E2T2 Practice Problems – Ch 26-30
1. A long straight wire carries a constant current I. The magnitude of the magnetic flux
through the illustrated rectangular loop of wire is
A) (0/4)2Il ln(b/a)
B) (0/4)4Il ln(b/a)
C) (0/4)Il ln[(a + b)/(b – a)]
D) (0/4)4Il ln[(b – a)/(b + a)]
E) (0/4)2Il ln[(b – a)/(b + a)]
2. Calculate the magnetic field at the center of a circular current loop of area 0.200 m2
divided by the magnetic field at the center of a square current loop of the same inside
area for the same current I value.
A) 1.00
B) 1.02
C) 0.246
D) 0.985
E) 0.785
3. A wire of radius 0.35 cm carries a current of 75 A that is uniformly distributed over its
cross-sectional area. The magnetic field B 0.20 cm from the center of the wire is
approximately
A) 2.5 mT
B) 8.8 T
C) 3.8 mT
D) 15 mT
E) 2.9 T
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4. By what percentage does the resistance of a copper wire ( = 3.9 10–3/K) increase
when its temperature increases from 20C to 100C?
A) 11%
B) 14%
C) 23%
D) 31%
E) 57%
Use the following to answer question 5:
5. Two long parallel wires are a distance d apart (d = 6 cm) and carry equal and opposite
currents of 5 A. Point P is distance d from each of the wires. Calculate the magnitude of
the magnetic field strength at point P.
A) 2.9 10–5 T
B) 8.5 10–6 T
C) 3.3 10–5 T
D) 1.7 10–5 T
E) none of the above
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6.
When the ammeter reads 7 A, the voltmeter reads
A) 2.8 V with Y at the higher potential.
B) 2.8 V with X at the higher potential.
C) 34 V with Y at the higher potential.
D) 34 V with X at the higher potential.
E) 60 V with X at the higher potential.
7. The apparatus in the figure consists of two parallel plates (shown on edge) and a large
magnet (not shown). The field of the magnet is uniform, perpendicular to the electric
field between the plates, and directed into the plane of the paper. The magnitude of B
is 0.40 T. Charged particles with speeds of 5.0 105 m/s enter this region through the
slit at the left and emerge through the exit slit at the right. What magnitude and
direction must the E field have so that positively charged particles entering from the
left will traverse to the exit slit undeviated?
A) 2.0 105 V/m up
B) 2.0 105 V/m down
C) 1.2 106 V/m down
D) 1.2 106 V/m up
E) 2.4 106 V/m down
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8. In this circuit, the current through the 5- resistor is approximately
A) 4A
B) 20 A
C) 10 A
D) 40 A
E) 8A
9. A wire of radius 0.35 cm carries a current of 75 A that is uniformly distributed over its
cross-sectional area. The magnetic field B at the surface of the wire is approximately
A) 8.4 mT
B) 4.3 mT
C) 6.7 mT
D) 2.3 mT
E) 5.7 mT
10. An open switch in an RL circuit is closed at time t = 0, as shown. The curve that best
illustrates the variation of potential difference across the inductor as a function of time is
A) 1
B) 2
C) 3
D) 4
E) 5
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11. A region of space contains a magnetic field of 500 G and an electric field of 3 106
N/C. The magnetic energy density in a cubical box of side = 20 cm in this region
is
A) 550 J/m3
B) 670 J/m3
C) 864 J/m3
D) 995 J/m3
E) None of these is correct.
Use the following to answer question 12:
12. The switch S is initially at position a for a long time. It is then switched to position b.
Describe what happens to the light bulb as a function of time when the switch is flipped
from a to b?
A) The light bulb was on but goes off immediately.
B) The light bulb was off and stays off.
C) The light bulb was on but its brightness decreases with time and eventually goes
off.
D) The light bulb was on and stays on.
E) The light bulb was off. It then lights up but the brightness decreases with time and
eventually goes off.
13. What is the average induced EMF between the ends of the wings of a plane flying at a
speed of 500 km/hr when the vertical component of Earth's magnetic field is 3 10–5 T?
The distance between the wingtips is 25 m.
A) 0.20 V
B) 0.050 V
C) 6.0 V
D) 0.10 V
E) none of the above
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14. In your four years at college you dry your hair everyday for 5 minutes with a 1200-W
hairdryer. If the average price of electrical energy was $0.085 per kilowatt-hour, how
much did it cost you during this time? (Assume 1 year = 365 days.)
