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Transcript
11.2-11.3 Electric Power Distribution, Generators and Motors New ideas for today •Magnetic induction •Lenz’s law •Transformers and power transmission •Motors and Generators Why such high voltage? Transformers! Observations about Power Distribution Household power is AC (alternating current) Power comes in voltages like 120V & 240V Power is transmitted at “high voltage” Power transformers are everywhere Power Consumption in wires Reminder: power consumption = current × voltage drop voltage = resistance × current power consumption = resistance × current2 So what? Wires waste power as heat Doubling current quadruples wasted power Better not transmit high current! AC DC vs. Edison Tesla Westinghouse AC = alternating current AC bulb on cord Current switches direction 60 times per second (in N. America) DC= “direct current” AC Power Transmission Power delivered to a city is: power delivered = current × voltage drop Power wasted in transmission wires is: power wasted = resistance × current2 For efficient power transmission: Use low-resistance wires (thick, short copper) Use low current and high voltage drop Can accomplish this with AC (alternating current) power transmission. Power lines 160-800 kV 7000 V 120 / 240 V neutral ground hot Voltage Hierarchy High voltage is dangerous High current is wasteful Use the following scheme: low voltage circuits in neighborhoods (120/240 V) medium voltage circuits in cities (7000 V) high voltage circuits across the countryside (155,000-765,000 V) Use transformers to change voltage Click me Electromagnetism II Magnetic fields created by Fundamental particles (dipoles) ― electrons, protons, neutrons… Moving electric charges (current) Electric fields created by Charges Electromagnetism II Magnetic fields created by Fundamental particles (dipoles) ― electrons, protons, neutrons… Moving electric charges (current) Electric fields created by Charges Changing magnetic fields (induction) Electromagnetic Induction EM cannon Changing magnetic field electric field Electric field in conductor current Current magnetic field Induced magnetic field opposes the original magnetic field change (Lenz’s law) Lenz’s Law Magnetic brake Pipe and magnet Transformer Transformer Alternating current in one circuit induces an alternating current in a second circuit Transfers power between the two circuits Doesn’t transfer charge between the two circuits Click me Current and Voltage Power arriving in the primary circuit must equal power leaving the secondary circuit Power = current × voltage A transformer can change the voltage and current while keeping the power unchanged! Secondary voltage = Primary voltage Secondary turns Primary turns Step Up Transformer More turns in secondary circuit so charge is pushed a longer distance Larger voltage rise A smaller current at high voltage flows in the secondary circuit Step Down Transformer Fewer turns in secondary circuit so charge is pushed a shorter distance Smaller voltage rise A larger current at low voltage flows in the secondary circuit Transformers can be dangerous… Clicker question You decide to use a transformer to increase the voltage from a battery, and hook it up in the circuit shown below. When you close the switch, 1.5 the voltage across the lightbulb is: (A) bigger than 1.5 V (B) smaller than 1.5 V (C) zero inductive charging B Electric Generators and Motors A generator provides electric power A generator requires a mechanical power A motor provides mechanical power A motor requires electric power Alternator Click me Electric Generator Rotating magnet Coil and magnet Induction flashlight Generator makes changing magnetic field induces AC current in the loop Converts mechanical power into electrical power Electric Motor Input AC power AC current makes changing magnetic field causes magnet to turn Converts electrical power into mechanical power A motor is a generator run backwards ! For next class: Read Section 13.1 See you next class!