CHAPTER 4—ELECTROMAGNETISM
TRUE/FALSE
1. Magnetic lines of force are also referred to as flux lines.
ANS: T
DIF: Easy
2. Coulomb’s Law can be applied to magnetism.
ANS: T
DIF: Medium
3. Electromagnets are permanent magnets.
ANS: F
DIF: Easy
4. Fleming's hand rules are based on electron flow.
ANS: F
DIF: Medium
5. Mutual induction occurs when two coils are connected in series, causing current to flow in the
secondary coil.
ANS: F
DIF: Medium
6. An AC generator produces a current that is expressed mathematically as a sinusoidal wave.
ANS: T
DIF: Easy
7. The apparent total resistance of an AC current is called impedance.
ANS: T
DIF: Easy
8. As the length of a conductor increases, the resistance increases.
ANS: T
DIF: Medium
MULTIPLE CHOICE
1. A magnetic field is produced by ____.
a. electric charges at rest
b. electric charges in motion
c. permanent magnets only
d. electric charges both at rest and in motion
ANS: B
DIF: Easy
2. An alternately expanding and contracting magnetic field is produced by ____.
a. a stationary magnet
c. a battery
b. a steady current flowing in a wire
d. none of the above
ANS: D
DIF: Medium
3. A diode electron tube ____.
a. is used for amplification
b. has a negatively charged anode
ANS: D
c. cannot reach saturation
d. can be used for rectification
DIF: Medium
4. The magnetic flux through a wire loop ____.
a. requires a time varying magnetic field
b. is maximum when the plane of the loop is perpendicular to the magnetic field
c. increases as the magnetic field decreases
d. none of the above
ANS: B
DIF: Medium
5. The charge carriers are considered to be a negative in a ____.
a. diode
c. N-type semiconductor
b. triode
d. P-type semiconductor
ANS: C
DIF: Medium
6. Electrons move southward in a vertical wire. The direction of the associated magnetic field directly
around the wire is ____.
a. clockwise
c. in the direction of the electron flow
b. counter-clockwise
d. opposite to the conventional current
ANS: B
DIF: Difficult
7. An induced current in a wire loop ____.
a. results from a constant magnetic flux
b. can result only from an alternately expanding and contracting magnetic field
c. is in such a direction that its effects oppose the change producing it
d. always decreases when the magnetic flux decreases
ANS: B
DIF: Medium
8. The process of electrons being emitted from heated metallic surfaces is called ____.
a. thermionic emission
c. equilibrium
b. thermionic remission
d. potential difference
ANS: A
DIF: Easy
9. The magnitude felt by a moving charge through a magnetic field would be least if the charge were
____.
a. moving directly into the magnetic field
b. moving directly away from the magnetic field
c. moving at a right angle to the magnetic field
d. both a and b
ANS: D
DIF: Difficult
10. A generator is a device that converts ____.
a. electrical energy into heat energy
b. heat energy into electrical energy
c. mechanical energy into electrical energy
d. electrical energy into mechanical energy
ANS: C
DIF: Easy
11. A simple DC generator has essentially the same construction as an AC generator except that a DC
generator has ____.
a. slip rings
c. brushes
b. a commutator
d. armature
ANS: B
DIF: Medium
12. A rectifier ____.
a. refers to a type of electromagnetic device
b. refers to a type of electromechanical device
c. converts DC to AC
d. converts AC to DC
ANS: D
DIF: Medium
13. A motor is a device that converts ____.
a. heat energy into electrical energy
b. electrical energy into mechanical energy
c. electrical energy into heat energy
d. mechanical energy into electrical energy
ANS: B
DIF: Easy
14. A transformer with more secondary windings than primary windings ____.
a. has a greater secondary voltage
c. has a greater power output than input
b. is a step-down transformer
d. none of the above
ANS: A
DIF: Medium
15. Which of the following is a component of a tube-type rectifier?
a. filament
c. primary windings
b. magnetic core
d. secondary windings
ANS: A
DIF: Medium
16. A bar magnet and a wire loop move with the same linear velocity. What is the voltage induced in the
wire?
a. relative to the angle of incidence
c. inversely proportional to the velocities
b. directly proportional to their velocities
d. zero
ANS: D
DIF: Medium
17. For a simple AC generator, the value of the induced voltage in the armature loop is maximum when
the orientation of the armature as compared to the magnetic field is ____.
a. at a node
c. 90 degrees
b. 45 degrees
d. 180 degrees
ANS: C
DIF: Medium
18. How many times does an AC current change direction in a single minute?
a. 1
c. 60
b. 2
d. 7,200
ANS: D
DIF: Medium
19. The efficiency of a transformer is not affected by ____.
a. eddy current loss
c. hysteresis loss
b. power loss
d. all of the above
ANS: D
DIF: Medium
20. American generators operate at ____.
a. 220 V
b. 120 AC
ANS: C
DIF: Easy
21. A stator consists of ____.
a. bars of copper around an iron core
b. commutator rings
ANS: D
c. 60 Hz
d. 30 A
c. a rotor
d. electromagnets
DIF: Medium
22. Conventional current flows ____.
a. from cathode to anode
b. from anode to cathode
c. in either direction
d. in the same direction as the electron flow
ANS: B
DIF: Medium
SHORT ANSWER
1. List the three ways to induce an EMF.
ANS:
move the conductor through a magnetic field, move the magnetic field across a stationary conductor,
and vary the magnetic field strength across a stationary conductor
DIF: Medium
2. The magnitude of an induced EMF depends on what four factors?
ANS:
the strength of the magnetic field, the speed of the motion between lines of force and the conductor,
the angle between the magnetic lines of force and the conductor, and the number of turns in the
conducting coil
DIF: Difficult