PHY2049ch31B%283-19-10%29

# PHY2049ch31B%283-19-10%29 - Alternating current (AC)...

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Alternating current (AC) circuits (Chapt. 31) t v i T 2T V C –V C v, i I –I

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The AC/DC wars The typical household 120V wall plug in the USA has three prongs. Ground Hot (narrow slot) Neutral (wide slot) For a correctly wired receptacle the wide slot is the Neutral (or Return) and should be near ground (0 V) (though I wouldn’t bet my life on it). The ground is literally earth ground, tied to a copper rod driven deep into the ground (6 – 10 ft). The narrow blade (for a correctly wired receptacle) is hot and has voltage that oscillates sinusoidally in time with respect to the neutral so that its output is, (t) 170Vsin(2 (60Hz)t) ξ
Time ξ (volts) ξ m = 170V –170V This has form, mm sin(2 ft) sin( t) ξ=ξ π ω With amplitude ξ m = 170 V and frequency f = 60 Hz This alternating potential causes the current in any circuit connected to it to alternate in time, so this is called an alternating current or AC system. t T 2T 3T 4T

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1) Why would we have settled on such a complex, time varying voltage to power our devices, and In the late 1800’s when the first electrical power networks in the US were being established Thomas Edison’s company was selling a direct current or DC system in which there would have been one fixed voltage coming out of the hot terminal. You may wonder: 2) What exactly about this corresponds to 120 V ? (remember called a 120 V outlet) George Westinghouse’s company sold AC systems and the two were in fierce competition (for contracts worth millions of dollars).
At times the battles turned ugly, with Edison claiming that the higher voltage AC systems were more dangerous . To prove it his employees held demonstrations electrocuting animals (large dogs, a horse) with AC voltage. * “Empires of Light” by Jill Jonnes In 1888 the state legislature of New York made electrocution that states official method of execution. Edison tried to have the electric chair labeled the Westinghouse chair in the press (reasoning that no one would want AC in their homes when it was so dangerous that it was used to execute people.) In fact, Edison aided development of the electric chair and found AC generators to use (after Westinghouse refused to sell his to prisons for that purpose). *

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Despite these (underhanded) efforts AC won out . The main reason for this is that AC power transformers that use magnetic induction can easily step the voltage up or down with minimal losses for long range power transmission, and power transmission is much more efficient at high voltage than at low voltage. To see why high voltage transmission is better consider the following circumstance. Suppose you have a 1 MW power plant and you want to transmit that power over as long a distance as possible. The transmission line wires have their own resistance , which will consume power. Suppose your wire has a resistance of 1 Ω /km How much wire would it take to dissipate all your power?
2 PI R = 2 P R I = Dividing this resistance by the resistance/km gives the cable length that dissipates all the power.

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## PHY2049ch31B%283-19-10%29 - Alternating current (AC)...

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