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Unformatted text preview: AC Electricity
Why AC? Distribution Getting Power to Our Homes Let's power our homes with DC power But want power plants far from home and ability to "ship" electricity across states UCSD: Physics 121; 2012 DC means direct current: just like what batteries deliver So power lines are long resistance no longer negligible long transmission line power plant home appliance Rwire looks like:
Winter 2012 Rload Rwire
2 UCSD: Physics 121; 2012 Power Dissipated in an Electricity Distribution System
150 miles 120 Watt Light bulb Power Plant on Colorado River 12 Volt Connection Box Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 /m 2 105 m = 20 Ohms We can figure out the current required by a single bulb using P = VI so I = P/V = 120 Watts/12 Volts = 10 Amps (!) Power in transmission line is P = I2R = 102 20 = 2,000 Watts!! "Efficiency" is = 120 Watts/4120 Watts = 0.3%!!! What could we change in order to do better? Winter 2012 3 UCSD: Physics 121; 2012 The Tradeoff The thing that kills us most is the high current through the (fixed resistance) transmission lines Need less current it's that square in I2R that has the most dramatic effect But our appliance needs a certain amount of power P = VI so less current demands higher voltage Solution is high voltage transmission Repeating the above calculation with 12,000 Volts delivered to the house draws only I = 120 Watts/12 kV = 0.01 Amps for one bulb, giving P = I2R = (0.01)220 = 20 10-4 Watts, so P = 0.002 Watts of power dissipated in transmission line Efficiency in this case is = 120 Watts/120.004 = 99.996%
Winter 2012 4 UCSD: Physics 121; 2012 DANGER! But having high voltage in each household is a recipe for disaster sparks every time you plug something in risk of fire not cat-friendly Need a way to step-up/step-down voltage at will can't do this with DC, so go to AC Winter 2012 5 UCSD: Physics 121; 2012 A way to provide high efficiency, safe low voltage:
step-up to 500,000 V step-down, back to 5,000 V ~5,000 Volts step-down to 120 V Winter 2012 High Voltage Transmission Lines Low Voltage to Consumers 6 UCSD: Physics 121; 2012 Transmission structures
three-phase "live" wires to house 500,000 230,000 long-distance 138,000 69,000 713,000 neighborhood Winter 2012 7 UCSD: Physics 121; 2012 Why is AC the solution? AC, or alternating current, is necessary to carry out the transformation To understand why, we need to know something about the relationship between electric current and magnetic fields Any current-carrying wire has a circulating magnetic field around it: Winter 2012 8 Electromagnet Coil UCSD: Physics 121; 2012 By arranging wire into a loop, you can make the magnetic fields add up to a substantial field in the middle looks just like a magnet Winter 2012 9 UCSD: Physics 121; 2012 Induced Current The next part of the story is that a changing magnetic field produces an electric current in a loop surrounding the field called electromagnetic induction, or Faraday's Law Winter 2012 10 UCSD: Physics 121; 2012 Transformer is just wire coiled around metal Magnetic field is generated by current in primary coil Iron core channels magnetic field through secondary coil Secondary Voltage is V2 = (N2/N1) V1 Secondary Current is I2 = (N1/N2) I1 But Power in = Power out negligible power lost in transformer Works only for AC, not DC If the primary wires and secondary wires don't actually connect, how does the energy get from the primary circuit to the secondary circuit?!
Winter 2012 11 Typical Transformers UCSD: Physics 121; 2012 transformers usually heavy due to iron core
Winter 2012 12 UCSD: Physics 121; 2012 Alternating Current (AC) vs. Direct Current (DC) AC is like a battery where the terminals exchange sign periodically! AC sloshes back and forth in the wires Recall when we hooked up a bulb to a battery, the direction of current flow didn't affect its brightness Although net electron flow over one cycle is zero, can still do useful work! Imagine sawing (back & forth), or rubbing hands together to generate heat Winter 2012 13 UCSD: Physics 121; 2012 = 170 Volts = -170 Volts Winter 2012 120 VAC is a root-mean-square number: peak-to-peak is 340 Volts! 14 UCSD: Physics 121; 2012 AC Receptacle Receptacles have three holes each Lower (rounded) hole is earth ground connected to pipes, usually green wire Larger slot is "neutral" for current "return" never far from ground white wire if wired correctly Smaller slot is "hot" swings to +170 and -170 black wire dangerous one Winter 2012 15 UCSD: Physics 121; 2012 Assignment Read 6.9.1 on grounding Winter 2012 16 ...
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