Unformatted text preview: UCSD Physics 12 Power Plants and Distribution
How we get our electricity UCSD Physics 12 The Importance of Electricity About 39% of our energy consumption is carried out at electrical power plants Sources are diversified: 57% coal 20% nuclear 10% petroleum 9% hydroelectric (3% of the input is hydro: it's efficient) 3% natural gas 0.5% geothermal 0.5% other (biomass, wind, solar) 2 Spring 2009 UCSD Physics 12 Common Themes 99.9% of these turn generators to make electricity all but solar photovoltaics 99.7% power generators work through turbine action all but wind, solar 90% of turbines powered by steam all but hydroelectric includes coal, petroleum, gas, nuclear, etc. Spring 2009 3 UCSD Physics 12 Hydroelectric power Spring 2009 4 UCSD Physics 12 A Hydroelectric "Battery" Spring 2009 5 UCSD Physics 12 Getting Electricity from Coal Spring 2009 6 UCSD Physics 12 Nuclear Plant Spring 2009 7 UCSD Physics 12 This covers 99% of our electricity production Petroleum and gas plants operate just like coal Most use steam in a "heat" engine--the subject of Chapter 3 All produce electricity through a generator spinning coils of wire within magnetic fields property of electromagnetism that a changing magnetic field through a loop of wire produces a voltage along the loop this voltage can drive a current and provide energy to an external circuit Spring 2009 8 UCSD Physics 12 The Generator Principle Loop of wire (conductor) rotates within stationary magnetic field this produces changing field requirement Brush contacts connect to rotating loops and carry current to external circuit In practice, wire makes many (thousands of) loops to get a larger voltage each loop adds to voltage Spring 2009 Simplest arrangement leads to alternating current (AC) 9 UCSD Physics 12 The Transformer Principle Transformers use similar principle to step-up or step-down voltage Current through loop produces magnetic field along axis of loop Alternating current produces changing magnetic field Magnetic field carried along iron core Secondary coil sees changing magnetic field and develops alternating voltage Ratio of voltages is just ratio of turns in the two coils: V2 = (N2/N1) V1 Allows transmission at high voltage, household at low voltage Spring 2009 10 UCSD Physics 12 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 Spring 2009 High Voltage Transmission Lines Low Voltage to Consumers 11 UCSD Physics 12 Transmission structures
three-phase "live" wires to house 500,000 230,000 long-distance 138,000 69,000 713,000 neighborhood Spring 2009 12 UCSD Physics 12 Measuring your electricity consumption All houses/apartments have energy meters to monitor electricity usage this is what the bill is based on Dials accumulate KWh of usage Disk turns at rate proportional to power consumption Kh value is the number of Watthours per turn (1 Wh = 3600 J) Example: one turn in 10 sec
(7.2 Wh) (3600 J/Wh)/(10 sec) = 2592 J/s 2.6 kW Takes 138.9 turns for 1 kWh Spring 2009 13 UCSD Physics 12 Example data from energy meter During the day at my house, the dial may take about 3 minutes to make a revolution. one revolution is 7.2 Wh = 7.2 3600 = 25,920 J 180 seconds per revolution means about 144 W computer, clocks, VCR, etc. Average usage is 16 kWh per day: 24 hours in day means average rate of 667 W For 2 people333 W each: 1/30th of our 10kW share means most of energy not used at home: industry and transportation are the big consumers on our behalves Spring 2009 14 UCSD Physics 12 References & Assignments Very good book on energy: ENERGY: A Guidebook, by Janet Ramage (more global perspective) A recent amazing book: Sustainable Energy--without the hot air, by David MacKay www.withouthotair.com (get book for free!) see 10-page synopsis for quick-read/intro Assignments Read Chapter 2 Homework #2 to be found on the web: go to Assignments page for link: get an early start! Extra Credit: Find your electrical utility meter, note the Kh constant, record the kWh reading at two times separated by some multiple of ~24 hours (record times); figure out average power being consumed, look at rate of wheel (time it) and convert this to an energy rate (power) in watts (will handle thru WebCT) Spring 2009 15 ...
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