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UCSD PHYS 10 - Energy In Our Daily Lives

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Energy in our Lives 04/30/2008Lecture 11 1UCSDPhysics 10Energy In Our Daily LivesEnergy In Our Daily LivesOur Energy Sources, Budgets,Our Energy Sources, Budgets,ExpendituresExpendituresSpring 2008 2UCSDPhysics 10Where Does Energy Come FromWhere Does Energy Come From••Ultimately, from the Big BangUltimately, from the Big Bang– Energy is, after all, conserved••In our daily lives: 93% Sun, 7% nuclearIn our daily lives: 93% Sun, 7% nuclear– Food energy: sunlight, photosynthesis– Hydroelectric energy: sunlight-driven water cycle (7%)– Fossil Fuels: Stored deposits of plant energy (85%)– Wind Energy: solar-driven weather (< 1%)– Solar Energy: well…from the sun, of course (< 1%)– Our nuclear energy, in essence, derives from productsof former stars (supernovae)Spring 2008 3UCSDPhysics 10World Energy Budget (annually)World Energy Budget (annually)7.27.228.728.7HydroelectricHydroelectricthen radiated awaythen radiated away2,000,0002,000,000Sun Abs. by EarthSun Abs. by Earth0.0080.0080.030.03Solar DirectSolar Direct0.030.030.130.13WindWind0.130.130.50.5GeothermalGeothermal0.40.41.61.6Biomass (burning)Biomass (burning)6.66.62626Nuclear EnergyNuclear Energy22.522.58989Natural GasNatural Gas23.223.29292CoalCoal40.040.0158158PetroleumPetroleumPercent of TotalPercent of Total10101818 Joules/yr Joules/yrSourceSourcecirca 2000Spring 2008 4UCSDPhysics 10Where does the sun get its energy?Where does the sun get its energy?••Thermonuclear fusion reactions in the sunThermonuclear fusion reactions in the sun’’s centers center– Sun is 16 million degrees Celsius in center– Enough energy to ram protons together (despite mutualrepulsion) and make deuterium, then helium– Reaction per mole 20 million times more energetic thanchemical reactions, in general4 protons:mass = 4.0294He nucleus:mass = 4.00152 neutrinos, photons (light)Energy in our Lives 04/30/2008Lecture 11 2Spring 2008 5UCSDPhysics 10EE = = mcmc22 in Sun in Sun••Helium nucleus is Helium nucleus is lighterlighter than the four protons! than the four protons!••Mass difference is 4.029 - 4.0015 = 0.0276 a.m.u.Mass difference is 4.029 - 4.0015 = 0.0276 a.m.u.– 1 a.m.u. (atomic mass unit) is 1.660510-27 kg– difference of 4.5810-29 kg– multiply by c2 to get 4.1210-12 J– 1 mole (6.0221023 particles) of protons  2.51012 J– typical chemical reactions are 100-200 kJ/mole– nuclear fusion is ~20 million times more potent stuff!Spring 2008 6UCSDPhysics 10Solar Energy Output FormsSolar Energy Output Forms••2% in neutrinos: very light, non-interactive2% in neutrinos: very light, non-interactive– more than ten billion per second course through yourfingernail– fly through earth, as if it weren’t even there– detected in rare interaction events in huge undergrounddetectors“Super-K” underground neutrino detector in Japan,half full of waterSpring 2008 7UCSDPhysics 10Solar Energy Output Forms, continuedSolar Energy Output Forms, continued••98% in light: photons98% in light: photons– Each photon takes about a million years to clear theannoying electrons in solar plasma– 8 minutes once free to reach earth••1370 Watts per square meter incident light power1370 Watts per square meter incident light power– Most makes it through atmosphere and reaches us here– That which is not reflected is re-radiated back to space• after warming us up– Hugely abundant: don’t have to drill or mine for itSpring 2008 8UCSDPhysics 10Where does the sunlight go?Where does the sunlight go?Energy in our Lives 04/30/2008Lecture 11 3Spring 2008 9UCSDPhysics 10Human Energy RequirementsHuman Energy Requirements••1,500 Calories per day just to be a couch-potato1,500 Calories per day just to be a couch-potato– 6,280,000 J••Average human power consumption is then:Average human power consumption is then:– 6.28 MJ / 86,400 seconds  75 W– We’re like light bulbs, constantly putting out heat••Need more like 2,000 Cal for active lifestyleNeed more like 2,000 Cal for active lifestyle– 100 W of powerSpring 2008 10UCSDPhysics 10Energy from FoodEnergy from Food••Energy from fat, carbohydrates, proteinEnergy from fat, carbohydrates, protein– 9 Calories per gram for fat– 4 Calories per gram for carbohydrate• Fiber part doesn’t count– 4 Calories per gram for protein••Calculate 63 fat, 84 CH, 40 protein Calculate 63 fat, 84 CH, 40 protein CalsCals– total is 187 Calories (180 is in the ballpark)••1 Calorie (kilo-calorie) is 4,184 J1 Calorie (kilo-calorie) is 4,184 J– 180 Cal = 753 kJ– set equal to mgh climb 1100 m vertically,assuming perfect efficiencySpring 2008 11UCSDPhysics 10Not So FastNot So Fast……••Human body isnHuman body isn’’t 100% efficient: more like 25%t 100% efficient: more like 25%– To put out 100 J of mechanical work, must eat 400 J– 180 Calorie candy bar only gets us 275 m, not 1100 m••Maximum sustained power output (rowing,Maximum sustained power output (rowing,cycling) is about 150-200 W (for 70 kg person)cycling) is about 150-200 W (for 70 kg person)– Consuming 600-800 W total, mostly as wasted heat– For 30 minutes 800 J/s 1800 s = 1.44 MJ = 343 Cal••Can burst 700 W to 1000 W for < 30 secCan burst 700 W to 1000 W for < 30 sec– put out a full horsepower momentarily!Spring 2008 12UCSDPhysics 10Most impressive display of human powerMost impressive display of human power••The Gossamer Albatross crossed the EnglishThe Gossamer Albatross crossed the EnglishChannel in 1979, powered by Bryan AllenChannel in 1979, powered by Bryan Allen– Flight took 49 minutes, wiped Bryan out!– Sustained power out ~250 WEnergy in our Lives 04/30/2008Lecture 11 4Spring 2008 13UCSDPhysics 10Aside: Human mass balanceAside: Human mass balance••No nuclear power in our stomachs, so mass isNo nuclear power in our stomachs, so mass isconservedconserved– mass in = mass out, assuming constant weight– burning Calories  losing weight, not directly, anyway••Breathing: an important element in mass balanceBreathing: an important element in mass balance– lose about a pound per day through nose/mouth!– breathe in O2, breathe out CO2: donating carbon to air– breathe in dry air, exhale moist air (H2O loss)••Trees get their mass through inverse processTrees get their mass through inverse processSpring 2008 14UCSDPhysics 10Human Energy Requirements SummarizedHuman Energy Requirements Summarized••We need


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