Transcript forcibly push - Cloudfront.net

Lectures for Conceptual Physics, 8th Ed.
1
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 simple…just rotate (move) a loop of wire in a
magnetic field and electricity was produced.
OR
AND
2
Here is a critical concept!
Magnet being moved.
Coil of insulated wire
The magnet
resists being
moved!
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).
3
Faraday’s 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.
=
-N
d
B
dt
Lenz’s 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 loop…you have to push.
4
More on Lenz’s 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.
That’s it! Tricky but true.
Picture from Serway, vol 2 p990
5
By the way, when you push current goes one way.
Then, when you pull, current goes the other way.
Generators and Alternating Current
Starting and stopping is hard.
p439
It’s easier to make something
rotate…the 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.
6
fig 24-6
Motor effect: moving
charges create a force.
Generator effect: pushing the
wire down creates a current.
7
It is rate of change of the
magnetic field, the magnetic
flux, that matters.
The rate of change, flux, is
greatest at c.
top
So, the strongest current is
produced at this part of the
cycle.
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
8
+
0
Maximum current
in one direction.
Maximum current in
the other direction.
Nikola Tesla, the scientist, along with George
Westinghouse, the astute business person,
provided the world with abundant electrical
power.
9
Turbogenerator Power
Power in.
p441
Fig 24-9
Power out.
This is the “turbo” part.
This is the generator part.
10
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.
11
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.
12
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. (Lenz’s 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
13
The answers are on p 445.
Study the answers…hint, hint!
14
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”.
15
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
Transformers “step-up” the
voltage at the production
plant.
V
And, transformers “stepdown” the voltage at the
customer end.
16
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 1860’s:
“ 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 447…”among the most brilliant physicists
of his time.”
And, finally, read the “In Perspective” section.
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The end