University:
Amherst College
Course:
PHYS 109 | Energy
Academic year:

2008
Views:

127

Pages:

18
Author:

Zerkyh5

Physics 9, January 31, 2008 •Energy in the News •Donuts and their equivalents –i.e. Energy Units and conversions Units of Energy • Joule = J = kg-m2/s2 =Nt-m. • British thermal unit = Btu = heat required to raise 1 pound of water 1 degree Fahrenheit = 1055 J. • QBtu = quadrillion Btu = 1015 Btu. • calorie = cal = heat required to raise one gram of water 1 degree Celsius = 4.184 J. • Food Calorie = Cal = 1000 calories = 4184 J. • Foot-pound = ft-lb = 1.356 J • Electron-volt = eV = 1.6 x 10-19 J • Kilowatt-hour = kWh = 3.6 x 106 J. Units and Conversion of Energy One slice of a 12” Medium Hand Tossed Pizza Hut cheese Pizza Each is listed as about 240 Cal = 1 BCD (Boston Cream Donut) = 1 ACD (Atkins Cider Donut). One Dunkin Donut’s Boston Cream Donut How many Joules are in 1 ACD? 1 BCD = 240 Cal x (4184 J/Cal) = 1.00 x 106 J = 1 MJ As you will see, there are plenty of Energy units. Lets drop the ACD in favor of the MJ. What can we do with 1MJ? How far could you ride a bike? Romer’s Energy Facts and Figures (appendix N) says that riding a bicycle consumes 80 BTU for every mile. 1 MJ x (1 BTU/1054J) = 950 BTU You could ride 950 BTU x (1 mile/80 BTU) = 12 miles How much gas has the equivalent energy of 1MJ? Energy in a barrel of gas is 5.2 x 106 BTU (Energy F+F, appendix H). 1 barrel = 42 gallons So energy/gallon = 5.2 x 106 BTU/barrel x (1 barrel/42 gallon) = 1.2 x 105 BTU/gallon So 950 BTU /(1.2 x 105 BTU/gallon) = 7.9 x 10-3 gallon of gas How far could you drive your car? Typical automobile mileage = 20 mpg 7.9 x 10-3 gallons x 20 mpg = 0.16 miles 0.16 miles x 5,280 ft/mile = 840 ft Another Concept - Power Power = rate at which energy is consumed, delivered or transferred. P = E/t Unit of Power: Joule/second = Watt How long can MJ power a 100 Watt light bulb? Solve for t = E/P. Time = 106 J/(100 J/s) = 10,000s 10,000 s x (1 hour/60 min) x (1 min/60 s) = 2.8 hour Yet another unit of energy 1 kW-hr = 1000 J/s x 3600 s = 3.6 MJ How many kW-hr in 1 MJ? 1 MJ = 1 MJ x 1 kW-hr /3.6 MJ = 0.28 kW-hr What is the cost of 1 MJ of energy? Electric bills are usually measured in kW-hr. Mine are about $0.15/kW-Hr. Cost for 1 MJ of electrical energy is 0.28 kW-hr x $0.15/(kW-Hr) = 4 cents For comparison, our 7.9 x 10-3 gallon of gas would cost about 7.9 x 10-3 gallon x $3.00/gallon = 2.4 cents. The donut cost $5.50/12 = $0.46 Consumer Prices for Electricity, Natural Gas, and Motor Gasoline for 2000 From the EIA QBtu – A really big unit of Energy 1 QBtu = 1015 Btu (Quadrillion Btu) About how many MJ is this? This is about 1012 MJ (a lot of donuts!) Note: Romer calls this a mQ? Note: Yearly expenditure of energy in the US is about 8 x 1019 Joules. What is the power consumption of the US in Watts? P = E/t = 8 x 1019 Joules/year 8 x 1019 Joules/year x (1 year/ 3.16 x 107s) = 2.5 x 1012 Joules/s = 2.5 x 1012 Watts What is the average American’s total power consumption? Power/person = 2.5 x 1012 Watts / 2.91 x 108 people = 8,600 Watts/person! How Many QBTU do we burn in year? 8 x 1019 Joules x (1QBTU/1.054 x 1018 J) = 76 QBTU Energy Consumption by Source, 1635-2000 (QBtu)

Physics 9 Energy in the News

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