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Instructor(s): F.E. Dunnam
PHYSICS DEPARTMENT
PHY 2054
Exam 2
Name (PRINT, last, first):
11 July 2011
Signature:
On my honor, I have neither given nor received unauthorized aid on this examination.
YOUR TEST NUMBER IS THE 5-DIGIT NUMBER AT THE TOP OF EACH PAGE.
(1) Code your test number on your answer sheet (use 76–80 for the 5-digit number). Code your name on your
answer sheet. DARKEN CIRCLES COMPLETELY. Code your UFID number on your answer sheet.
(2) Print your name on this sheet and sign it also.
(3) Do all scratch work anywhere on this exam that you like. Circle your answers on this test form. At the end of the
test, this exam printout is to be turned in. No credit will be given without both answer sheet and printout.
(4) Blacken the circle of your intended answer completely, using a #2 pencil or blue or black ink. Do not
make any stray marks or some answers may be counted as incorrect.
(5) The answers are rounded off. Choose the closest to exact. There is no penalty for guessing. If you believe that no correct
answer is listed, leave this item blank!!
(6) Hand in the answer sheet separately.
Useful (??) Constants:
1. A flat conducting coil (area = 5 cm2 ) of copper wire mounted near the earth’s
surface and connected to a voltmeter detects, via induced emf in the coil, a
vertical lightning bolt striking the earth. Suppose that the lightning bolt strikes
the ground 100 m distant from the coil, and the bolt has an initial current of
10 mega amperes, dropping to zero in 10 microseconds. If the voltmeter reads
30 volts, how many turns of wire are in the coil and how should the coil be
oriented so as to maximize the reading?
(1)
(2)
(3)
(4)
(5)
60, plane of coil
60, plane of coil
30, plane of coil
30, plane of coil
none of these
perpendicular to earth’s surface
parallel to earth’s surface
parallel to earth’s surface
perpendicular to earth’s surface
kaboom
µ0 = 4π × 10−7 Tm/A
²0 = 8.85 × 10−12 C2 /(Nm2 )
−19
electron charge = −1.6 × 10 C
electron mass = 9.11 × 10−31 kg
V=volt
N=newton
J=joule
m=meter
W=watt µ = “micro-” = 10−6
A = ampere “pico” = 10−12 n = “nano” = 10−9 m = “milli” = 10−3 proton mass = 1.67 × 10−27 kg
c = 3 × 108 m/s
“mega” = 106
coil
100 m
(not to scale)
2. The figure sketches the clockwise spiral path of a charged particle moving in a
magnetic field B that is perpendicular to the particle velocity. If the particle
speed is unchanging, the situation shown represents:
(1)
(2)
(3)
(4)
(5)
a negatively charged particle; B is increasing
a negatively charged particle; B is decreasing
a positively charged particle; B is decreasing
a positively charged particle; B is increasing
constant B, particle polarity irrelevant
3. A 5-turn flat coil of conducting wire has inductance L. The inductance of a similar coil having 20 turns is
√
(1) 16L
(2) 4L
(3) L/4
(4) 8L
(5) none of these
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4. An RLC series circuit, connected to a sinusoidal ac source E of frequency120 Hz, is at resonance. Under these conditions:
(1)
(2)
(3)
(4)
(5)
the
the
the
the
the
voltage across R equals the applied voltage.
voltage across R is zero.
voltage across C is zero.
voltage across L equals the applied voltage.
applied voltage and currrent differ in phase by 90 degrees.
