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Transcript
Ch 25
The Question: If an electric current in a wire
produces a magnetic field, is the reverse true?
…i.e. Does a magnetic field produce a current?
The Answer: Let’s try it out!
no current
• If magnet is stationary, _________
• If magnet is moving in, _________
current
is produced
_________________________
current
•If magnet is moving out, ________
_____________________________________________
is produced in opposite direction
•If the poles of the magnet are
changes direction
reversed,current
___________________
Additional Observations:
• If 2 magnets are used (twice the magnetic field
strength), the current will _________.
double
• If the magnet is moved more slowly, the current will
be ________.
smaller
• If the magnet is continuously moved back and
forth, ___________
alternating ________
current is created!
To summarize all of our observations:
changing magnetic field
• When there is a _________
around a conductor (the coil of wire), a current
________
is induced in the conductor.
• The amount of current induced is related to the
magnetic field and the _____
rate at
strength of the ______________
which the field changes.
Michael Faraday, in England, in
1831 was the first to discover this
phenomenon, termed
ELECTROMAGNETIC INDUCTION.
Faraday’s Law quantifies the phenomenon:
The induced voltage (which ultimately can
produce a current) in a coil is proportional to
the product of:
rate at which the magnetic field
• The _____
strength ________
changes within the coil, and the
Number of loops
•________________in
the coil.
From Nova’s “Einstein’s Big Idea”
E is for Energy…
The Story of Michael Faraday
<iframe title="YouTube video player" width="640" height="390"
src="http://www.youtube.com/embed/yVDHKKTC4tA"
frameborder="0" allowfullscreen></iframe>
Michael Faraday
An application of Electromagnetic Induction:
OK… so NOT the
“more than meets the
eye” kind, but the
electrical transformer….
Purpose of Transformers: Used to
change or “transform” the voltage in a circuit.
How do they work?
Demo: A pair of coils are
placed side-by-side. One
is connected to a battery
(DC) and one is
connected to an ammeter.
When the switch is closed (or opened) in the
tiny, BRIEF surge of current
primary coil, a
__________________________
occurs in the secondary coil
But, WHY?
• When the switch closes, the current increases
changing current produces a
from zero. This ________
____________________
changing magnetic field around the primary coil.
•This changing magnetic field is “felt” by the
nearby secondary coil, which according to
voltage in the
Faraday’s Law, will induce a _________
secondary coil, (which then produces a current)
The effect is enhanced (larger
current produced in secondary)
if an iron core is added…. DEMO
Why do transformers use AC, and not DC?
The key to obtaining a current in the
secondary coil is to establish a changing
magnetic field in the primary coil. Instead of
using a battery (DC) and switching the
current on/off repeatedly (a little impractical!!)
alternating current
to accomplish this, _____________________
in the primary coil is used.
If DC was used in the primary coil, the
(zero Volts)
output would be Nothing
__________________!
So… HOW does the voltage change?
If we assume 100% efficient transformers…
Now, if the two output loops are joined in
sum
series, the voltages _____.
Since the secondary coil had more windings
higher
(loops), the secondary voltage was ________.
Step-up transformer.
This is called a _________
Did we just “create” electrical energy?
Of course NOT
____________!!
Energy is always
conserved
_________.
Since POWER is the
rate at which energy is consumed,
_____
POWER is then, also, conserved….
Pprimary  Psecondary
I PVP  I SVS
For a “Step-Up” transformer..…
↓
so… as Voltage ↑, Current ___.
(Assuming 100%
efficient transformer)
IV
P P
V
 IS
S
Step-Up vs. Step-Down Transformers
More
_____ Windings
on Secondary
Coil
Less Windings
_____
Step-Down on Secondary
Coil
Step-Up
Secondary
Voltage
↑
__
IV
P P
V
 IS
S
Secondary
V I V
Voltage
IP
↓
__
P
S
S
The secondary voltage is proportional to the
number of secondary windings according to…
VP
NP

VS
NS
Where…
NP = # of windings on primary coil
NS = # of windings on secondary coil
Simulation at http://micro.magnet.fsu.edu/electromag/java/transformer/index.html
A Model of a transformer: Demo…
Iron core
(won’t work
without it!)
Assembled
View
“Exploded” View
• 1 V AC applied to inner (primary) coil produces
13
___ V AC on outer (secondary) coil
• PREDICTION: 3 V AC applied to inner (primary) coil
39 V AC on outer (secondary) coil,
will produce ___
because the secondary coil must have ____
13 times the
number of windings as the primary coil.
Note: If the input voltage is applied to the outer coil, it
down
becomes the primary, and the transformer is a step-______.
Electrical Power Distribution
A Typical Scenario…
With the use of transformers, electrical
power is sent across long transmission lines
at ______
HIGH voltages. WHY?
If the voltage is high, the current will be relatively
____,
low so there will be less power loss in the lines.
“The War of the Currents”
In the 1880’s, there was a debate/ battle between
two great scientists – Nikola Tesla and Thomas
Edison over whether to use AC or DC for electric
power distribution.
Tesla:
Advocate
for AC
Edison:
Advocate
for DC
Tesla won because with ____,
AC a transformer can be used to
_______
DC voltage can’t be “transformed” to a
change the voltage. ____
higher voltage which would be necessary to reduce the
power/heat loss in the distribution lines. Edison’s answer to that
problem? “Just build a power station every mile or so”!!
From “Tesla – Master of Lightning”
The Story of Nikola Tesla and the
“War of the Currents”
Tesla ACDC
Overview of Historical Figures
• Faraday: motors, generators,
transformers
• Maxwell: mathematical equations
– Electrostatics
– Changing magnetic fields
– Changing electrical fields
– Electromagnetic radiated waves
Edison: used principles to produce commercial
products
• Michelson: first Nobel prize for Physics in
America – speed of light – relativity,
proved speed of light is same regardless
of your motion; becoming the heart of
Einstein’s Special Theory of Relativity
History-Invention Assignment
• Each student is to complete all 5
Statements: counts as test grade
• I will assign one statement to each of five
groups whose responsibility is to present
the material to the class starting Friday
Sign at Niagara Falls, Summer 2008
Example: A 2W power line
supplies power to a small
subdivision at 8000 V and
100 Amps.
• Calculate the total power supplied to
the subdivision.
P  IV  (100 A)(8000V ) 
0 A)(8000V )  800,000W
• Calculate the power loss in the
distribution line.
First, derive a useful formula…
P  IV  I  ____
I R   ______
I R
2
Plost  I R  100 A 2W  20,
2
2
Plost  20,000W
(2.5% of the supplied power would be lost.)
• If a transformer was used to step-up the
voltage by a factor of 10 (to 80,000 V),
calculate the new power loss in the
distribution line.
↓ by 10 times … I= 10A
If V ↑ by 10 times, I ______________________.
Or…
I PVP  I SVS
100 A8000V   I S 80,000V 
I S  10 A
22




Plost

10
A
2
W

200
W
lost
I ↓ by 10 times, but the power loss was
reduced by a factor of 100
____. WHY?