Download Phy C April exam 2011 Revised

NJSL Physics C - April 2011
1. A resistor (resistance R) and an inductor (inductance L) are
connected into a circuit at points A and B as shown. The voltage
across AB varies so that the current through the resistance is given
by i(t) = a + b∙t, where a and b are constants and t is time. The voltage across AB as a function of
time is given by
.
a. aR + bL +Rbt
b. aL + bR +Lbt
c. aLR + LRbt
d. aL+Rbt
e. (R + L)(a + bt)
For questions #2 and #3 use the following information:
Two straight conducting rails form a 45o angle as shown. A conducting bar in
contact with the rails starts at the vertex at time t = 0 and moves with a constant
velocity v along the rails. A uniform magnetic field B is directed out of the page
and perpendicular to the plane of the triangle formed by the rails and the bar.
2. What is the magnitude of the emf induced in the triangle as a function of time t?
a. Bv2t2 b. Bv2
c. Bvt
d. Bv2t
e. Bv
3. What is the direction of the current induced in the triangle?
a. clockwise b. counterclockwise
4. What is the magnetic force on a wire of length L carrying a current i in the positive x
⃗?
⃗ = 𝐵𝑦 𝑗 + 𝐵𝑧 𝑘
direction in a region with a magnetic field 𝐵
⃗?
a. 𝐹 = 𝑖𝐿𝐵𝑦 𝑗 + 𝑖𝐿𝐵𝑧 𝑘
b. 𝐹 = 𝑖𝐿𝐵𝑦 𝑖 + 𝑖𝐿𝐵𝑧 𝑗?
c. 𝐹 = 𝑖𝐵𝑦 𝑖 + 𝑖𝐵𝑧 𝑗?
⃗?
d. 𝐹 = 𝑖𝐿𝐵𝑧 𝑗 + 𝑖𝐿𝐵𝑦 𝑘
e. 0
5. In the Bohr model of the hydrogen atom, the electron (charge e) moves around the proton at a
speed v in a circle of radius r. Considering the orbiting electron to be a small current loop,
determine the maximum torque applied on this current loop by an external magnetic field B.
a. 0.5evrB
b. evrB
c. evB/2r
d. evB/r
e. er/vB
6. An electric generator contains a coil N turns of wire, each forming a rectangular loop of area A.
The coil is placed entirely in a uniform magnetic field with magnitude B and initially
perpendicular to the coil's plane. What is the maximum value of the emf produced when the loop
is spun at radial speed ω about an axis perpendicular to the magnetic field?
a. BAN
b. 0
c. BA
d. BAN
e. B2AN
7. 1 Weber is the same as
a . T/m
b. T∙s
.
c. V/s
d. T/m2
e. V∙s
8. A toroid has a square cross section with the length of an edge equal to
the radius of the inner surface. The ratio of the magnitude of the
magnetic field at the inner surface to the magnitude of the field at the
outer surface is
.
a. ¼
b. 4
c. ½
NJSL Phy C April 2011 Exam
d. 2
e. 1
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A single-turn coil, L on each side and
resistance R, moves freely into a region of a
uniform magnetic field B as shown below. The
only significant forces on the coil are magnetic
caused by the magnetic field B.
9. Which of the graphs to the right correctly
describes the velocity of the coil as a function
of time if the right edge of the coil enters the
field at t = 0? (The coil remains in the field for
the entire time represented on the graphs)
10. What is the initial current induced when the right edge of the coil in #9 starts entering the region
of the field B?
a. 0
b. vBL/R
c. 2vB/RL d. B2L2/B
e. vBR/L
11. The quantity (B2/µo) has units of
a. J/H
b. H/m3
c. J
d. J/m3
.
e. J/m
12. An electric field is associated with every
.
a. magnetic field
b. time-dependent magnetic field
c. object moving in a magnetic field
d. magnetic flux density
e. conductor moving in a magnetic field
13. The figure to the right shows a circuit with two identical resistors (marked 1
and 2) and an ideal inductor. The switch is closed and kept closed for a long
time. Is the current in resistor 1 greater than, less than, or equal to the
current in resistor 2 at the moment the switch is re-opened?
a. more b. less
c. the same
14. The primary coil shown to the right has a radius r, length L and the number
of turns per meter, n. The secondary coil is wound over the primary as
shown and has a total of N turns. Determine the voltage in the secondary
coil when the current in the primary coil changes at a rate of di/dt.
