EXAM 3
PHYS 121
1. What is the radius of curvature of the path of a 3.0-keV proton in a perpendicular magnetic field of magnitude 0.80 T?
a.
b.
c.
d.
e.
9.9 mm
1.8 cm
2.3 cm
3.4 cm
7.6 mm
2. An electron follows a circular path (radius = 15 cm) in a uniform magnetic field (magnitude = 0.3 mT). What is the period of this
motion?
a.
b.
c.
d.
e.
0.12 µs
1.2 ms
0.18 µs
1.8 ms
1.8 µs
3. A proton moving with velocity of 5x106 m/s at an angle 450 to both the z axis and x axis enters a magnetic field B = 0.02 T
pointing out of the page. What is the force on the proton due to the magnetic field?
a.
b.
c.
d.
e.
(1.6 i -1.6 k)x10-14N
(-1.6 i -1.6 j)x10-14N (b)
(1.8 i +1.6 k)x10-14N
(-1.6 j -1.6 k)x10-14N
(1.2 i -1.6 j)x10-14N
4. A current loop is oriented in three different positions relative to a uniform magnetic field. In position 1 the plane of the loop is
perpendicular to the field lines. In position 2 and 3 the plane of the loop is parallel to the field as shown. The torque on the loop is
maximum in
a.
b.
c.
d.
e.
only position 1.
only position 2.
only position 3
positions 2 and 3.
all three positions.
5. A circular coil (radius = 0.40 m) has 160 turns and is in a uniform magnetic field. When the orientation of the coil is varied
through all possible positions, the maximum torque on the coil by magnetic forces is 0.16 N  m when the current in the coil is 4.0
mA. What is the magnitude of the magnetic field?
a.
b.
c.
d.
e.
0.37 T
1.6 T
0.50 T
1.2 T
2.5 T
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EXAM 3
PHYS 121
6.
What is the magnitude of the magnetic field at point P if a = 5 mm and b = 12 mm and current I = 16 A?
a.
b.
c.
d.
e.
0.28 mT
0.44 mT
0.85 mT
1.08 mT
1.50 mT
7. The figure shows a cross section of three parallel wires each carrying a current of 15 A. The currents in wires A and C are out of
the paper, while that in wire B is into the paper. If the distance R = 5.0 mm, what is the magnitude of the force on a 4.0-m length of
wire C?
a.
b
c.
d.
e.
90 mN
54 mN
30 mN
18 mN
36 mN
8. The segment of wire (total length = 6R) is formed into the shape shown and carries a current I. What is the magnitude
of the resulting magnetic field at the point P?
a.
b.
c.
d.
e.
µ0I
8R
µ0I
2R
µ0I
4R
µ0I
2R
µ0 I
8R
a
2
9. The coil shown in the figure has 2 turns, a cross-sectional area of 0.20 m , and a
2
field (parallel to the axis of the coil) with a magnitude given by B = (4.0 + 3.0t ) T,
where t is in s. What is the potential difference, VA – VC, at t =3.0 s?
a.
b.
c.
d.
e.
–7.2 V
+7.2 V
–4.8 V
+4.8 V
–12 V
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EXAM 3
PHYS 121
10. The circuit shown below is in a uniform magnetic filed that is into the page and decreasing in
magnitude at the rate 250 T/s. What is the net current in the circuit?
a.
b.
c.
d.
e.
0.08 A
0.24 A
0.40 A
0.60 A
0.96 A
11.
In the arrangement shown, a conducting bar of negligible resistance slides along horizontal, parallel, frictionless conducting
rails connected as shown to a 2.0-Ω resistor. A uniform 1.5-T magnetic field is perpendicular to the plane of the paper. If L = 60 cm,
at what rate is thermal energy being generated in the resistor at the instant the speed of the bar is equal to 4.2 m/s?
a. 8.6 W
b. 7.8 W
c. 7.1 W
d. 9.3 W
e. 1.8 W
12. A uniform magnetic is normal to the plane of a single, circular loop of wire, 12 cm in radius and of 0.06  resistance. At what
rate must the magnetic field change with time if an induced current of 2 A is to appear in the loop?
A. 1.00 T/s
B. 1.42 T/s
C. 1.76 T/s
D. 2.65 T/s
E. 3.00 T/s
Workout problems Show your work.
Workout problem1 ( 3 points). Suppose the switch has been closed for a time interval sufficiently long for the capacitor to become
fully charged. Assume C = 10 μF, R1 = 5.0 kΩ, R2 = 15.0 kΩ, R3 = 10.0 kΩ, and Ε = 18 V .
a. What is the intial charge on the capacitor? Ans._______________________
b. At t = 0 the switch is opened. Find the time interval required for the charge on the
capacitor to fall to half its initial value.
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EXAM 3
PHYS 121
Answer a) Q = 108 μC
b) t = 0.14 s
Workout problem 2 ( 2 points).
are 20 cm apart.
Two long wires are oriented so that they are perpendicular to each other, and at their closest, they
a) What is the magnetic field in unit vector notation at a point midway between them if the top wire carries a current of 20-A and
the bottom wire carries current of 10-A? Ans.______________________
b) What is the magnitdue of the magnetic field at point P? Ans.__________________
Answer: a) B = (4x10-5 T)i + (3x10-5 T) k
Answer : b) B = 5.0 x10-5 T
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EXAM 3
PHYS 121
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