PHY122 – Midterm 2 10.22.2012 Before you begin:
• Use black pencil.
• Write and bubble your SBU ID Number at bottom left.
• Fill bubbles fully and erase cleanly if you wish to change!
• 23 Questions, each question is 10 points.
• Each question has at most one correct answer. Marking 2 or more bubbles will automatically give
you 0 points.
1. My exam version is:
A
B
C
D
E
2. A doubly charged ion (q = +2e) with velocity 6.9 x 106 m/s moves in a path of radius 0.30 cm in a
magnetic field of 0.80 T in a mass spectrometer, as shown below. What is the mass of this ion?
A) 1.1 x 10-­‐28 kg B) 6.7 x 10-27 kg
C) 1.1 x 10-26 kg
D) 8.2 x 10-27 kg
E) 3.3 x 10-28 kg
The centripetal force equals the force on the positive ion in the magnetic field. The mass equals qvB/r = 1.1 x 10-­‐28 kg. The charge = 2(1.6 x 10-­‐19 C). 3. A wire in the plane of a page carries a current I directed towards the top of the page, as shown below.
If the wire is located in a uniform magnetic field B directed out of the page, the force on the wire
from the resulting magnetic field is:
#7
#10
A) directed into
B) directed out of C) directed to the D) directed to the E) zero.
the page.
the page.
right.
left.
Using the right hand rule, point your thumb in the direction of the current and your fingers should come out of the page in the direction of the magnetic field. The force on the wire is directed in the direction of your palm, which is to the right. 1
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4. Radiation from the Sun reaches the Earth, far above the atmosphere) at a rate of about 1350 W/m2.
Assume that this is a single electromagnetic wave and calculate the maximum value of the electric
field.
A) 0.71 kV/m
B) 1.0 kV/m
C) 1.4 kV/m
D) 2.0 kV/m
E) 2.7 kV/m
The maximum amplitude of the electric field can be determined by the equation: I = (1/2)cεo(Eo)2.
5. An electron is in a uniform magnetic field B that is directed out of the page, as shown below. When
the electron is moving in the plane of the page in the direction indicated by the arrow, the force on the
electron is directed:
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A) toward the
right.
B) out of the
page.
C) toward the
left.
D) toward the top
of the page.
E) toward the
bottom of the
page.
APyour
PhysicsAP Exam Multiple Choice Questions
Using the right hand rule, point
thumb in theMagnetostatics
direction of the
velocity of the charge and your fingers
should come out of the page,#1
representing the direction of the field. The direction of the
#4force points in
the same direction as the back of your hand, which is the opposite of the direction of the force for a
positive charge.
6. How many windings must a solenoid at length 80 cm have in order to establish a magnetic field of
strength 0.2 T inside a solenoid, if it carries a current of 20 A? A) 1000
B) 6400
C) 10,000
D) 32,000
E) 64,000
The magnetic field inside a solenoid can be determined using the equation B = µoI(N/L). 7. Two long parallel wires carry currents of 20 A and 5.0 A in opposite directions. The wires are
separated by d = 0.20 m. What is the magnitude of the magnetic field midway between the two wires
at point P?
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A) 1.0 x 10-5 T
B) zero
C) 3.0 x 10-5 T
D) 4.0 x 10-5 T
E) 5.0 x 10-5 T
At point P, the field due to the currents in both wires points downward. Therefore, each magnetic field
can be calculated and added together: Bnet = µo(20)/(2π x 0.10m) + µo(5.0)/(2π x 0.10m)
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8. A square loop of side 0.20 m has a total resistance of 0.60 Ω. The loop is positioned in a uniform
magnetic field B of 0.030 T, which increases to 0.20 T in 0.50 seconds. The field is directed into the
page and perpendicular to the plane of the page, as shown below. Calculate the induced emf. A) 0.0068 V
B) 0.068 V
C) 0.0024 V
D) 0.016 V
E) 0.014 V
The induced emf can be calculated with Faraday’s Law: ε = (0.20 – 0.03 T)(0.20m)2/(0.50 sec) 9. Using the same information from question #8: Calculate the magnitude and direction of the induced
current.
A) 0.0040 A,
B) 0.023 A,
C) 0.023 A,
D) 0.027 A,
E) 0.027 A,
counterclockwise
clockwise
counterclockwise
clockwise
counterclockwise
The induced current will be counterclockwise to counteract the increasing magnetic field; an induced field
will come out of the page with a counterclockwise current. We can use Ohm’s Law to find the magnitude
of the current: 0.014 V = I (0.60 Ω). 10. What is the wavelength of an x-ray whose frequency is 1.0 x 1018 Hz?
A) 3.3 x 10-2 nm
B) 3.0 x 10-1 nm
C) 3.3 nm
D) 3.0 x 10-1 nm
E) 3.0 x 1035 nm
The wavelength can be calculated by using c = λf. There is a typographical error, so both answers B and D are correct. 11. The secondary coil of a neon sign transformer provides 7500 V at 10.0 mA. The primary coil operates
on 120 V. What current does the primary draw?
A) 0.625 A
B) 0.625 mA
C) 0.160 A
D) 1.66 A
E) 0.160 mA
The current can be found by using the relationship between current and voltage in primary and secondary
transformers: I2 = (V1/V2)I1. When substituting values, (10.0 x 10-3 A) = (120 V/7500 V)I1.
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12. A positively charged particle is moving with a constant velocity through a region with both a
magnetic and electric field. The magnetic field and the motion of the particle are shown below.
Towards what direction must the electric field be to cause the particle to move at a constant velocity?
