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Instructor: Prof. Andrey Korytov, Prof. Paul Avery
PHYSICS DEPARTMENT
PHY 2054
Exam 2
October 29, 2014
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 loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal
sides, each with length L = 15 cm as shown in the figure. The triangle lies within a 0.7 T
uniform magnetic field in the plane of the triangle and perpendicular to the hypotenuse.
The magnetic force on either of the two sides has a magnitude of:
(1) 0.15 N
(2) 0.21 N
(3) 0.30 N
(4) 2.8 N
B
(5) 1.4 N
2. Refer to the previous problem. What is the magnitude of the torque on this loop due to the magnetic field (in N·m)?
(1) 15.8 × 10−3
(2) 11.1 × 10−3
(3) 22.5 × 10−3
(4) 45.0 × 10−3
(5) 84.3 × 10−8
3. An electrical device with an equivalent resistance of 100 Ω has a maximum instantaneous voltage rating of 60 V. Assume
that it is plugged into a standard 120 V electrical outlet in series with a resistance R. What is the approximate minimum
value of R needed to keep the instantaneous voltage across the device within its safety limits?
(1) 183 Ω
(2) 100 Ω
(3) 55 Ω
(4) 1240 Ω
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4. A 14.0 g conducting rod of length 1.30 m and resistance 8.0 Ω slides freely downward between
two vertical conducting rails without friction. The entire apparatus is placed in a 0.43 T uniform
magnetic field. Ignore the electrical resistance of the rails. What is the terminal velocity of the
rod? (g = 9.8 m/s2 )
(5) 283 Ω
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m/s
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1.96
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(1)
(2)
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(4)
(5)
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5. A transformer consists of a 300-turn primary coil and a 2000-turn secondary coil. If the current in the secondary coil is
9.00 A, what is the primary current?
(1) 60.0 A
(2) 1.35 A
(3) 35.4 A
(4) 25.0 A
(5) 47.4 A
6. A hair dryer has a power rating of 1200 W when plugged into a standard 110 V AC receptacle. How much current does
it draw if it is connected instead to a 110 V DC source?
(1) 10.9 A
(2) 7.71 A
(3) 13.4 A
(4) 25.1 A
(5) 15.6 A
7. The number of turns is tripled for an ideal solenoid, and its length is doubled while holding its cross-sectional area
constant. If the old inductance is L, what is the new inductance?
(1) 9L/2
(2) 3L/2
(3) 2L/3
(4) 9L/4
8. In the circuit shown, what is the current through the 15 V battery?
(1) 1.18 A upwards
(2) 3.41 A downwards
(3) 1.12 A upwards
(5) 3L/4
+
+
4Ω
8Ω
12 V
(4) 2.79 A upwards
15 V
2Ω
(5) 1.83 A downwards
9. An ammeter with full scale deflection for I = 20.0 A has an internal resistance of 31.5 Ω. We need to use this ammeter
to measure currents up to 30.0 A. What size resistor should be used to protect the ammeter?
(1) 63.0 Ω
(2) 47.0 Ω
(3) 31.5 Ω
(4) 15.8 Ω
10. An electromagnetic flowmeter is used to measure blood flow rates during surgery. Blood containing Na+ ions flow to the left through an artery with an inner diameter of D = 4.00 mm. The
artery is in a downward magnetic field of B = 0.100 T and develops a Hall voltage VH = 80.0µV
across its diameter. What is the volumetric flow rate of the blood in (m3 /s)?
(1) 2.51 × 10−6
(2) 3.20 × 10−6
(3) 4.07 × 10−6
(4) 5.03 × 10−6
(5) 77.3 Ω
B
v
(5) 1.01 × 10−5
11. An electromagnet is made by inserting a soft iron core into a solenoid. The solenoid has 1,700 turns, radius 2.4 cm, and
length 13.0 cm. When 0.5 A of current flows through the solenoid, the magnetic field inside the iron core has magnitude
0.43 T. What is the relative permeability κB of the iron core?
(1) 52.3
(2) 35.7
(3) 41.8
(4) 63.5
(5) 74.1
12. Four long parallel wires pass through the corners of a square with side 14.1 cm as shown in the
figure. All four wires carry the same magnitude of current I = 5.7 A in the directions indicated.