𝑑𝐼
a. 𝜇𝑜 𝜋𝑟 2 𝑛𝑁
b. 𝜇𝑜 𝜋𝑟 2
𝑑𝑡
d. 0
e. none of these
c. 𝜇𝑜 𝜋𝑟 2 𝑛𝑁
𝑑𝐼
𝑑𝑡
15. Two parallel wires carry the same current and attract each other with a force
F per unit length. If both currents are tripled and the distance between them
is doubled, then the force per unit length is
.
a. 2F/9 b. 3F/4 c. 4F/3 d. 9F/2
e. 9F/4
NJSL Phy C April 2011 Exam
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16. An electron (mass m, charge e) moves in a circle of a radius r inside
a long solenoid. The circle is concentric with the solenoid and the
electron moves with a speed v in the plane perpendicular to the
magnetic field of the solenoid. The solenoid has n turns per meter.
What is the current in the solenoid?
a.
𝑚𝑣
𝑒𝜇𝑜 𝑟𝑛
mvr
b. eμ n
o
c.
eμo n
mvr
d.
mrv2
en
e.
mv2
enr
17. An electron (mass m, charge e) with kinetic energy K moving along the
positive direction of an x axis enters a region in which a uniform electric field
of magnitude E is in the negative y direction. What is the magnitude of the
minimum uniform magnetic field needed to keep the electron moving straight
along the x-axis?
a.
𝑒𝑚𝐸
√𝐾
b.
emE
√𝐾
c.
𝑒𝐸√
2𝐾
d. 𝐸√
𝑚
𝑚
2𝐾
e. 𝑒𝑚𝐸√𝐾
18. In the figure to the right, a current i is set up in a long hairpin
conductor formed by bending a wire into a semicircle of radius
R. Point A is the center of the semicircle. What is the magnitude
of at point A?
a.
𝜇𝑜 𝐼(𝜋+2)
4𝜋𝑅
b.
𝜇𝑜 𝐼
4𝜋𝑅
c.
𝜇𝑜 𝐼
𝜋𝑅
d. 0
e. none of these
19. Which of the following correctly represents the relationship of the magnetic field outside a long
straight current-carrying wire at a distance R from the axis of the wire?
a. B  1/R2
b. B  1/R3/2
c. B  1/R3
d. B  1/R
e. B  R
20. A rectangular loop of wire is placed midway between two long straight
parallel conductors as shown. The conductors carry currents i1 and i2 as
indicated. If i1 is increasing and i2 is constant, then the induced current
in the loop is
.
a. clockwise
b. zero
c. depends on i1 + i2
d. depends on i1 - i2
e. counterclockwise
NJSL Phy C April 2011 Exam
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For questions #21 and 22 refer to the following circuit diagram:
Initially, the switch was open for a long time. The fluxes of the
inductors are completely isolated, that is the flux of one coil does not
pass through the other coil.
21. Find the rate of change of current in the resistor immediately after the
switch is closed.
a. 4000 A/s
b. 6000 A/s c. 8000 A/s d. 9000 A/s e. 0
22. Find the current in the resistor long time after the switch is closed.
a. 0 b. 0.6 A c. 0.8 A
d. 1.2 A
e. 1.6 A
23. How many time constants after t = 0 does the current in an inductor or the charge on a capacitor
reach 95% of their respective maximum current or charge?
a. 1
b. 2
c. 3
d. 4
e. 5
24. The area A is perpendicular to the three parallel wires shown here.
Determine the magnetic field circulation around the boundary of the
area A.
a. 0
b. you need to know the area A
c. you need to know the length of the path
d. 60π x 10-7 Wb
e. 20π x 10-7 Wb
The drawing below shows four situations in which two very long wires
are carrying the same current, although the direction of the currents may be different.
The point P in the drawings is equidistant from each wire.
25. Which one (or more) of these situations gives rise to a zero net magnetic field at P?
a. 1, 3 and 4
b. 2 and 4
c. 2 and 3 d. only 2 e. only 1
NJSL Phy C April 2011 Exam
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