CTF-17.
A single circular loop of wire of radius R
surrounds a long solenoid of radius r, turns
per length n, carrying a current I. The
diagram shows the end-on view of the
situation. What is the magnetic flux
through the single wire loop?
A: µonI πR2
B: µoI r2/(2R)
C: πR2(µoI/2πR)
D: µonI πr2
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R
r
I
solenoid
E: None of these.
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CTF-18. The following
A
questions refer to the
L
B
V
circuit shown which has
R
R
two resistors, both with
resistance R, an inductor
with inductance L, a battery with constant voltage V, and a
switch which can be in position A or B.
Suppose the switch has been in position A for a long time
and is then switched to position B.
1. Immediately after the switch is thrown to B, what is the
current in the inductor?
A) zero B) V/R C) V/(2R)
D) 2V/R E) None
Immediately after the switch is thrown to B, what is the
voltage across the inductor L? a)
A) zero B) V
C) 2V
D) V/2 E) None
2.
CTF-19.
A hand-cranked electrical generator is attached to a resistor
R. The generator is turned at a constant rate which causes
an AC current to flow in the resistor. Suddenly,the value of
the resistance R increases. Does the generator get easier or
harder to turn?
A) Easier.
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B) Harder.
C) No change in difficulty.
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CTF-20. An radio tower is emitting a single-wavelength
(monochromatic) EM wave in all directions (isotropically).
A technician measures the electric field amplitude E at 100
meters from the tower and again at 400m from the tower.
The intensity of an EM wave is proportional to the square
of the amplitude of the E-field: I ∝ E2. What is the ratio
E100/E400?
E measured
at 100m
A) 2
B) 4
C) 16
E measured
at 400m
D) 162 = 256
CTF-21. A head lamp of power P sends rays
of light in the forward direction only. At a
distance R in front of the lamp, the intensity of
P
the light is _________ 4πR 2 (fill in the
blank)
A) greater than
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B) less than
C) equal to
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CTF-22. A ray of light is sent into an optic fiber at an
angle such that the angle with the normal to the pipe is θ.
The fiber has an inner core with index ni and an outer
cladding with index no. If no/ni is not large enough there
will be no internal reflection. In order for internal
reflection to possibly occur, the ratio no/ni must be
no
θ
A) positive
D) less than one
ni
B) negative C) greater than one
E) equal to one
CTF-23. Is the image formed by the lens in your eye a real
imagage or a virtual image?
A) real
B) virtual
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