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Instructor: Profs. P. Avery, Z. Qiu
PHYSICS DEPARTMENT
PHY 2054
Exam 2
March 20, 2013
Signature:
Name (PRINT, last, first):
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.
DIRECTIONS
(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 student 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. 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 with scratch work most questions demand.
(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 the answer sheet may not read properly.
(5) The answers are rounded off. Choose the closest to exact. There is no penalty for guessing.
>>>>>>>>WHEN YOU FINISH <<<<<<<<
Hand in the answer sheet separately.
Constants
−12
−31
ǫ0 = 8.85 × 10
F/m me = 9.11 × 10
kg
mp = mn = 1.67 × 10−27 kg e = 1.6 × 10−19 C
k = 9 × 109 N m2 /C2 µ0 = 12.56 × 10−7 H/m NA = 6.02 × 1023 atoms/mole c = 3 × 108 m/s
nH2 O = 1.333
micro = 10−6
nano = 10−9
pico = 10−12
1. A voltage surge produces 140 V for a moment in a standard 120 V line. What will temporarily be the output of a 100-W
light bulb assuming its resistance does not change?
(1) 136 W
(2) 100 W
(3) 140 W
(4) 117 W
(5) 120 W
2. A 105 µF capacitor supplies 17 J of energy to an electronic flash. What is the resistance of the lamp if the current falls
to 35% of its initial value in 1.4 ms?
(1) 12.7 Ω
(2) 31.0 Ω
(3) 8.5 Ω
(4) 4.4 Ω
(5) 2.9 Ω
3. A 2.00 kW heating element from a hair drier designed to operate at 120 V has a length of 80.0 cm. A 10.0 cm section
of the element is removed. What power is used by the now shortened heating element at 120 V?
(1) 2290 W
(2) 1750 W
(3) 2200 W
(4) 1810 W
(5) 1960 W
4. A particle of mass 4.3 × 10−21 kg moves in a uniform magnetic field of B = 2.4T and follows a circular path. If one
revolution is completed in 4.2 × 10−7 s, what is the magnitude of the particle’s charge?
(1) 2.7 × 10−14 C
(2) 4.3 × 10−15 C
(3) 1.5 × 10−13 C
(4) 1.1 × 10−14 C
5. Four long straight wires, each with current 12 A, overlap to form a 12 cm × 24 cm rectangle,
as shown. What is the magnitude of the magnetic field at the center point P?
(5) none of these
3
4
1
P
2
(1) 8.0 × 10−5 T
(2) 4.0 × 10−5 T
(3) 1.2 × 10−4 T
(4) 6.0 × 10−4 T
(5) zero
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6. The figure shows a mass spectrometer in which charged particles of different masses
enter a region through a slit and move perpendicular to a uniform magnetic field permeating the region (the gray dots represent the magnetic field pointing out of the page).
The particles are measured by a detector when they strike the bottom of the region.
Assume 12 C and 14 C singly charged ions are accelerated through the same potential
difference before entering the spectrometer. If the 12 C ions strike the detector 3.0 m
from the slit, at one point will the 14 C ions hit it?
(1) 3.24 m
(2) 3.50 m
(3) 3.00 m
(4) 4.00 m
(5) 4.50 m
7. In the figure, a long wire carries a steady current of 12 A. Later, a second current is initiated
in a circular loop of radius 3.5 cm with its center 10 cm from the long wire. What current
must flow in the loop to make the net magnetic field at its center point have the same
magnitude but opposite direction as the original field?
(1) 2.67 A
(2) 5.35 A
(3) 1.34 A
(4) 12.0 A
(5) 24.0 A
8. An inductor and a 2.5 Ω resistor are wired in series to a 24.0 V ideal battery. A switch in the circuit is closed at time 0,
at which time the current is 0. At a time 2.5 sec after the switch is thrown the potential difference across the inductor
is measured to be 5.0 V. What is the value of the inductance?
