Chapter 20 Electromagnetism
300 points (12.5 points each)
1. 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
2. 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.
3. What in a DC motor reverses the polarity and current each half-cycle?
b. solid ring commutator
c. a split ring commutator
d. a CRT
4. 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
5. 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.
6. 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?
7. 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
8. The source of the earth's magnetic field is thought to be due to the motion of charged particles in the earth's
c. inner core.
d. outer core.
9. The Earth reversed its magnetic field about how many years ago?
10. 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
11. 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? (give a degree and a heading – eg 45o South of North)
12. When a voltage is applied for display on an oscilloscope CRT, it is usually shown:
13. 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
14. A wire moves across a magnetic field. The emf 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.
15. 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.
16. 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.
17. 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.
18. 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
19. 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.
20. 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?
21. A step-down transformer is needed to reduce a primary voltage of 120 V AC to 6 V AC. What turns ratio is required?
22. 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)
23. 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? ______
24. A Weber per second is equivalent to what electrical unit?