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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 current current is produced current is produced in opposite direction current changes direction

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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 changing current magnetic fieldrate

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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. rate changes Number of loops

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From Nova’s “Einstein’s Big Idea” E is for Energy… The Story of Michael Faraday Michael Faraday

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An application of Electromagnetic Induction: OK… so NOT the “more than meets the eye” kind, but the electrical transformer….

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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

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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 changing changing magnetic field voltage

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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)

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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 higher Step-up

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Did we just “create” electrical energy? ____________!! Energy is always _________. Since POWER is the _____ at which energy is consumed, POWER is then, also, conserved…. (Assuming 100% efficient transformer) For a “Step-Up” transformer..… so… as Voltage ↑, Current ___. Of course NOT conserved rate ↓

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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… Where… N P = # of windings on primary coil N S = # of windings on secondary coil More ↓ ↑ Less

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Simulation at

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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-______ down 1 : 13 winding ratio!

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Electrical Power Distribution A 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

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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” TeslaAC DC

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From “Tesla – Master of Lightning” The Story of Nikola Tesla and the “War of the Currents” Tesla ACDC

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Pictures from Vacation to Niagara Falls, Summer 2008

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Sign at Niagara Falls, Summer 2008

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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.

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First, derive a useful formula… Calculate the power loss in the distribution line. (2.5% of the supplied power would be lost.)

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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… I ↓ by 10 times, but the power loss was reduced by a factor of ____. WHY? ↓ by 10 times … I = 10A 100

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