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Power transmission Voltage difference What do magnets have to do with electricity? time - Power loss to wires - Delivering Power - transformers - Creating electrical current generators. 1 Power transmission - Power loss to wires I 120 V \/\/\/\/\/ • Voltage dropped in wire = IRwire • Power wasted in wire = I2Rwire • Significant when: – I is large (supplying lots of appliances) – R is large (long wires) Long wires, some R Close heater switch I increases V1 drops Light bulb dims, wire gets warm V1 How can we efficiently supply a town with power from a power station 30 miles away? • P = I ×V • Transmit power at high V and low I Voltage drop smaller (IRwire) Power wasted much smaller (I2Rwire) Power distribution questions Voltage supplied by power company Voltage supplied to home. (some voltage drop in wires) power plant Q: Power plant decides to deliver power of 10,000 W to power a house: How much current needed if voltage at home is 100 V? A: Power to house = current x voltage supplied to home: P = IV. I = Power/Voltage = 10,000 W/100 V = 100 A Q: At this current, what is power loss in wires if Rwire = 1 ohm? a) 100 W, b) 10 W, c) 1000 W, d) 10,000 W, e) 100,000 W 4 Power distribution questions Voltage supplied by power company Voltage supplied to home. (some voltage drop in wires) power plant Power plant still delivers 10,000 W to power a house, but now adjusts voltage supplied so the voltage at home is 10,000 Volts. Q: What changes compared with home voltage of 100 Volts ? Current through wire needed to supply power will be ---------. Voltage drop across segments of wire will be ----------. Power going into heating the wires will be ----------. a) same, same, same d) less, less, less b) less, same, less c) more, same, more e) more, more, more. 5 Distributing power at high voltage Voltage supplied by power company Voltage supplied to home. power plant Advantage: Massive reduction in power loss in wires Disadvantage: Kills people Solution: • Transmit at high V over long distances • Reduce to low V near houses • Change V up and down efficiently with AC and transformers • See Blm for history of power transmission 6 Alternating current (AC) look at wall outlet with Oscilloscope (measures voltage difference) Oscilloscope Voltage difference A B No voltage diff Current = 0 Amps Voltage at A larger than at B +170V 0 -170V time US- 60 hertz (60 oscillations/s) 120 Vrms (av. voltage diff) Europe-50 Hz, 230 Vrms Voltage at B larger than at A 7 Does AC work the same as DC? 1. In light bulbs and heaters? a) yes, b) no 2. In computers, cell phones, and electronics? a) yes, b) no 8 Transformers in the power distribution system 5000V 500,000 V (on towers) substation power plant Transformers enable us to: • Change voltage easily • Transfer power between circuits so one house doesn’t effect next. 120 V short wires into houses 7200 V running around town. 9 How do transformers work? • Convert AC voltage up and down • Made of two coils of wire (around a core) Secondary coil (out) Primary coil (in) AC current in primary coil (e.g. from power company) produces AC current in secondary coil (e.g. current in your house) Two Steps: A) Changing electric current in primary coil produce changing magnetic field B) Changing magnetic field produces a current in the secondary coil 10 What is a magnet and a magnetic field? • Natural phenomenon closely related to electricity • North and south poles, opposite poles attract • Important difference: Magnetism has no monopoles (like + and - charges.) North and South poles are hooked together. ALWAYS. • A magnetic field describes the force on a north pole of a magnet at each location in space Compass is a little bar magnet. Earth is a big bar magnet. N end of compass needle attracted to S end of earth magnet. 11 How can we make a magnet? 1. Magnetic material (e.g. Iron) - Electrons behave like tiny bar magnets - Usually paired in opposite orientations – cancel out - Iron retains some unpaired electrons – billions of atomic magnets combine to make a big magnet 2. Electric currents produce magnetic fields - Magnetic field around a coil of wire is much like that around a bar magnet - Electromagnet Coil of wire Bar magnet Producing magnets using electric currents North pole compass with I = 0 DC power supply Q: What direction will compass point if turn on current to 5 amps? a. b. c. d. e. could be b or d. explain reasoning, then do experiment 13 DC power supply Conclusion: Current through coil of wire produces magnetic field (electromagnet). Magnetic field B depends on as equation shorthand B = k I N = (constant)(current)(number of turns) 14 Back to transformers Conclusion so far: - Steady current in primary coil will produce a steady B field - Direction of B field depends on direction of current -Changing (AC) current will produce a changing B field – step A Next: -What effect does the changing B field have on the secondary coil? Secondary coil (out) Primary