A) $4.10
B) $8.90
C) $12.4
D) $22.8
E) $15.6
15. In this circuit, the batteries have negligible internal resistance and the ammeter has
negligible resistance. The energy delivered by the 18-V battery in 4 s is approximately
A) 30 J
B) 45 J
C) 72 J
D) 141 J
E) 163 J
Use the following to answer question 16:
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16. A rectangular coil moving at a constant speed v enters a region of uniform magnetic
field from the left. While the coil is entering the field, the direction of the magnetic
force is
A) 1
B) 2
C) 3
D) 4
E) 5
17. A 20-F capacitor is charged to 200 V and is then connected across a 1000- resistor.
The amount of charge on the capacitor after 10 ms is approximately
A) 14 mC
B) 2.4 mC
C) 0.80 mC
D) 1.6 mC
E) 3.2 mC
18. A circular 20-turn coil with a radius of 10 cm carries a current of 3 A. It lies in the xy
plane in a uniform magnetic field B = 0.4 T iˆ + 0.3 T k̂ . The coil rotates so that its
magnetic moment, , is aligned with the magnetic field. The potential energy of the
system when the coil's magnetic moment is aligned with the field is
A) 0.282 J
B) –0.461 J
C) 0.461 J
D) –0.282 J
E) –0.564 J
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19. A wire of length L, cross sectional area A and resistance R is now drawn out by pulling
such that its new resistance is 25R. What is its new cross sectional area? (Assume the
density remains constant.)
A) 1.00 10–1 A
B) 6.67 10–1 A
C) 4.00 10–1 A
D) 2.50 10–1 A
E) 2.00 10–1 A
Use the following to answer question 20:
20. The copper strip in the figure is in a magnetic field of 2.5 T and the current flowing in it
is 15 A. The number density of free electrons (q = 1.6 10–19 C) in copper is 8.47
1022 electrons per cubic centimeter. The Hall voltage is
A) 2.6 V
B) 1.9 mV
C) 3.5 V
D) 4.2 mV
E) None of these is correct.
21. For copper, = 8.93 g/cm3 and M = 63.5 g/mol. Assuming one free electron per
copper atom, what is the drift velocity of electrons in a copper wire of radius 0.625 mm
carrying a current of 3 A?
A) 3.54 10–4 m/s
B) 1.80 10–4 m/s
C) 4.26 10–4 m/s
D) 7.52 10–4 m/s
E) 2.46 10–4 m/s
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22. A circular, 20-turn coil has a radius of 5.0 cm. What is the magnitude of the magnetic
moment of the coil when it carries a current of 2.5 A?
A) 1.5 A · m2
B) 0.16 A · m2
C) 2.0 10–2 A · m2
D) 0.39 A · m2
E) 3.9 kA · m2
23. You place a single loop of wire 0.50 m by 0.30 m perpendicular to a field of 2.0 T. In
30 ms you turn the loop until it is parallel with the field. The average emf induced in
this loop is
A) 0.30 V
B) 10 V
C) 5.0 V
D) 67 V
E) 20 V
24. An alpha particle has a charge of +2e (e = 1.6 10–19 C) and is moving at right angles to
a magnetic field B = 0.27 T with a speed v = 6.15 105 m/s. The force acting on this
charged particle is
A) zero
B) 5.3 10–14 N
C) 3.3 105 N
D) 2.7 10–14 N
E) 4.8 105 N
25. What is the current through the circuit in the figure?
A) +0.83 A
B) –0.50 A
C) +0.50 A
D) +0.55 A
E) –0.92 A
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26. A circular loop of wire 1.0 cm in radius carries a current of 30 A. The magnetic field at
the center of the loop is
A) 1.9 mT
B) 2.4 mT
C) 3.8 mT
D) 12 T
E) 48 T
27. A rectangular loop of wire (0.10 m by 0.20 m) carries a current of 5.0 A in a
counterclockwise direction. The loop is oriented as shown in a uniform magnetic field
of 1.5 T. The force acting on the upper 0.10-m side of the loop is
A) 1.5 N
B) 0.75 N
C) 0.50 N
D) 0.15 N
E) zero
28. A 100-turn coil has a radius of 7.50 cm and a resistance of 50.0 . At what rate must a
perpendicular magnetic field change to produce a current of 5.00 A in the coil?
A) 275 T/s
B) 134 T/s
C) 329 T/s
D) 141 T/s
E) 106 T/s
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29. A long solenoid with 20 turns per centimeter has a core of iron-silicon. When the
current is 1.50 A, the magnetic field inside the core is 4.50 T. The applied field B app is
approximately
A) 1.88 mT
B) 3.77 mT
C) 4.32 mT
D) 3.87 mT
E) 1.93 mT
30. Each of three identical electric heaters is rated at 1000 W when operated across 100-V
lines. When the three are connected in series across a 120-V line, at what rate is
electric energy converted to heat energy? (Neglect variation of resistance with
temperature.)
A) 0.48 kW
B) 1.0 kW
C) 1.2 kW
D) 3.0 kW
E) 3.6 kW
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Answer Key
1. A
2. D
3. A
4. D
5. D
6. C
7. B
8. C
9. B
10. E
11. D
12. E
13. D
14. C
15. D
16. C
17. B
18. D
19. E
20. E
21. B
22. D
23. B
24. B
25. C
26. D
27. B
28. D
29. B
30. A
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