Lectures for Conceptual Physics, 8th Ed.

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Lectures for Conceptual Physics, 8th Ed.
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Electromagnetic Induction p436
1820 Hans Oersted showed that current affected a
magnet.
1831 Michael Faraday and Joseph Henry made
electricity from magnets. Made it possible to
light up cities at night and ruined the sleep
habits of the new era.
It was simplejust rotate (move) a loop of wire
in a magnetic field and electricity was produced.
OR
AND
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Here is a critical concept!
The magnet resists being moved!
Magnet being moved.
Coil of insulated wire
Resistor
The resistor is converting electric power into
heat (work).
The source of that power is the person. She has
to forcibly push the magnet into the coil.
The power she puts into making current comes out
as heat (work).
The electricity just connects her work (input) to
the work done (output).
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Faradays Law p438
The induced voltage in a coil is proportional to
the product of the number of loops and the rate
at which the magnetic field changes within those
loops.
d
- N
B
dt
Lenzs Law Heinrich Lenz 19th cent.
The induced voltage and the magnetic flux have
opposite sign.
i.e. the magnetic field created by the induced
voltage opposes the magnetic flux in the loopyou
have to push.
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More on Lenzs Law
Moving the magnet to the right increases the
magnetic field lines in the loop.
The induced current creates a magnetic field
which opposes this increase.
The repelling magnetic fields explain why you
have to push.
Thats it! Tricky but true.
Picture from Serway, vol 2 p990
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By the way, when you push current goes one way.
Then, when you pull, current goes the other way.
Generators and Alternating Current p439
Starting and stopping is hard.
Its easier to make something rotatethe loop.
(fig 24-5)
Rotating the loop induces current in the loop.
This is the input.
The heat and light from the bulb is the output.
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fig 24-6
Motor effect moving charges create a force.
Generator effect pushing the wire down creates
a current.
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It is rate of change of the magnetic field, the
magnetic flux, that matters.
The rate of change, flux, is greatest at c.
So, the strongest current is produced at this
part of the cycle.
top
AND, the next time the loop assumes the c
position, the top will be on the bottom and
current will be going the other way.
Fig 24-7
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0
-
Maximum current in the other direction.
Maximum current in one direction.
Nikola Tesla, the scientist, along with George
Westinghouse, the astute business person,
provided the world with abundant electrical power.
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Turbogenerator Power p441
Fig 24-9
Power out.
Power in.
This is the turbo part.
This is the generator part.
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MHD Power p441
MagnetoHydroDynamic generator
N
S
Loud but very efficient.
No moving parts. But, only recently have we had
materials that can stand the high temp.s.
Heat from exhaust is used to boil water for a
conventional generator.
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Transformers p442
Transform voltage from low to high or high to low.
A galvanometer (voltmeter)
Putting an iron core through the coils focuses
the magnetic field lines.
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More on Transformers
On the left side (the primary side) a 1 VAC input
is operating.
It is critical that the input voltage is
alternating, changing directions.
The magnetic field produced by the primary coil
causes an opposing magnetic field in the
secondary coil. (Lenzs Law)
The fluctuating secondary magnetic field creates
an alternating current in the secondary coil.
Primary voltage
Secondary voltage

Number of primary turns
Number of secondary turns
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The answers are on p 445.
Study the answershint, hint!
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Self-Induction p445
Imagine a large coil with a current running
through it. Things are normal. The current is
steady and creating a large magnetic field.
Did I mention that this is called an
electromagnet?
Then you suddenly open the switch. (Cut off the
current.)
The magnetic field with all its energy collapses
quickly producing a giant current in the other
direction which zaps across the switch.
This phenomenon is also called back emf, or
back electromotive force.
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Power Transmission p446
Large currents in wire create heat and loss of
power.
So electricity is sent long distances at high
voltage and small currents.
I V I V
Transformers step-up the voltage at the
production plant.
And, transformers step-down the voltage at the
customer end.
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Field Induction p447
A more fundamental way of looking at all the
preceding is to focus on fields not currents and
voltages. Fields are not encumbered by wires.
The whole electricity/magnetism thing was
officially unified by James Clark Maxwell in the
1860s
A magnetic field is created in any region of
space in which an electric field is changing with
time. The magnitude of the induced magnetic
field is proportional to the rate at which the
electric field changes. The direction of the
induced magnetic field is at right angles to the
changing electric field.
Note the comment about Richard P. Feynman at the
bottom of pag 447among the most brilliant
physicists of his time.
And, finally, read the In Perspective section.
The end