21. Which one of the following principles best describes the basic operation of an electromagnet?

A. Current is induced in a conductor that is moving through a magnetic field
B. Like magnetic poles repel, while unlike poles attract
C. Current flowing through a conductor generates a magnetic field
D. Like magnetic poles attract, while unlike poles repel
E. None of the above

22. The transistor has a collector current of 2 mA. If the current gain is 135, what is the base current?

A. 14.8 µA
B. 10.3 µA
C. 7.8 µA
D. 7.75 µA
E. None of the above

23. Current and voltage are said to be in phase when:

A. their waveforms are both ac or both dc
B. they have the same amplitude and frequency
C. their waveforms cross through zero and reach positive and negative peaks at the same time
D. All of the above
E. None of the above

24. Silicon atoms combine into an orderly pattern called a

A. Crystal
B. Covalent bond
C. Semiconductor
D. Valence orbit
E. None of the above

25. The rotating coil of wire in an electric motor is called the:

A. armature
B. brush
C. commutator
D. loop
E. None of the above