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Electromagnetic Induction and Electromagnetic
Waves
Topics:
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Electromagnetic induction
Lenz’s law
Faraday’s law
The nature of electromagnetic waves
The spectrum of electromagnetic
waves
Sample question:
The ultraviolet view of the flowers on the right shows markings that
cannot be seen in the visible region of the spectrum. Whose eyes are
these markings intended for?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-1
Lenz’s Law
Lenz’s law There is an induced
current in a closed, conducting
loop if and only if the magnetic flux
through the loop is changing. The
direction of the induced current is
such that the induced magnetic
field opposes the change in the
flux.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-13
Using Lenz’s Law
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-14
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-18
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say
about the current in the loop?
B. The loop has a counterclockwise current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-19
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-20
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-21
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after the
switch is closed, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-26
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after the
switch is closed, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-27
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Long after the switch
is closed, what can we say about the current in the lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-28
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Long after the switch
is closed, what can we say about the current in the lower loop?
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-29
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after the
switch is reopened, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-30
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after the
switch is reopened, what can we say about the current in the lower
loop?
B. The loop has a counterclockwise current.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-31
Into the Field
A five-turn rectangular loop is moved through a uniform field at 2 m/s as shown
below.
1.What is the maximum magnetic flux through the loop during its motion
through the field? The loop is 5 cm long and 3 cm wide.
2.The loop takes 100 ms to completely enter the field. Sketch a graph of the
magnetic flux through the loop in the interval from t=0 to t=150 ms. Label
values of flux. (Assume the loop begins to enter the magnetic field at t = 0
s)
3.Compute the emf in the loop while it is entering the field.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-32
Additional Examples
2. The outer coil of wire is 10 cm long, 2 cm in diameter, wrapped
tightly with one layer of 0.5-mm-diameter wire, and has a total
resistance of 1.0 Ω. It is attached to a battery, as shown, that
steadily decreases in voltage from 12 V to 0 V in 0.5 s, then
remains at 0 V for t > 0.5 s. The inner coil of wire is 1 cm long, 1
cm in diameter, has 10 turns of wire, and has a total resistance
of 0.01 Ω. It is connected, as shown, to a current meter.
a. As the voltage to the outer coil begins to decrease, in which
direction (left-to-right or right-to-left) does current flow
through the meter? Explain.
b. Draw a graph showing the current in the inner coil as a
function of time for 0 ≤ t ≤ 1 s. Include a numerical scale on
the vertical axis.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-55
Generators
• A generator is a device that converts
mechanical energy to electric energy.
• Rather than move a straight wire through
a magnetic field, it is more practical to
rotate a coil of wire. As the coil rotates,
one edge always moves upward through
the electric field while the other edge
moves downward.
• The motion of the wire induces a current,
which is then removed by brushes that
press up against rotating slip rings.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Generators
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Generators
• As the coil in a generator rotates, the
sense of emf changes, giving a
sinusoidal variation of emf as a function
of time.
• The alternating sign of the voltage
produces an alternating current, AC.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Electric Generators
A sinusoidal emf is induced in the rotating
loop (N is the number of turns, and A the area
of the loop):
(21-5)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.