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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 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?