Transcript Chapter 29 - galileo.harvard.edu

Chapter 29
Electromagnetic Induction
&
Faraday’s Law
Introduction
• Our approach
– What is electromagnetic induction?
• Faraday’s Law
• Lenz’s Law
– Applications and consequences
•
•
•
•
•
EMF induced in a moving conductor
Electric generators
Back EMFs & counter torques
Eddy currents
Transformers & power transmission
– E-field with a changing B-field
Electromagnetic Induction
• What do we mean by electromagnetic
induction?
• Sketch an example
• Make induction happen with equipment
given; draw some conclusions
Faraday’s Law of induction
• What is magnetic flux (in words and
mathematically)?
• What is the relation between magnetic flux
and induced emf? (mathematically and in
words)
• What are some examples of ways a
changing magnetic flux induces an emf?
Lenz’s Law
• What is the direction of the induced
current? That is, how does the direction
depend on the physical situation that
induces the current?
• What are some examples of the behavior
described by Lenz’s Law?
• Precisely state Lenz’s Law.
EMF induced in moving
conductor
• How is an emf induced in a conductor
moving in a magnetic field? (Sketch a
picture and describe this phenomenon.)
• What is the mathematical relationship
among relevant quantities in this situation?
• How is a current induced in a conductor
moving in a magnetic field? (Sketch a
picture and describe this phenomenon.)
EMF induced (cont’d)
• How much force is needed to move the
conductor (when one side of a rectangular
loop moves) to cause the current?
• How is this consistent with conservation of
energy?
Electric Generators
• In general, how do generators work?
• In general, what’s the main difference
between dc and ac generators?
• In general, what’s the main difference
between a dc generator and an alternator?
Back EMF & Counter Torque
• What is back emf, and how in general
does it arise in a motor?
• What is motor overload, and how is it
understood in terms of back emf?
• What is counter torque, and how in
general does it arise in a generator?
Eddy Currents
• What are eddy currents, and how do they
arise?
• How are eddy currents used?
– http://www.doitpoms.ac.uk/tlplib/recyclingmetals/eddy_current.php
• How are they minimized?
Transformers & Power
Transmission
• What is a transformer? (Make a sketch)
• How is the transformer equation derived?
• What assumptions go into deriving the
transformer equation?
• What is meant by “step-up” and “stepdown” transformers, and how in general is
this difference produced?
• How and why are transformers used?
Electric Field from Changing
Magnetic Field
• Using the fact that an emf is related to an
electric field, re-express Faraday’s Law in
terms of the electric field.
• How is this relation more general?
• What is the electric field produced
between the circular poles of an electromagnet with alternating current?
• Are electrical forces due to changing
magnetic fields conservative? Explain.
The end