Chapter 20 Electromagnetism
On-Line Quiz
300 points (12.5
points each)
19. A
mass spectrometer needs:
a. only a magnetic field
b. only an electric field
c. colinear magnetic and electric
fields
d. perpendicular magnetic and
electric fields
20. What
fundamental fact underlies the operation of essentially all electric motors?
a. Opposite electric charges attract
and like charges repel.
b. A current-carrying conductor
placed perpendicular to a magnetic field will experience a force.
c. Alternating current and direct
current are both capable of doing work.
d. Iron is the only element that is
magnetic.
e. A magnetic north pole carries a
positive electric charge, and a magnetic south pole carries a negative electric
charge.
21. What
in a DC motor reverses the polarity and current each half-cycle?
a. fuse
b. solid ring commutator
c. a split ring commutator
d. a CRT
22. The
torque on the rotating loop of wire in a DC motor _____ as the loop rotates.
a. is constant in magnitude
b. is zero
c. varies in magnitude
d. is inversely proportional to the
current
23. In
a pair of accelerating plates, such as found inside a CRT, the electrons are
emitted
a. from the cathode which is
positive, toward the anode which is positive.
b. from the cathode which is
negative, toward the anode which is positive.
c. from the anode which is positive,
toward the cathode which is negative.
d. from the anode which is negative,
toward the cathode which is positive.
24. An
electron enters the velocity selector of a mass spectrometer in the direction
shown in the sketch. If it emerges
along the path shown, which plate (A or B) has the higher potential?
25. The
earth's magnetic field at the magnetic equator is approximately
a. 104
T
b. 1/2 T
c. 10-5
T
d. 10-3
T
26. The
source of the earth's magnetic field is thought to be due to the motion of
charged particles in the earth's
a. mantle.
b. crust.
c. inner core.
d. outer core.
27. The
Earth reversed its magnetic field about how many years ago?
a. 11.
b. 700.
c. 700,000.
d. 70,000,000.
28. The
pole of the compass which points toward the Earth's SOUTH geographic pole is a
a. magnetic north pole
b. magnetic south pole
c. positive charge pole
d. magnetic barber pole
29. If
you find yourself in a locality where the magnetic declination is "15.oEast,"
in which direction is true north in relation to a compass needle?
a. 15ø WEST
of the needle
b. 15ø EAST
of the needle
c. 15ø SOUTH WEST
of the needle
d. 15ø SOUTH EAST
of the needle
30. When
a voltage is applied for display on an oscilloscope CRT, it is usually shown:
a. horizontally
b. diagonally
c. vertically
31. A
coil of 160 turns and area 0.20 m2 is
placed with its axis parallel to a magnetic field of 0.4 T. The magnetic field changes from 0.4 T in the x-direction to
0.4 T in the negative x-direction in 2 s. If
the resistance of the coil is 16 W, at what rate is power generated in
the coil? (Hint: Remember that e
= Voltage and then you need to know how to find power if you know Voltage and W.)
a. 2.56 W
b. 10.2 W
c. 12.8 W
d. 0.8 W
32. A
wire moves across a magnetic field. The
emf (electromotive force = voltage) produced in the wire depends on
a. the field's magnetic flux.
b. the length of the wire.
c. the orientation of the wire with
respect to the magnetic field vector.
d. all of the above.
33. Faraday's
law of induction states that the emf induced in a loop of wire is proportional
to
a. the magnetic flux.
b. the magnetic flux density times
the loop's area.
c. the time variation of the
magnetic flux.
d. current divided by time.
34. Doubling
the number of loops of wire in a coil produces what kind of change on the
induced emf, assuming all other factors remain constant?
a. The induced emf is 4 times as
much.
b. The induced emf is 3 times as
much.
c. The induced emf is twice as much.
d. There is no change in the induced
emf.
35. A
horizontal metal bar rotates at a constant angular velocity w about a vertical
axis through one of its ends while in a constant magnetic field B that is
directed down. The emf induced
between the two ends of the bar is
a. constant and proportional to the
product Bw.
b. constant and proportional to the
product Bw2.
c. constant and proportional to the
product B2w2.
d. none of the above.
36. A
horizontal metal bar that is 2 m long rotates at a constant angular velocity of
2 rad/s about a vertical axis through one of its ends while in a constant
magnetic field of 5 x
10-5
T. If the magnetic field vector
points straight down, what emf is induced between the two ends of the bar?
a. 5.6 x
10-3
V
b. 3 x
10-3
V
c. 2 x
10-4
V
d. 1.6 x
10-4
V
37. A
generator coil rotates through one revolution 60 times each second.
The frequency of the emf is
a. 30 Hz.
b. 60 Hz.
c. 120 Hz.
d. cannot be determined from the
information given.
38. An
ideal transformer has 60 turns on its primary coil and 300 turns on its
secondary coil. If 120 V at 2 A is
applied to the primary, what voltage and current are present in the secondary?
a. 400 V; 0.4 A
b. 500 V; 0.4 A
c. 600 V; 0.4 A
d. 700 V; 0.4 A
39. A
step-down transformer is needed to reduce a primary voltage of 120 V AC to 6 V
AC. What turns ratio is required?
a. 50:1
b. 40:1
c. 30:1
d. 20:1
40. Consider
an outer loop of wire surrounding an inner loop of wire. Suppose the current in the outer loop is counterclockwise and
decreasing. What is the direction
of the induced current in the inner loop? (Hint:
Remember the law of conservation of energy must apply in all cases)
a. clockwise
b. counterclockwise
41. A
transformer with 120. turns in its secondary supplies 12. volts at 220.ma to a
toy train. The primary is connected
across a 120.V wall outlet.
a) How many turns are in the primary?
__________
b) What is the primary current? __________
c) What power is delivered by the wall
outlet? ______
a. a)
1.2 x 103
b) 20.0 mA c) 2.91 watts
b. a)
1.2 x 103
b) 22.0 mA c) 2.64 watts
c. a)
1.2 x 103
b) 22.0 mA c) 3.78 watts
d. a)
1.2 x 103
b) 18.0 mA c) 3.55 watts
42. A
Weber per second is equivalent to what electrical unit?
a. Watt
b. Ampere
c. Volt
d. Tesla