coil (in) Two Steps: A) Changing electric current in primary coil produce changing magnetic field B) Changing magnetic field produces a current in the secondary coil 15 Producing voltages and currents using magnets. North South Bulb lights up if we move coil in and out of magnet Q: What will happen if I move coil more slowly? a) brighter, b) dimmer, c) same 16 Useful Phet on induced voltage c) b) a) Move bar magnet up across front of coil. Voltage will be biggest when a) just starting , b) half way across c) lined up with middle of coil. 17 Producing electric currents using magnets. North South Bulb lights up if we move coil in and out of magnet Q: What will happen if I use coil with 3 turns instead of 500? a) brighter, b) dimmer, c) same (discuss reasoning) 18 Back to transformers Conclusion: - Changing the magnetic field through secondary coil will give a voltage drop across it. - If secondary coil is part of a complete circuit, current will flow – step B -Transformer physics complete! Secondary coil (out) Primary coil (in) Two Steps: A) Changing electric current in primary coil produce changing magnetic field B) Changing magnetic field produces a current in the secondary coil Transformer rule Assume all B is channeled from primary through secondary Rate of change of B is the same in both coils V = k DB/Dt, per loop the voltage per loop is the same for primary and secondary: Vout / Nsecondary = Vin / Nprimary Which leads to the transformer rule: Vout = Vin x Nsecondary/Nprimary or Vout / Vin = Nsecondary/Nprimary Vin Vout Primary Secondary 20 Transformer rule for current Vout = Vin × Nsecondary / Nprimary In an ideal transformer, no power (P = IV) is wasted: Iin Vin = Iout Vout Iout = Iin ×Vin / Vout = Iin × Nprimary / Nsecondary Increase voltage Decrease current and vice versa Iin Vin Vout Iout Primary Secondary 21 Transformer construction detail - The core. B field from coil spreads out a lot, like in simulation for bar magnet. Means less B goes through second coil. Less current, wastes power. current in B current out What will happen to light bulb? iron core concentrates B (sucks it in), more changing B through second coil, bigger induced voltage bigger current out! Core does not carry current! 22 Changing voltages Vout = Vin × Nsecondary / Nprimary Step up transformer: Nsecondary > Nprimary Q: If Vin 5000V AC, Nprimary = 50 and Nsecondary = 5000, what is Vout ? a) 50V b) 500V c) 5000V d) 50,000 V e) 500,000 V Changing voltages Vout = Vin × Nsecondary / Nprimary Step down transformer: Nsecondary < Nprimary Q: If Vin 120V AC, Nprimary = 500 and Nsecondary = 50, what is Vout ? a) 12,000V b) 1200V c) 120V d) 12V e) 1.2 V Q: What would happen to a 40W lightbulb if wired to the secondary? a) Filament burns out b) Same brightness as if wired to mains c) Just lights up a bit d) No light at all Transformer summary Primary coil (in) Secondary coil (out) 1) Oscillating current in primary creates oscillating B field 2) Iron core concentrates B field, improving coupling between primary and secondary no wasted power. 3) Oscillating B through secondary coil creates voltage which drives a current through bulb etc. Transformer rule assumes perfect coupling (real transformers pretty close) Vsec = Vprimary x (Nsec/Nprimary) Also Isec = Iprimary x (Nprimary/Nsec) (since P=IV is constant) step up transformer – increases voltage – decreases current step down transformer – decreases voltage – increases current 25 Electric power generation N Q: How did I generate power earlier in class? S A: By moving a coil relative to a magnetic field. Power plant generators: - Use steam or water to spin magnets past coils (or vice-versa). - Like transformer, but changing B created by moving magnet S magnets N I, V out N S S N N S iron core spinning turbine 26 Generator demo http://phet.colorado.edu/simulations/sims.php?sim=Generator 1. How does frequency of voltage oscillation depend on how fast magnet is spun? a) twice magnet rotation frequency, b) same, c) half d) unrelated, e) 4 times rotation frequency. 2. How does size of voltage depend on how fast spun? a) unrelated, b) faster gives more V, c) faster gives less V 27 How is turbine driven in a real power plant? Hydroelectric turbine E = mgh, power = energy/sec = mass/sec x gh ~ 40% efficient Pelectrical out = .4 (mass water/s x gh) h S N S N [In a wind or wave driven generator, the wind/waves turn the turbine directly] N S N S 28 Nuclear/fossil fuel power plant boiler turbine I cooling pond • Fuel is used to boil water and make steam pressure • Steam rotates the turbine 29 How is energy conserved in a power plant? The induced current in the coil produces a magnetic field that acts back on the rotating magnet to oppose its motion. (Lenz’s Law) Thus mechanical energy is taken from the rotor and converted to electrical energy. N S Generator demo - open switch, no current to light bulb. - closed switch, current flows through light bulb In which case is it hardest to turn the generator? 30