Physics 112
Exam 2
Summer 2013
Name________________
Physics 112
Summer 2013
Exam 2
Monday, July 15
Directions:
This 60-minute exam consists of twenty multiple-choice questions. This test is worth 20% of
your final grade. (One point is equal to 1% of the final grade.)
The questions on this test are not in order of difficulty.
You must mark all of your answers on both your test and answer sheet.
In marking the multiple choice bubble sheet use a number 2 pencil. Do not use ink. If you did
not bring a pencil, ask for one. Fill in the appropriate circles completely. If you need to change
any entry, you must completely erase your previous entry.
Carefully read each question and its five possible answers. Select one and only one answer for
each question. Choose the answer that is closest to the correct one.
When you are finished with the exam, return the bubble sheet, the exam itself, and scratch paper
that you used for the exam.
Good luck!
Physics 112
Exam 2
Summer 2013
21. A proton has a speed of 3.0 × 106 m/s in a direction perpendicular to a uniform magnetic
field, and the proton moves in a circle of radius 0.20 m. What is the magnitude of the
magnetic field? (Proton mass 1.67× 10-27kg)
A) 0.08 T
B) 0.16 T
C) 0.24 T
D) 0.32 T
E) 0.64 T
22. Negative charge Q is moving with velocity v in the magnetic field B as shown below (with
vectors v and B on the surface of the page). What is the direction of the magnetic force?
A) Out of the page
B) Into the page

C) Parallel to the vector B
D) Force is zero
E) It is not enough information to answer the question

v
Q<0

23. An electric field of 1.60 kV/m and magnetic field of 0.400 T act on a moving electron to
produce no net force. Calculate the minimum speed of the electron.
A)
B)
C)
D)
E)
2.0  10 3 m / s
3.0  10 3 m / s
4.0 103 m / s
5.0  10 3 m / s
6.0  10 3 m / s
24. The force on a current-carrying wire in a magnetic field is the strongest when
A) The current is parallel to the field lines
B) The current is at a 30° angle with respect to the field lines
C) The current is at a 45° angle with respect to the field lines
D) The current is at a 60° angle with respect to the field lines
E) The current is perpendicular to the field lines

B
Physics 112
Exam 2
Summer 2013
25. A 1.5-m length of wire carrying 4.5 A of current is oriented horizontally. At that point on the
Earth’s surface, the dip angle of the Earth’s magnetic field makes an angle of 38° to the wire.
Estimate the magnitude of the magnetic force on the wire due to the Earth’s magnetic field of
5.5  105 T at this point.
A) 1.2  10 4 N
B) 2.3  10 4 N
C) 3.4  10 4 N
D) 4.5  10 4 N
E) 1.2  10 5 N
26. A current-caring circular coil is in uniform magnetic field. The maximum magnitude of the
torque on the coil occurs when the angle between the field and magnetic moment is
A)
B)
C)
D)
E)
0
45
90 
135 
180 
27. Two long parallel wires are 4.0 cm apart. Each wire carries the current 10 A in the same
direction. Find the magnetic field halfway between the wires.
A)
B)
C)
D)
E)
1  10 4 T
2 10 4 T
3  10 4 T
4  10 4 T
zero
28. Two long parallel wires carry currents of 20 A and 5.0 A in opposite directions. The wires
are separated by 0.20 m. At what point between the two wires are the contributions to the
magnetic field equal?
A) 0.040 m from the 20 A wire
B) 0.080 m from the 20 A wire
C) 0.12 m from the 20 A wire
D) 0.16 m from the 20 A wire
E) 0.18 m from the 20 A wire
Physics 112
Exam 2
Summer 2013
29. The magnetic field inside an air filled solenoid is B. The cross-sectional area of the solenoid
is doubled, keeping the current flowing through the solenoid and the number of turns per unit
length unchanged. What happened with the magnetic field inside solenoid?
A)
B)
C)
D)
E)
Remains the same
Doubles
Quadruples
Halves
Devides by 4
30. A bar magnet falls through a loop of wire with the north pole entering first. As the north pole
enters the wire, the induced current will be (as viewed from above)
A)
B)
C)
D)
E)
Zero
Clockwise
Counterclockwise
To top of loop
A current whose direction cannot be determined from the information given
31. A stationary loop of wire is placed in a magnetic field directed into the page as shown below.
The current in the loop of wire is:
A) Clockwise if the magnitude of the magnetic field is decreasing
B) Clockwise if the magnitude of the magnetic field is increasing
C) Clockwise if the magnitude of the magnetic field is increasing or
decreasing
D) Counterclockwise if the magnitude of the magnetic field is
increasing or decreasing
E) No current will flow through the loop if the magnetic field is
increasing or decreasing
x x x x x x x x x
x x x x x x x x x
x x x x x x x x x
x x x x x x x x x
32. The north pole of the earth's magnetic field is at the geographic south pole. A compass is a
small magnet whose north pole end is drawn in the approximate direction of:
A)
B)
C)
D)
E)
The geographic south pole along the lines of the magnetic field
The geographic north pole along the lines of the magnetic field
The geographic south pole against the lines of the magnetic field
The geographic north pole against the lines of the magnetic field
The geographic north pole perpendicular to the lines of the magnetic field
Physics 112
Exam 2
Summer 2013
33. A uniform magnetic field B is perpendicular to the area bounded by the U shaped conductor
and a movable metal rod of length l. The rod is moving along the conductor at a speed v. The
total resistance of the loop is R. What is power needed to move the rod?
B 2l 2 v 2
A) P 
R
B) P 
Bl 2 v 2
R
C) P 
B 2 lv 2
R
B 2l 2 v
R
Blv
E) P 
R
D) P 
34. An airplane with a wing span of 60 m flies horizontally at a location where the downward
component of the Earth's magnetic field is 6.0 × 10-5 T. Find the magnitude of the induced emf
between the tips of the wings when the speed of the plane is 225 m/s.
A)
B)
C)
D)
E)
0.41 V
0.61 V
0.81 V
1.2 V
1.4 V
35. A wire loop is in a uniform magnetic field. Current flows in the wire loop, as shown. What
does the loop do?
A)
B)
C)
D)
E)
Moves to the right
Moves up
Remains motionless
Rotates
Moves out of the page
36. A 4.0-mH coil carries a current of 5.0 A. How much energy is stored in the coil's magnetic
field?
A)
B)
C)
D)
E)
2.0 mJ
10 mJ
20 mJ
50 mJ
80 mJ
Physics 112
Exam 2
Summer 2013
37. After how many time constants does the current in a LR circuit reach 90% of its maximum
value?
A)
B)
C)
D)
E)
1.2
2.3
3.4
4.5
never
38. An inductance coil operates at 240 V and 60.0 Hz. It draws 12.8 A. What is the coil’s
inductance?
A)
B)
C)
D)
E)
1.0  10 2 H
2.0  10 2 H
3.0  10 2 H
4.0  10 2 H
5.0  10 2 H
39. Unpolarized light passes through three successive ideal polarizers, each of whose axis makes
a 30° angle with the previous one. What fraction of the light intensity is transmitted?
A)
B)
C)
D)
E)
1/4
1/8
3/8
9/32
27/64
40. An electromagnetic wave in vacuum has a frequency of 1.00 MHz. What is the wavelength
of the wave?
A)
B)
C)
D)
E)
1.00 cm
3.00 cm
1.00 m
3.00 m
300 m
Physics 112
Exam 2
Summer 2013
Record Sheet
You may fill in this sheet with your choices, detach it and take it with you after the exam for
comparison with the posted answers
21
31
22
32
23
33
24
34
25
35
26
36
27
37
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39
30
40