122 sect. B – wilkes MC
There are 12 questions, each worth 5 points. Mark answers on your scanner sheet, and also here.
1.
Two positively charged bodies are moving in opposite directions on parallel paths that
lie in the xz plane. Their speeds are equal and their trajectories are equidistant from the
x axis. The magnetic field at the origin, due to the motion of these charged bodies will
be
A) in the +x direction.
D) in the -y direction.
B) in the +y direction.
E) zero.
C) in the -x direction.
Ans: B r.hand rule on each particle’s velocity points upward at origin.
2.
The current in a wire along the x axis flows in the positive x direction, as shown. If a
proton, located as shown in the figure, has an initial velocity in the positive z direction,
it experiences
A) a force in the direction of positive x.
D) a force in the direction of positive y.
B) a force in the direction of negative x.
E) no force.
C) a force in the direction of positive z.
Ans: E at location shown, B points same direction as v, so v x B=0
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122 sect. B - wilkes
3. Two parallel wires carry currents I1 and I2 = 2I1 in the same direction. Which of the
following shows the relationship of the magnitudes of force F1 on wire 1, and force F2
on wire 2?
A) F1 = F2 B) F1 = 2F2 C) 2F1 = F2 D) F1 = 4F2 E) 4F1 = F2
Ans: A Newton’s 3rd! or apply B force law to get the same result.
4. The two wires in a DC power cable are 5 mm apart and carry currents of 4 A in opposite
directions. If the cable is 1.5 m long, calculate the force between the wires and state if
this force is attractive or not.
A) 9.6 ! 10–4 N, attractive
D) 9.6 ! 10–4 N, repulsive
B) 9.6 ! 10–7 N, attractive
E) 1.1 ! 10–8 N, repulsive
C) 1.1 ! 10–8 N, attractive
Ans: D apply eqn 27-14, opposite currents repel (apply RHR, or recall demo in class)
5. A long solenoid of radius 3 cm has 1100 turns per m. If the solenoid wire carries a
current of 1.5 A, then calculate the magnetic field inside the solenoid.
A) 2.1 ! 10–3 T
D) 7.0 ! 10–2 T
B) 1.0 ! 10–3 T
E) none of the above
–4
C) 1.7 ! 10 T
Ans: A apply eqn 27-10
6. The plane of a circular, 200-turn coil of radius 5.25 cm is perpendicular to a uniform
magnetic field produced by a large electromagnet. This field is changed at a steady rate
from 0.650 T to 0.150 T in 0.0100 s. What is the magnitude of the EMF induced in the
coil?
A) 110 V B) 170 V C) 1.7 V D) 26 V E) 87 V
Ans: E apply eqns 28-4 and -5
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122 sect. B - wilkes
7.
A copper ring lies in the yz plane as shown. The magnet's long axis lies along the x axis.
Induced current flows through the ring as indicated. The magnet
A) must be moving away from the ring.
B) must be moving toward the ring.
C) must be moving either away from or toward the ring.
D) is not necessarily moving.
E) must remain stationary to keep the current flowing.
Ans: B
8. A rectangular coil moving at a constant speed v enters a region of uniform magnetic
field from the left, as shown above. While the coil is entering the field, the direction of
the magnetic force on it is in which of the directions shown in the box above?
A) 1 B) 2 C) 3 D) 4 E) 5
Ans: C flux in coil is increasing, so induced current opposes change: pushes coil out
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122 sect. B - wilkes
9. 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; you can regard the plane as a
conducting rod.
A) 0.20 V B) 0.050 V C) 6.0 V D) 0.10 V E) none of the above
Ans: D apply eqn. 28-7; don't forget to convert km/hr to m/sec!
10. After you measure the self-inductance of a solenoid, you unwind it, cut the wire in half,
and then rewind one half of the wire into a solenoid with the same diameter and length,
but half the number of turns. What is the self-inductance of the new coil, relative to the
original one?
A) it is the same
D) it is halved
B) it is doubled
E) it is quartered
C) it is quadrupled
Ans: E apply eqn. 28-13: L is proportional to n2
11. An LR circuit has a resistance R = 25 ", an inductance L = 5.4 mH, and a battery of
EMF = 9.0 V. How much energy is stored in the inductance of this circuit when a steady
current is achieved?
A) zero B) 0.35 J C) 0.35 mJ D) 0.70 mJ E) 0.97 mJ
Ans: C In steady state, L acts like a short, I=V/R=0.36, apply eqn. 28-21
12.
The time constant for the RL circuit is of the order of a few seconds. Describe what
happens to the light bulb when the switch S is closed.
A) The bulb comes on immediately and then goes off over a few seconds.
B) The bulb comes on gradually over a few seconds and stays on.
C) The bulb does not come on at all.
D) The bulb comes on gradually over a few seconds and then goes off instantaneously.
E) The bulb comes on gradually over a few seconds and then goes off gradually over a
few seconds.
Ans: B Apply eqn. 28-25: Inductor causes current to build up slowly.
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