Clicker Question
Power and Energy
You are paid at a rate of $15 / hour.
You are saving to buy a $3000 ticket to the Super Bowl.
How many hours do you have to work?
A)
B)
C)
D)
E)
20 hours
100 hours
200 hours
3000 hours
None of the above is correct
Power is the rate of doing work.
It is the energy given to or taken away from an object per time.
If it helps to write Watts as Joules/second that is fine.
Energy  Power  time
Energy
Power 
time
time 
Total Amount  Rate  time
time 
Total Amount
$3000

 200hours
Rate
$15 / hour
Important Notice
Last Name, First Name
Assignment #
Course Name
They must be clearly typed (or very neatly written) in the
upper left corner of all turned in assignments – homework,
extra credit, etc. Otherwise penalty of -2.
Also, for future assignments, all extra credit papers must be
typed and homework if hand written must be very neat…
Energy
Power
Power is a rate. It is not the same as energy!
Reading Assignment
Start reading all of Chapter 4 for the end of this week.
Read 4.1-4.4 for Wednesday.
Homework Assignment
Homework Assignment #2
is due Wednesday, January 27, 2015 at 4:30 pm.
It must be handed in to the Wooden Box on the Duane G2B level.
Problem #4 had 18^8 which should be 10^8.
-- My office hours are Monday 1-2 pm,
Tuesday 11 am –12 pm
Also available by appointment – just email me.
-- The class Teaching Assistants will also be available in the
Physics Help Room (downstairs on the Duane 2B level).
Prasanth Prahladna : Tuesdays 3-4 pm, Paul Quelet :
Wednesdays 10-11 am, Ivan Kurz: Wednesdays 11 am – 12 pm
Even More Energy Units…
Where is the energy given on this label?
New Energy Unit = Food Calorie or Calorie
Calorie and calorie!
calorie
Energy to change the
temperature of 1 gram of water by 1 °C
1 Food Calorie = 4184 Joules
Calorie or Food Calorie (or kilo-calorie)
Energy to change the temperature of
1 kilo-gram of water by 1°C
* Thus a Food Calorie is actually the same as 1000 calories
(which is the physics or chemistry term).
1
British Thermal Unit (BTU)
Energy required to change the
temperature of 1 pound of water by 1°F
So really that is 1000
physics calories!
Clicker Question
The best coal (Anthracite) which is 88% carbon, has an
energy content of 13,300 BTU / lb.
So how many degrees can you raise the temperature of
260 pounds of water with the energy from one pound of
coal?
A)
B)
C)
D)
E)
5 degrees Fahrenheit
50 degrees Fahrenheit
250 degrees Fahrenheit
3000 degrees Fahrenheit
40 degrees Celsius
1 Jelly donut
(250 Calories)
13,300 BTU raises the temperature of 1 lbs of water by 13,300 0F
OR
13,300 BTU raises the temperature of 260 lbs of water by
13,300 / 260 = 50 0F
What if we compare this 1 million Joules from the donut, to the
Potential Energy of a 80 kg person at a given height (H).
How many Joules of energy is this?
 1000 calories   4.184 Joules 
250 Calories  


 1 Calorie   1 calorie 
 106 Joules !
My Great Idea (or Not)
I have a Concept2 Rowing Machine at home.
If I row for 20 minutes, it says I have burned
“250 Food Calories” (like the one jelly donut).
What is that height H?
PE  mgH
PE
(10 6 Joules )
H

mg (80kg  9.8m / s 2 )
H
CO2 + H2O + Energy
6
PE
(10 Joules )

mg (80kg  9.8m / s 2 )
Hike up
Quandry Peak
H  1275 meters or 4000 feet
That is about 106 Joules in 1200 seconds or
a Power of 800 Watts.
However, I am only about 10% efficiency at
producing useful work (the chain pulling a
wheel), and thus I only could get out about 80
Watts of electricity generation (for example).
So instead of paying $100 for my 1000
kWatt-hours of electricity last month, I could
have just rowed for 12,000 hours ~ 500 days!
Biological mechanism needed for efficient conversion.
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Temperature Units
Heat and Energy
Key Definitions…
Temperature (T) has an absolute lowest value (Absolute Zero).
Thermal Energy – kinetic energy of electrons,
molecules, atoms in random motion (not all going in
the same direction)
The Kelvin temperature scale has T=0 at Absolute Zero.
Temperature - relates to the average kinetic energy
of these microscopic particles (higher T, faster
motion).
Different units from energy, so this is not an energy!
Heat – the flow of energy from higher temperature
to lower temperature objects
Lord Kelvin
T (Kelvin) = T (Celsius) + 273
Thus, absolute zero is T(Celsius) = -273 0C
T (Celsius) = (5/9) x [T (Fahrenheit) – 32]
Thus:
212 0F = 100 0C (boiling water)
32 0F =
0 0C (freezing water)
73 0F = 23 0C (room temperature)
-459 0F = -273 0C (absolute zero)
Temperature is not the same as heat or thermal energy!
Heat refers to the flow of energy between two materials due
to the random motion (kinetic energy) of the atoms and
molecules.
Temperature is what you measure with a thermometer and it
tells you something about the “average energy” of the atoms
and molecules.
Internal energy of air molecule at
room temperature?
T = 20 0C = 293 Kelvin
Boltzmann Constant
Kinetic Theory of Gases
610-21
KE (molecules) = 1.5 kB T =
Air molecules in 1 ft3 = 7.21023
Joules
Internal energy in 1 ft3 = 4300 Joules !
KE = ½
mv2
Why isn’t T = KE?
Molecules rotate too, etc.
How fast are these air molecules moving?
Mass molecule ~ 8 x 10-26 kg,
KE = ½ mv2  velocity v ~ 400 m/s = 800 miles per hour!
Can you get at this 4300 Joules of energy and get it to do
work? Why doesn’t it hurt when they hit you at 800 mph?
Heat Transfer Mechanisms
Conduction – exchange of energy from higher
temperature object to lower temperature object via
direct contact
Convection – exchange of energy via circulating fluid (gas
or liquid).
Think of the wind. Often very hard to calculate.
Radiation – electromagnetic radiation
also called electromagnetic waves can
travel through empty space and
transfer energy
Do EM waves really carry energy?
Think about a sun burn….
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