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Commutator Field
magnet 2 1 012 2 1 012
N
Brush

S Armature N
S S

S

e –

e – N N

FIGURE 6.32 A current is induced in a coil of wire moved
e – e – through a magnetic field. The direction of the current depends on
the direction of motion.
FIGURE 6.31 A schematic of a simple electric motor.

Electromagnetic induction occurs when the loop of wire

are now repelled for another half-turn. The commutator again moves across magnetic field lines or when magnetic fi eld lines
reverses the polarity, and the motion continues in one direction. move across the loop. The magnitude of the induced voltage is

An actual motor has many coils (called windings) in the armature proportional to (1) the number of wire loops passing through the

to obtain a useful force and many commutator segments. Th is gives magnetic field lines, (2) the strength of the magnetic fi eld, and
the motor a smoother operation with a greater turning force. (3) the rate at which magnetic field lines pass through the wire.

6.5 ELECTROMAGNETIC INDUCTION
CONCEPTS Applied
So far, you have learned that (1) a moving charge and a current-

carrying wire produce a magnetic field and (2) a second mag- Simple Generator
netic fi eld exerts a force on a moving charge and exerts a force
1. Make a coil of wire from insulated bell wire (#18
on a current-carrying wire as their magnetic fi elds interact.
copper wire) by wrapping 50 windings around a
Soon after the discovery of these relationships by Oersted

cardboard tube from a roll of paper. Tape the coil at
and Ampère, people began to wonder if the opposite eff ect was
several places so it does not come apart, and discard
possible; that is, would a magnetic field produce an electric cur-

the cardboard tube.

rent? The discovery was made independently in 1831 by Joseph 2. Make a current-detecting instrument from a magnetic
Henry in the United States and by Michael Faraday in England. compass and some thin insulated wire (the thinner
They found that if a loop of wire is moved in a magnetic fi eld, or if the better). Wrap the thin insulated wire in parallel

the magnetic field is changed, a voltage is induced in the wire. Th e windings around the compass. Make as many parallel

voltage is called an induced voltage, and the resulting current in windings as you can, but leave enough room to see

the wire is called an induced current. The overall interaction is both ends of the compass needle. Connect the wire
called electromagnetic induction. ends to the coil you made in step 1.
3. Orient the compass so the needle is parallel to the wire
One way to produce electromagnetic induction is to move
around the compass. When a current passes through
a bar magnet into or out of a coil of wire (Figure 6.32). A gal-
the coil of wire, the magnetic field produced will cause
vanometer shows that the induced current flows one way when

the needle to move, showing the presence of a current.
the bar magnet is moved toward the coil and flows the other

4. First, move a bar magnet into and out of the stationary
way when the bar magnet is moved away from the coil. Th e coil of wire and observe the compass needle. Second,

same effect occurs if you move the coil back and forth over a move the coil of wire back and forth over a stationary
stationary magnet. Furthermore, no current is detected when bar magnet and observe the compass needle.

the magnetic field and the coil of wire are not moving. Th us, 5. Experiment with a larger coil of wire, bar magnets of
electromagnetic induction depends on the relative motion of greater or weaker strengths, and moving the coil at

the magnetic field and the coil of wire. It does not matter which varying speeds. See how many generalizations you
moves or changes, but one must move or change relative to the can make concerning electromagnetic induction.
other for electromagnetic induction to occur.
6-23 CHAPTER 6 Electricity 161