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!• If magnet is stationary, _________
• If magnet is moving in, _________ _________________________
•If magnet is moving out, ________ _____________________________________________
•If the poles of the magnet are reversed, ___________________
no currentcurrent
is produced
current is produced in opposite direction
current changes direction
Additional Observations:• If 2 magnets are used (twice the magnetic field strength), the current will _________.
• If the magnet is moved more slowly, the current will be ________.
• If the magnet is continuously moved back and forth, ___________ ________ is created!
To summarize all of our observations:• When there is a _________ magnetic field around a conductor (the coil of wire), a ________ is induced in the conductor.
• The amount of current induced is related to the strength of the ______________ and the _____ at which the field changes.
double
smaller
alternating current
changingcurren
t
magnetic field
rate
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:
• The _____ at which the magnetic field strength ________ within the coil, and the
•________________in the coil.
ratechanges
Number of loops
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 primary coil, __________________________ occurs in the secondary coil
a tiny, BRIEF surge of current
But, WHY?• When the switch closes, the current increases from zero. This ________ current produces a ____________________ around the primary coil.
•This changing magnetic field is “felt” by the nearby secondary coil, which according to Faraday’s Law, will induce a _________ in the secondary coil, (which then produces a current) The effect is enhanced (larger current produced in secondary) if an iron core is added…. DEMO
changingchanging magnetic
field
voltage
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!!) to accomplish this, _____________________ in the primary coil is used.
Why do transformers use AC, and not DC?
If DC was used in the primary coil, the output would be __________________!
alternating current
Nothing (zero Volts)
So… HOW does the voltage change?If we assume 100% efficient transformers…
Now, if the two output loops are joined in series, the voltages _____.
Since the secondary coil had more windings (loops), the secondary voltage was ________. This is called a _________ transformer.
sum
higherStep-up
Did we just “create” electrical energy?
____________!! Energy is always _________. Since POWER is the _____ at which energy is consumed, POWER is then, also, conserved….
secondaryprimary PP (Assuming 100% efficient transformer)
For a “Step-Up” transformer..… so… as Voltage ↑, Current ___.
Of course NOTconserved
rate
SSPP VIVI SSPP VI IV
↓
SSPP VI IV
Step-Up vs. Step-Down Transformers
Step-Up
_____ Windings on Secondary
Coil
Secondary
Voltage __
Step-Down
_____ Windings on Secondary
Coil
Secondary
Voltage __
The secondary voltage is proportional to the number of secondary windings according to…
S
P
V
V Where…
NP = # of windings on primary coil
NS = # of windings on secondary coil
More
↓
↑Less
SSPP VI IV
S
P
N
N
Simulation at http://micro.magnet.fsu.edu/electromag/java/transformer/index.html
A Model of a transformer: Demo…
“Exploded” View
Assembled View
• 1 V AC applied to inner (primary) coil produces ___ V AC on outer (secondary) coil
• PREDICTION: 3 V AC applied to inner (primary) coil will produce ___ V AC on outer (secondary) coil, because the secondary coil must have ____ times the number of windings as the primary coil.
Iron core (won’t work without it!)
Note: If the input voltage is applied to the outer coil, it becomes the primary, and the transformer is a step-______.
13
3913
down
1 : 13 winding
ratio!
Electrical Power DistributionA Typical Scenario…
With the use of transformers, electrical power is sent across long transmission lines at ______ voltages. WHY? If the voltage is high, the current will be relatively ____, so there will be less power loss in the lines.
HIGH
low
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
_______ won because with ____, a transformer can be used to change the voltage. ____ voltage can’t be “transformed” to a 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”!!
“The War of the Currents”
Tesla ACDC
Sign at Niagara Falls, Summer 2008
Example: A 2 power line supplies power to a small subdivision at 8000 V and 100 Amps.
• Calculate the total power supplied to the subdivision.
RIPlost2
First, derive a useful formula…
• Calculate the power loss in the distribution line.
__________ IIVP RI RI 2
WPlost 000,20(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.
If V ↑ by 10 times, I ______________________.
Or… SSPP VIVI VIVA S 000,808000100
AIS 10
I ↓ by 10 times, but the power loss was reduced by a factor of ____. WHY?
↓ by 10 times … I= 10A
100