in the primary coil
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Transcript in the primary coil
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!
current
• If magnet is stationary, no
_________
• 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 _________
current
around a conductor (the coil of wire), a ________
is induced in the conductor.
• The amount of current induced is related to the
field and the _____
rate at
strength of the magnetic
______________
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.
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
a tiny, BRIEF surge of current
primary coil, ____________________________
occurs in the secondary coil
But, WHY?
• When the switch closes, the current increases
from zero. This ________
changing current produces a
____________________
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 in
to accomplish this, ___________________
the primary coil is used.
If DC was used in the primary coil, the
Nothing (zero Volts)!
output would be ___________________
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!)
“Exploded” View
Assembled
View
1 : 13
winding
ratio!
• 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”!!
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)(8000 V )
0 A)(8000 V ) 800,000 W
• 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 20,000 W
(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 A8000V 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?