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Electromagnetic Induction and Electromagnetic
Waves
Topics:
• 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
Electromagnetic Induction
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Slide 25-8
Motional emf
  vlB
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Slide 25-9
Magnetic Flux
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Slide 25-10
Checking Understanding
A loop of wire of area A is tipped at an angle  to a uniform
magnetic field B. The maximum flux occurs for an angle   0.
What angle  will give a flux that is ½ of this maximum value?
A.   30
B.   45
C.   60
D.   90
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-11
Answer
A loop of wire of area A is tipped at an angle  to a uniform
magnetic field B. The maximum flux occurs for an angle   0.
What angle  will give a flux that is ½ of this maximum value?
C.   60
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-12
Faraday’s Law
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Slide 25-15
21.5 Electric Generators
A generator is the opposite of a motor – it
transforms mechanical energy into electrical
energy. This is an ac generator:
The axle is rotated by an
external force such as
falling water or steam.
The brushes are in
constant electrical
contact with the slip
rings.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
21.5 Electric Generators
A dc generator is
similar, except that it
has a split-ring
commutator instead of
slip rings.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
21.5 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.
Checking Understanding
A long conductor carrying a current runs next to a loop of wire. The
current in the wire varies as in the graph. Which segment of the
graph corresponds to the largest induced current in the loop?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-16
Answer
A long conductor carrying a current runs next to a loop of wire. The
current in the wire varies as in the graph. Which segment of the
graph corresponds to the largest induced current in the loop?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-17
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
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Slide 25-14
Saul notes on using Lenz’s Law
Lenz’s law says that the B-field from the induced current will oppose
the change in magnetic flux from the source B-field.
A. Find direction of B-field vector in loop
B. Determine if magnetic flux from source is increasing or
decreasing
C.
•
If magnetic flux from source is decreasing, direction of
Binduced is same as Bsource
•
If magnetic flux from source is increasing, direction of
Binduced is opposite of Bsource
Use RHR 3 to find direction of induced current
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-23
Right Hand Rules (RHR) for magnetic forces & fields
Right-hand rule 1 (RHR 1) => for finding magnetic force
FB= q*v_vector x B_vector (Cross-Product Rule)
Point right hand in the direction the charges are moving (current or velocity)
Rotate your right hand until you can point your fingers in the direction of the
magnetic Field
Thumb points in direction of force for + charge
Force is in opposite direction for - charges
(Text references pp 804-806)
Right-hand rule 2 (RHR 2) => Finding direction of B from I
Point thumb of right hand in direction of current I,
B-field lines curl in direction of fingers
(Text references pp 795 & 797)
Right-hand rule 3 (RHR 3) =>
Finding direction of current in a loop from direction of B-field
Point thumb of right hand in direction of B-field
Fingers of right hand curl in direction of current
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 25-23