5. In the circuit shown, V0 = 45 volts, R1 = 10Ω, R2 = 5Ω, and L = 0.3H.
Immediately after switch S is closed, the battery current is:
(1) 3 A
(2) 4.5 A
(3) 9 A
(4) 0 (zero)
(5) insufficient data
6. A long time after the switch is closed in the above circuit, the current through the inductor will be approximately
(1) 4.5 A
(2) 3 A
(3) 9 A
(4) 0 (zero)
(5) insufficient data
7. If the switch has remained closed for, say, 100 time constants, the energy stored in the inductor is
(1) 3 J
(2) 4.5 J
(3) 1.35 J
(4) 122 J
135
(5) √ J
2
8. An electron is moving southward in a region where the magnetic field is directed northward. The magnetic force exerted
on the electron is:
(1) zero
(2) up
(3) down
(4) east
(5) west
9. Two parallel long straight wires carry the same current and repel each other with a force F per unit length. If both
currents are doubled and the wire separation tripled, the force per unit length becomes:
(1) 4F/3
(2) 2F/9
(3) 2F/3
(4) 4F/9
(5) 6F
10. An 8.0-mH inductor and a 2.0-Ω resistor are wired in series to a 20-V ideal battery. A switch in the circuit is closed
at time 0, at which time the current is 0. A long time after the switch is thrown the potential differences across the
inductor and resistor are:
(1) 0, 20 V (2) 20 V, 0 (3) 10 V, 10 V (4) 16 V, 4 V (5) unknown since the rate of change of the current is not given
11. In an ideal 8:1 step-down transformer the primary power is 10 kW and the secondary current is 25 A. The primary
voltage must therefore be
(1) 3200
(2) 25,600
(3) 400
(4) 50
(5) 6.25
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12. The direction of travel of an electromagnetic wave such as that produced by an antenna is oriented in what direction
with respect to the associated electric (E) and magnetic (B) fields?
(1)
(2)
(3)
(4)
(5)
perpendicular to both E and B
parallel to both E and B
parallel to E, perpendicular to B
parallel to B, perpendicular to E
none of these
13. In the circuit shown at right, R = R = 10kΩ, C = 200µF, and E = 40 V.
When S is closed, the time constant for charging C is (in seconds):
S
R
C
E
R
(1) 2
(2) 4
(3) 0.5
(4) 1
(5) none of these
14. For the above circuit, when S is open, the time constant for discharge of capacitor C (in seconds) is
(1) 4
(2) 2
(3) 0.5
(4) 1
(5) none of these
15. Many do-it-yourself outdoor home lighting kits are ‘low-voltage’ systems, using a transformer that steps-down the 120V
household voltage to 24V in the interest of safety. Suppose that in such a system the combined resistance of the outdoor
lights is 2 ohms. What will be the current in the secondary and primary coils of the transformer, respectively?
(1) 12, 2.4
(2) 2.4, 12
(3) 6, 12
(4) 12, 6
(5) 48, 10
16. Two identical electric lamps A and B are hooked together as shown in the
diagram. When the switch at the center is closed, what now happens to the
intensity (brightness) of each lamp compared to its initial intensity? [Hint:
lamp intensity is proportional to energy dissipated.]
(1)
(2)
(3)
(4)
(5)
A
A
A
A
A
brighter ; B dimmer
brighter ; B brighter
dimmer ; B dimmer
dimmer ; B brighter
stays the same ; B stays the same
17. The current-carrying loop in the diagram is released from rest in the uniform
magnetic field shown. About which axis does it rotate?
(1)
(2)
(3)
(4)
(5)
x-axis
y-axis
z-axis
There is no torque on the loop so it does not rotate.
Depends on whether the charge carriers are positive or negative.
18. Identical currents I flow in the long straight conductor and the circular loop
shown in the diagram. If r = 2cm and I = 3A, find the magnetic field strength
in T at the center of the circular loop.
(1) 7.9 × 10−5
(2) 1.1 × 10−4
(3) 9.4 × 10−5
(4) 1.5 × 10−5
(5) zero
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19. The figure shows a uniform magnetic field which is normal to the plane of a
conducting circular loop of resistance 1.5Ω and radius 0.024 m. The magnetic
field has a magnitude of 3.0 T and is directed up out of the paper. The area
of the rectangular portion of the loop is negligible compared to that of the
circular loop. What is the average current in the loop if the magnitude of the
magnetic field is doubled in 0.4 s?
(1)
(2)
(3)
(4)
(5)
9.0 × 10−3
2.8 × 10−3
4.5 × 10−3
4.5 × 10−3
9.0 × 10−3
A,
A,
A,
A,
A,
clockwise
clockwise
clockwise
counter-clockwise
counter-clockwise
20. A single circular loop of 1.0 m radius carries a current of 10.0 mA. It is placed
in a uniform magnetic field of magnitude 0.50 T that is directed parallel to
the plane of the loop as suggested in the figure. What is the magnitude of the
torque exerted on the loop by the magnetic field?
(1) 1.57 × 10−2 N·m
(2) 3.14 × 10−2 N·m
(3) 6.28 × 10−2 N·m
(4) 9.28 × 10−2 N·m
(5) 10 N·m
THE FOLLOWING QUESTIONS, NUMBERED IN THE ORDER OF THEIR APPEARANCE ON THE ABOVE
LIST, HAVE BEEN FLAGGED AS CONTINUATION QUESTIONS: 6 7 14