10
Energy UCSD
Physics In Our Daily Lives
Our Energy Sources, Budgets, Expenditures
UCSD
Physics 10
Where Does Energy Come From
Ultimately, from the Big Bang
Energy is, after all, conserved
In 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 products of former stars (supernovae) 2
Spring 2008
UCSD
Physics 10
World Energy Budget (annually)
Source Petroleum Coal Natural Gas Hydroelectric Nuclear Energy Biomass (burning) Geothermal Wind Solar Direct Sun Abs. by Earth Spring 2008 1018 Joules/yr 158 92 89 28.7 26 1.6 0.5 0.13 0.03 2,000,000
circa 2000
Percent of Total 40.0 23.2 22.5 7.2 6.6 0.4 0.13 0.03 0.008 then radiated away 3
UCSD
Physics 10
Where does the sun get its energy?
Thermonuclear fusion reactions in the sun's center
Sun is 16 million degrees Celsius in center Enough energy to ram protons together (despite mutual repulsion) and make deuterium, then helium Reaction per mole 20 million times more energetic than chemical reactions, in general
4 protons: mass = 4.029 He nucleus: mass = 4.0015
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2 neutrinos, photons (light)
Spring 2008
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UCSD
Physics 10
E = mc2 in Sun
Helium nucleus is lighter than the four protons! 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! 5
Spring 2008
UCSD
Physics 10
Solar Energy Output Forms
2% in neutrinos: very light, non-interactive
more than ten billion per second course through your fingernail fly through earth, as if it weren't even there detected in rare interaction events in huge underground detectors
Spring
"Super-K" underground neutrino detector in Japan, half full of water 2008 6
UCSD
Physics 10
Solar Energy Output Forms, continued
98% in light: photons
Each photon takes about a million years to clear the annoying electrons in solar plasma 8 minutes once free to reach earth
1370 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 it
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UCSD
Physics 10
Where does the sunlight go?
Spring 2008
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UCSD
Physics 10
Human Energy Requirements
1,500 Calories per day just to be a couch-potato
6,280,000 J
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 lifestyle
100 W of power
Spring 2008
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UCSD
Physics 10
Energy from Food
Energy 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 Cals
total is 187 Calories (180 is in the ballpark)
1 Calorie (kilo-calorie) is 4,184 J
180 Cal = 753 kJ set equal to mgh climb 1100 m vertically, assuming perfect efficiency
Spring 2008
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UCSD
Physics 10
Not So Fast...
Human body isn'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 (rowing, output 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 sec
put out a full horsepower momentarily!
Spring 2008
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UCSD
Physics 10
Most impressive display of human power
The Gossamer Albatross crossed the English Channel in 1979, powered by Bryan Allen
Flight took 49 minutes, wiped Bryan out! Sustained power out ~250 W
Spring 2008
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UCSD
Physics 10
Aside: Human mass balance
No nuclear power in our stomachs, so mass is conserved
mass in = mass out, assuming constant weight burning Calories losing weight, not directly, anyway
Breathing: 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 process
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UCSD
Physics 10
Human Energy Requirements Summarized
We need chemical energy from food to run
Ultimate source is sun, long chain of events to twinkies Constantly burn energy at rate of 75-100W We spend energy at about 25% efficiency Maximum sustained power is 150-200 W
actually burn 4 times this due to inefficiencies
Spring 2008
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UCSD
Physics 10
Chemical Energy: Gasoline
Gasoline and other combustibles are about as energy-rich as the fat we eat: 11 Calories/gram
Jet fuel, crude oil, kerosene, you name it
Spring 2008
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UCSD
Physics 10
Fuel Efficiency
Can calculate miles-per-gallon based on this info:
30 m/s requires 50 kW to fight air drag (Lecture 8) Go one mile in 54 seconds at this speed (67 m.p.h.) 50 kW 54 seconds = 2.68 MJ = 640 Calories Assuming 30% engine efficiency (lots of heat), need 640 3.3 = 2100 Calories, or 192 grams of fuel One gallon is 3.5 kg ~20 miles-per-gallon!
Improvement via aerodynamic drag reduction
also helps to go slower (v2 dependence)
Spring 2008
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UCSD
Physics 10
Energy Expenditure
Per capita energy production in U.S. at > 10 kW
times 86,400 seconds per day is about 1 GJ per day!
1,000,000,000 J per day per person 250,000 Calories
Demands 23 kg (6 gallons) of gas per day per person Or equivalently 38 kg (85 lb) of coal (at 6 Cal/gram)
Most of this expenditure is industrial
Production of consumer goods
Most residential/commercial energy used for heat
Spring 2008
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UCSD
Physics 10
Solar Alternative
Once fossil fuels are exhausted (coming soon!), need alternative production source Straight to solar may be smart 1370 W/m2 incident on earth, 900 W/m2 typically available to ground panel in full sun
take day/night and clouds into consideration: 200 W/m2 average silicon photovoltaics about 15% efficient 30 W/m2
Each person would need 300 square meters of panels to cover all of our nation's energy needs
for just our electricity needs, would need square in desert 100 miles on a sidenot impossible!
Spring 2008
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UCSD
Physics 10
References and Assignments
References
Energy and the Environment, Rinstinen & Kraushaar Energy, by Gordon Aubrecht, Prentice Hall, 1995 Energy: A Guidebook, by Janet Ramage (British)
Course on subject: Physics 12: Energy & Environ.
Spring Quarters (I'll teach Spring 2009)
Midterm Reviews:
Wed. 4/30 6:30 PM to 8:20 PM; Pepper Canyon 122 (Tom) Thu. 5/01 8:00 PM to 9:50 PM; Center 212 (Jim)
Scantron form # 101864-PAR-L & No. 2 pencil Assignments:
HW for 5/09: Hewitt 7.E.42, 7.P.9, 6.R.16, 6.R.19, 6.R.22, 6.R.23, 6.E.8, 6.E.12, 6.E.43, 6.P.6, 6.P.12, 8.R.29, 8.E.47, 8.P.9
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