A) upward
B) downward
C) left
D) right
E) out of the
page
Using the right hand rule, if the field goes into the page (direction of fingers) and the charge is moving to
the right (direction of thumb), the magnetic force on the positive charge must be upward (direction of
palm). Therefore, the electrostatic force must be downward. For a positive charge, this force and the
electric field are in the same direction.
13. A metal rod of length L is pulled downward with constant velocity v through a uniform magnetic field
B that points into the page. What is the potential difference between points a and b?
A) zero
B)
1
BLv , with
2
point a at higher potential
C)
1
BLv , with
2
point b at higher
potential
D) BLv , with
point a at higher potential
E) BLv , with
point b at higher potential
The potential difference for motional emf is equal to BLv. Using the right hand rule, point your fingers
into the page in the direction of the field, and your thumb in the direction of the velocity. Your palm
points in the direction of the force on the positive charges. Therefore, the right side of the bar (point b)
will have a higher potential than the left side (point a).
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14. Electrons with a range of speeds move in a region of uniform magnetic field, perpendicular to the
field. Which of the following is true?
A) All the
electrons will
move in circles of
the same radius.
B) The magnetic C) The magnetic D) The magnetic E) All the
field will cause
field will cause
force does zero
electrons
the faster
the slower
work on the
experience the
electrons to slow
electrons to speed electrons.
same centripetal
down more
up more quickly
force.
quickly than the
than the faster
slower electrons.
electrons.
Since the electrons are moving in a magnetic field in the absence of other significant forces, they will
experience uniform circular motion and move at constant velocity with different radii. The magnetic force
does zero work on the charges since the force is directed perpendicular to the tangential velocity of the
charges.
15. A rectangular wire loop is at rest in a uniform magnetic field B of magnitude 2 T that is directed out
of the page. The loop measures 5 cm by 8 cm, and the plane of the loop is perpendicular to the field,
as shown below. The total magnetic flux through the loop is:
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A) zero
B) 2 x 10-3 T•m2 C) 8 x 10-3 T•m2 D) 2 x 10-1 T•m2 E) 8 x 10-1 T•m2
The magnetic flux is equal to the product of the magnetic field strength and area of the loop:
(2T)(0.05 m)(0.08 m).
16. A flexible conducting loop is placed in a magnetic field with the plane of the loop perpendicular to
the field. Which of the following will NOT induce a current in the loop?
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#9
A) increasing the B) stretching the
C) moving the
D) removing the
E) rotating the
magnitude of the
loop, making a
loop parallel to
loop from the
loop about a
field
larger circle
the field
field
diameter
A current will be induced if the area within the magnetic field is changed, if the field strength varies, or if
the loop is rotated. Moving the loop parallel to the field will not induce a current.
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17. Two long parallel wires, each 1 m long, carry currents of 5.0 A and 8.0 A in the opposite direction.
The wires are separated by 0.30 m. Find the magnetic force between the two wires.
A) 2.7 x 10-5 N,
B) 7.2 x 10-5 N,
C) 2.7 x 10-5 N,
D) 7.2 x 10-5 N,
E) 3.6 x 10-5 N,
repulsive
repulsive
attractive
attractive
repulsive
Wires that carry current in opposite directions will repel each other, since their magnetic fields are in the
same direction in the space between them. The magnitude of this force can be calculated:
F = (µo)(1 m)(5.0 A)(8.0 A)/(2π x 0.30 m). 18. What is the peak voltage in an AC circuit where the rms voltage is 120 V?
A) 84.8 V
B) 120 V
C) 170 V
D) 340 V
E) 360 V
Peak voltage is equal to the product of the root-mean-square voltage (120 V) and the square root of 2.
19. When the transmission axes of two polarizing filters are perpendicular to each other, what is the
percentage of the incident light which will pass through the two filters?
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%
When the polarizing filter and the analyzer are aligned perpendicular to each other, no light may pass
through.
20. What is the wavelength corresponding to the most intense light emitted by a giant star of surface
temperature 5000K?
A) 576 nm
B) 578 nm
C) 580 nm
The peak wavelength is equal to (2.9 x 106)/5000 K = 580 nm.
D) 582 nm
E) 585 nm
21. At what rate are photons emitted by a 50-W sodium vapor lamp? Assume that the lamp’s light is
monochromatic and of wavelength 589 nm.
A) 1.0 x 1020
B) 2.5 x 1020
C) 2.0 x 1020
D) 1.5 x 1020
E) 3.0 x 1020
photons/s
photons/s
photons/s
photons/s
photons/s
The energy of each photon can be found by solving for E = hc/λ, which equals 3.38 x 10-­‐19 J. Since the vapor lamp emits 50-­‐W, which is 50 J per second, we can take 50 and divide by the energy for each photon to find the number of emitted photons per second. 6
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#9
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22. A wire of constant length is moving in a constant magnetic field, as shown below. The wire and the
velocity vector are perpendicular to each other and are both perpendicular to the field. Which of the
following graphs best represents the potential difference ε between the ends of the wire as a function
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of the speed v of the wire?
#9
#7
A)
B)
#8
C)
D)
E)
#8
The potential difference is directly proportional to the velocity of the conductor perpendicular to the
magnetic field.
#8 23. A typical, energy-efficient refrigerator dissipates energy at a rate of 350 W. Assuming the refrigerator
runs 24 hours per day, what is the annual cost of electricity at $0.15/kWh?
A) $1260
B) $1.26
C) $46
D) $230
E) $460
Multiplying 0.350 kW by 24 hours and 365 days gives us 3066 kwh in one year; this value can be
multiplied by $0.15 per kWh to get a yearly cost of $460.
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