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)