© 2001 John Wiley & Sons
Electric motors spin the parts of many household machines. Sometimes this rotary motion
is obvious, as in a fan or mixer, but often it’s disguised, as in the agitator of a washing
machine or the window opener of an automobile. Motors come in a variety of shapes and
sizes, each appropriate to its task. No matter how much torque or power a motor must
provide, you can probably find one that’s suitable. Some motors operate from direct cur-
rent and can be used with batteries while others require alternating current. There are
even motors that work on either type of current.
In this section, we’ll examine the basic mechanisms of electric motors. We’ll look at
what makes a motor turn and how motors differ from one another. As we do, we’ll revisit
many of the relationships between electricity and magnetism that we’ve been studying
since Chapter 8.
Questions to Think About: How can magnetic forces cause something to spin? If mag-
netic forces cause motors to spin, why can’t a motor be built exclusively from permanent
magnets? What determines which way a motor turns? Why are some motors safe for use
with flammable chemicals while others are not?
Experiments to Do: Take a look at several different electric motors. You’ll find motors in
a cassette tape recorder, a room fan, a kitchen mixer, and in the starter of a car. Motors in
the cassette recorder and the automobile starter both run on DC power, but one consumes
far more electric power and provides far more mechanical power than the other. What
aspects of a starter motor allow it to handle so much power?
Motors in the fan and the mixer both run on AC power, but their internal structures
are quite different. The fan motor starts slowly and gradually picks up speed while the
mixer motor rotates at almost full speed only moments after you turn it on. How do the
CHAPTER 9. ELECTRODYNAMICS