Transcript Slide 1

Lecture PowerPoint
Physics for Scientists and
Engineers, 3rd edition
Fishbane
Gasiorowicz
Thornton
© 2005 Pearson Prentice Hall
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Chapter 30
Faraday’s Law
Main Points of Chapter 30
• Magnetic induction
• Induced current and Lenz’s law
• Motional emf
• Eddy currents
• Induced electric fields
• Generators
30-1 Faraday’s Discovery and the Law of
Induction
• Faraday wanted to know if magnetic
fields would create electric fields
• Steady fields did not
• But changing fields did!
30-1 Faraday’s Discovery and the Law of
Induction
There are many ways to change the
magnetic flux through a surface:
• Move the magnet
• Turning current on or off in one loop
induces current in another
• Move the loop
• Change the shape (and the area) of the
loop
30-1 Faraday’s Discovery and the Law of
Induction
• A changing magnetic flux through a loop
induces a current around the loop
• Faraday’s law of induction:
(30-2)
30-1 Faraday’s Discovery and the Law of
Induction
The direction of the induced current is
given by Lenz’s law:
The induced current is in a direction such
that the magnetic field it creates tends to
oppose the change that created it
If it were in the other direction, energy
would not be conserved
30-2 Motional emf
• Created by a conductor moving in
a magnetic field
• “Loop” is imaginary but real emf is
induced in conductor
30-2 Motional emf
• Motional emf:
(30-5)
Eddy currents: produced in
conductor moving in magnetic field
• Can make a very effective brake
• If you don’t want a brake, eddy
currents can be foiled by cutting
holes (slots) in conductor
30-3 Forces and Energy in Motional emf
Consider loop moving from region of constant
magnetic field into region of no magnetic field.
Flux changes as loop crosses boundary.
Force tends to slow loop down (drag force)
Magnitude of force:
(30-8)
30-4 Time-Varying Magnetic Fields
A changing magnetic field induces an
electric field
Electric field can be calculated given
sufficient symmetry
30-5 Generators
An external force rotates a loop in a magnetic
field
An emf is induced:
(30-14)
Induced power:
(30-16, 18)
30-5 Generators
The sinusoidally varying current and
voltage are characteristic of
electricity from generators
Electricity from batteries is constant
Summary of Chapter 30
Faraday’s law: A changing magnetic
flux induces an emf:
(30-2)
Lenz’s law: Induced currents produce
magnetic fields that tend to cancel the flux
changes that induce them.
Motional emf is induced when conductor
moves through magnetic field