Find the magnetic field at the center of the square.
(1) 32.3 µT
(2) 45.7 µT
(3) 52.9 µT
(4) 16.2 µT
(5) 8.1 µT
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13. The armature of an AC generator is a rectangular coil 10.5 cm × 4.0 cm with 210 turns. It is immersed in a uniform
magnetic field of magnitude 1.5 T. If the amplitude of the emf in the coil is 30.0 V, at what angular speed (in rad/sec)
is the armature rotating?
(1) 22.7
(2) 16.1
(3) 60.5
(4) 32.1
(5) 27.4
14. A dc motor has coils with resistance of 13 Ω and is connected to an emf of 127 V. When the motor operates at full speed,
the back emf is 63 V. If the current is 6.1 A with the motor operating at less than full speed, what is the back emf at
that time?
(1) 47.7 V
(2) 79.3 V
(3) 49.2 V
(4) 58.4 V
(5) 53.1 V
15. A coil has an inductance of 0.31 H and a resistance of 43 Ω. The coil is connected to a 6.4 V battery. After a long time
elapses, the current in the coil is no longer changing. What is the energy stored in the coil?
(1) 3.43 mJ
(2) 23.0 mJ
(3) 6.74 mJ
(4) 6.50 mJ
(5) 4.42 mJ
16. A circuit breaker trips when the rms current exceeds 10.3 A. How many 60 W lightbulbs, connected in parallel, can be
run on this circuit without tripping the breaker? (The rms voltage of the circuit is 110 V.)
(1) 18
(2) 15
(3) 19
(4) 25
(5) 17
17. A sample containing carbon (atomic mass 12 u) and oxygen (16 u) is placed in a mass spectrometer. The ions all have
the same charge and are accelerated through the same potential difference before entering a uniform magnetic field. For
each ion, the spectrometer measures the distance between the enterance point and the place where it strikes the bottom
of the region. If the carbon ions strike the detector at a distance of 9.14 cm from the enterance point, at what distance
will oxygen ions strike the detector?
(1) 10.6 cm
(2) 12.2 cm
(3) 6.9 cm
(4) 7.9 cm
(5) 16.3 cm
18. The current I in a long wire is going up as shown in the figure and increasing in magnitude.
What is the direction of the induced current in the left loop and the right loop? List the direction
of the induced current in the left loop first. (CW = clockwise, CCW = counterclockwise)
(1)
(2)
(3)
(4)
(5)
I
CW, CCW
CCW, CW
CW, CW
CCW, CCW
There is no induced current.
19. In the circuit shown in the figure, the switch is in position 1 for a long time. It is then
moved to position 2. How long will it take for the charge on the capacitor to reach 50 µC
after the switch is moved to its new position? The values are C = 21 µF, R1 = 5000 Ω
and R2 = 25 Ω. The battery has a voltage of 50 V.
1 2
+
C
R1
(1) 1.60 ms
(2) 320 ms
(3) 2.84 ms
(4) 7.12 ms
(5) 4.23 ms
R2
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20. A toroid is like a solenoid that has been bent around in a circle until its ends meet, as
shown in the figure (inner radius 4 cm, outer radius 8 cm). What is the magnitude of the
magnetic field inside a toroid of 1000 turns carrying a current 0.5 A at a distance 6 cm from
the center of the toroid? (Hint: Use Ampere’s Law applied to a closed loop in the toroid
mid-plane, as shown by a dashed line.)
(1) 1.67 mT
(2) 0.172 mT
(3) 2.53 mT
(4) 4.92 mT
(5) 0.876 mT