(1) 4.0 H
(2) 27 H
(3) 3.6 H
(4) 2.0 H
(5) 1.57 H
9. Note that an incorrect version was given (B field was specified vertical instead of horizontal). What is shown here is a
corrected version. All students got credit for this problem. A conducting square loop of mass 20 g and sides 5 cm lies on
a table within a magnetic field of magnitude 2.5 T pointing horizontally. What is the minimum current that must flow
in the loop to cause one side of the loop to rise off the table? (g = 9.8 m/s2 )
(1) 0.78 A
(2) 2.54 A
(3) 3.4 A
(4) 3.9 A
(5) 4.8 A
10. Four long parallel wires are arranged on a plane, as shown in the attached figure,
with 2.0 cm gaps between them. Each wire carries a 6.0 A current in the direction
indicated by the arrow. On the wire labeled C, what is the magnetic force per
meter?
A
B
C
D
(1) 1.8 × 10−4 N/m
(2) 5.6 × 10−4 N/m
(3) 1.2 × 10−2 N/m
(4) 9.2 × 10−4 N/m
(5) 3.0 × 10−5 N/m
11. A uniform 4.5-T magnetic field passes through the plane of a circular wire loop of radius 18 cm. What flux passes
through the loop when the direction of the 4.5-T field is at a 30◦ angle to the normal of the loop plane (in T · m2 )?
(1) 0.40
(2) 0.46
(3) 5.0
(4) 0.23
(5) 0.26
12. A 500-turn circular coil with a radius of 20 cm is mounted on a rotating frame that rotates at 3.18 turns/sec in the
presence of a 0.050-T uniform magnetic field that is perpendicular to the axis of rotation. What is the maximum emf in
the coil?
(1) 63 V
(2) zero
(3) 212 V
(4) 108 V
(5) 31 V
13. Four lightbulbs, each rated at 90 W for a standard 120 V AC receptacle, are placed in series and operated from a single
120 V receptacle. What is the total power emitted by the lightbulbs assuming that their internal properties remain the
same?
(1) 22.5 W
(2) 90 W
(3) 360 W
(4) 45 W
(5) 5.6 W
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14. A 12.0 mH inductor and a 5.0 Ω resistor are wired in series to a 28.5 V ideal battery. A switch in the circuit is closed
at time 0, at which time the current is 0. At a time 3.5 msec after the switch is thrown the potential difference across
the inductor is:
(1) 6.6 V
(2) 14.3 V
(3) 22 V
(4) 9.7 V
(5) 28 V
15. Refer to the previous problem. After a long time, what is the energy stored in the inductor?
(1) 0.19 J
(2) 0.10 J
(3) 0.051 J
(4) zero
(5) 0.14 J
16. The circuit shown has a 4 V battery in series with a 10 Ω resistor and is in the shape of a square
with sides 12 cm. It lies within a uniform magnetic field pointing into the page. If the current in
the circuit is 0.80 A counterclockwise, at what rate is the magnitude of the magnetic field changing
and is it increasing or decreasing?
(1)
(2)
(3)
(4)
(5)
280
280
830
830
140
T/s,
T/s,
T/s,
T/s,
T/s,
increasing
decreasing
increasing
decreasing
decreasing
+
17. A rod with resistance R lies across frictionless conducting rails in a constant uniform magnetic field B, as shown. Assume the rails have negligible resistance. The rod is pulled to
the right by a person with constant power P . After a long time, what is the resulting speed
of the rod?
(1)
√
RP /BL
(2) P/RBL
(3) P/RB 2 L2
(4) R/P BL
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BP /RL
18. A long straight wire and a rectangular conducting loop lie in the same plane as shown. If the current
in the long wire is increasing, what is the direction of the net force exerted on the loop?
(1) Right
(2) Left
(3) Up
(4) Down
(5) Zero
19. A transformer consists of a 500-turn primary coil and a 2000-turn secondary coil. If the current in the secondary is 3.00
A, what is the primary current?
(1) 12.0 A
(2) 1.33 A
(3) 0.75 A
(4) 48 A
(5) 36 A
20. A toaster has a power rating of 1600 W at 120 V. When plugged into a non-standard 60 V AC receptacle, what is the
maximum instantaneous current the toaster will experience?
(1) 9.42 A
(2) 6.67 A
(3) 19.2 A
(4) 14.1 A
(5) 26.7 A