Electromagnetic Induction

Ch 25 Electromagnetic Induction 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

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! double smaller alternating current To summarize all of our observations: changing • 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. current 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. rate changes Number of loops

An application of Electromagnetic Induction: Transformers 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) changing changing magnetic field voltage 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 changingmagnetic 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. alternating current If DC was used in the primary coil, the output would be __________________! 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 _____. sum Since the secondary coil had more windings (loops), the secondary voltage was ________. This is called a _________ transformer. higher Step-up

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

Step-Up vs. Step-Down Transformers More ↑ Less ↓ The secondary voltage is proportional to the number of secondary windings according to… 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 1 : 13 winding ratio! “Exploded” 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. 13 39 13 Note: If the input voltage is applied to the outer coil, it becomes the primary, and the transformer is a step-______. down

Electrical Power Distribution A Typical Scenario… With the use of transformers, electrical power is sent across long transmission lines at ______ voltages. WHY? HIGH If the voltage is high, the current will be relatively ____, so there will be less power loss in the lines. low

“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. Edison: Advocate for DC Tesla: Advocate for AC Tesla AC _______ 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”!! DC

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.

Calculate the power loss in the distribution line. First, derive a useful formula… (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… ↓ by 10 times … I= 10A I↓ by 10 times, but the power loss was reduced by a factor of ____. WHY? 100

Electromagnetic Induction