Unformatted text preview: AC Electricity
Our Everyday Power Source Getting Power to Our Homes Let's power our homes with DC power DC means direct current: as from batteries UCSD: Physics 8; 2007 But want power plants far from home and ability to "ship" electricity hundreds of miles Power lines are long resistance becomes large... long transmission line power plant home appliance Rwire looks like: Rwire Rload
2 UCSD: Physics 8; 2007 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 (huge) If I join two wires in series what is the resistance?
3 UCSD: Physics 8; 2007 The Tradeoff The problem is the high current through the (fixed resistance) transmission lines Need less current 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 = 4 UCSD: Physics 8; 2007 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 change voltage before reach house can't do this with DC, so go to AC Why do we use high Voltage transmission lines? 5 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 UCSD: Physics 8; 2007 step-down to 120 V High Voltage Transmission Lines Low Voltage to Consumers 6 UCSD: Physics 8; 2007 Transmission structures
three-phase "live" wires to house 500,000 230,000 long-distance 138,000 69,000 713,000 neighborhood 7 UCSD: Physics 8; 2007 Why is AC the solution? AC, or alternating current, is necessary to carry out the transformation (change) of voltage 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: Right hand rule: thumb points along current flow, fingers give B field
8 Electromagnet Coil UCSD: Physics 8; 2007 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 9 UCSD: Physics 8; 2007 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 10 UCSD: Physics 8; 2007 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 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?!
11 AC = alternating (changing) Current DC = direct current = constant Typical Transformers UCSD: Physics 8; 2007 What does a transformer do? If I need 6 Volts.. I can make an electric current by.. 12 UCSD: Physics 8; 2007 Alternating Current (AC) vs. Direct Current (DC) AC is like a battery where the terminals exchange sign rapidly 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 average electron displacement after one cycle is zero can still do useful work! Imagine sawing (back & forth), or rubbing hands together to generate heat 13 UCSD: Physics 8; 2007 = 170 Volts = -170 Volts 120 V AC is the square root of the mean of the squared voltage. We use the RMS to get the power = VI. The power is the same as a constant DC V=120 V. The AC voltage has top peak to bottom minimum range (peak-to-peak) of 340 Volts!
14 UCSD: Physics 8; 2007 AC Receptacle Receptacles have three holes each Lower (rounded) hole is earth ground connected to pipes, usu. 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 When I want to calculate the power for an AC circuit..
15 UCSD: Physics 8; 2007 Three-Phase Alternating Current (AC) Three wires, each carrying the same AC, but delayed from each other by 1/3 of a cycle. The power available is also constant = 1.5 x peak power from any one phase. Neutral = sum of the three voltages, which is constant Some large motors are made to work from all 3 phases 16 ...
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