Transcript 3-10-09 Thermodynamics

Bell Ringer
Name:
3/10/2009
What date is the Brewers home
opener?
Response
New Seating Chart
• Find your new seat for the grading period
Change to late assignment policy
•
Everything is now going to be posted to the
web
– Lectures
– Labs & solutions
– Homework assignments & solutions
– Tests and quizzes & solutions
– Once a solution is posted, the lab or
homework will no longer be accepted
– The exception is for excused absences
Labs
• Labs are not recess for middle schoolers
Extra Credit
• Need one volunteer from each class to learn
enough about the smartboard software to
demonstrate how to draw using it.
• This will be for our next concept map lesson
Heat of Fusion Lab Review
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Each student started with 10
-1, if the hypothesis was blank
-1, if the predicted final temperature had
incorrect units or was outside a reasonable
range
-1, if there was no mass for the Styrofoam cup
-1, if there was no readings for mass of the
water
-1/2, if there was a mass reading for the water
but I couldn’t tell if it included the cup or not
Heat of Fusion Lab Review
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-1, if there was no initial temperature reading
-1, if there was no mass reading for the ice
-1/2, if there was a mass reading for the ice but
I couldn’t tell if it included the cup or not
-1, if there was no reading for the final
temperature
The second page was graded as extra credit
+1/4, if #1 indicated the heat was absorbed by
the ice
Heat of Fusion Lab Review
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+1/2, if #1 indicated the heat was absorbed by
the ice then transferred to the surroundings
+1/4, if #2 had the proper values substituted
into the formula
+1/4, if #3 showed the Q from #2 was divided
by the proper ice mass
+1/4, if #4 had all proper values substituted
into the formula
+1/4, if #5 indicated that was much greater
than c
+1/4, if #6 used the value from #4 or #3 to
compute an error
Heat of Fusion Lab Review
Masses
Cup
Cup + Water
Water
Cup + Ice + Water
Optional
Ice
Ice + Water
Heat of Fusion Lab Review
Temperatures
Water only
Water + Ice
Heat of Fusion Lab Review
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Calculating total energy for process
Energy gained by ice = Energy lost by water
Qgained  Qlost
 cal 
Qduring _ change  Qafter _ change  Qgained Qlost  mwater 1 o T
 g C
Qduring _ change  mice H f
Qafter _ change  mice cT
 cal 
mice H f  mice 1 o T  Qgained
 g C
Chapter 23 Homework
Extra credit:
Use Q  H f mice for the phase change and Q  mcT
for the change as water, then add them together.
cal
at phase change Q  H f mice  80
 100 g  8000 cal
g
after phase change

cal
Q  mcT  100 g  1 o  30 o C  0 o C
g C
cal
Q  mcT  100 g  1 o  30 o C  3000 cal
g C
Qtotal  8000 cal  3000 cal  11000 cal



Thermodynamics
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Thermodynamics - the study of heat
movement
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Thermodynamics concentrates on the
macroscopic (big) world, rather than the
microscopic (tiny) world
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Thermodynamics provides us with theories for
operating heat engines
Absolute Zero
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No upper limit on how quickly molecules can
move because of kinetic energy
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There is a definite lower limit called absolute
zero. The kinetic energy approaches zero.
Absolute Zero
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Which is larger, a Celsius degree or a Kelvin
degree?
Absolute Zero
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Which is larger, a Celsius degree or a Kelvin
degree?
– Neither, they are both the same.
Absolute Zero
Absolute Zero
Dry Ice
Water Freezes
Water Boils
Iron Melts
Incandescent Bulb
Sun’s Surface
Lightening
Center of Sun
Hydrogen Bomb
Kelvins
0
Celsius
-273
195
273
373
-78
0
100
1811
2500
6000
28000
1538
2227
5727
27727
20000000
100000000
Thermodynamics History
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In the 17th century heat was thought to be an
invisible fluid called caloric
Caloric flowed from hot to cold objects
Caloric was conserved in its interactions
Thermodynamics History
• If objects only get warm because of heat
transferred from another object, how do my
hands get warm if I rub them together?
Thermodynamics History
• If objects only get warm because of heat
transferred from another object, how do my
hands get warm if I rub them together?
– something you learned about a while back,
friction
– recall that work = force * distance
– and friction is a force acting over a distance
Thermodynamics History
• Metal workers noticed heat transfer while drilling
cannons a long time ago.
• It took a while for anyone to realize that friction
from the drill bits was making the cannon barrels
hot.
Thermodynamics History
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In the 1840’s it became understood that the
flow of heat was nothing more than the flow of
energy and caloric theory was abandoned
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James Joule used the paddle-wheel apparatus
to compare heat energy with mechanical
energy
Thermodynamics History
James Joule tried to measure the
expected increase of water moving
over a waterfall and landing in the
pool below.
His measurements did not confirm
his estimates, so he developed the
paddle-wheel apparatus to use
instead
Thermodynamics History
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Today heat is viewed as a form of energy
which can neither be created nor destroyed
First Law of Thermodynamics
Whenever heat is added to a system, it
transforms to an equal amount of some other
form of energy
First Law of Thermodynamics
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What happens when you strike a penny with a
hammer, besides it getting dented and flatter?
First Law of Thermodynamics
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What happens when you strike a penny with a
hammer, besides it getting dented and flatter?
– It gets hot
– Energy is converted from potential to
thermal in form
First Law of Thermodynamics
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Back to rubbing our hands together. What is
the effect of the work done?
First Law of Thermodynamics
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Back to rubbing our hands together. What is
the effect of the work done?
– Our hands get warm
– Work is converted from potential to thermal
in form
• Can thermal energy be easily converted into
work?
First Law of Thermodynamics
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Our book restates the first law of
thermodynamics as:
Heat added = increase in
+ external work
internal energy
First Law of Thermodynamics
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If a hollow object is heated on a stove, and it
doesn’t move, then the heat is transferred to
the inside to the object increasing its kinetic
energy
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But if the object can perform mechanical work,
then the internal kinetic energy would be
lessened
– An example of this would be a piston in a
steam engine
First Law of Thermodynamics
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In a steam engine, the piston is forced to move
because the steam pushes on the piston as it
expands
– The piston moves and work is performed
– The energy transferred to the piston will be
reduced by the amount of work done
Heat added = increase in
+ external work
internal energy
First Law of Thermodynamics
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Can the process move in the other direction,
that is, work adding heat into the process?
First Law of Thermodynamics
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Can the process move in the other direction,
that is, work adding heat into the process?
– Yes, think of a mechanical tire pump being
used to inflate a tire
– It gets hot
Adiabatic Processes
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Compression or expansion of a gas, when no
heat enters or leaves is said to be adiabatic.
Adiabatic changes occur so rapidly that the
heat has little time to enter or leave.
– The best example is the cylinders in an
automobile engine.
Adiabatic Processes
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In the automobile engine, the gases in the
piston chamber expand and contract so quickly
that most of the heat stays in the chamber
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We have a repeating cycle of:
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Work being done on the gas by compressing it
Gas gaining internal energy and warming
The gas expanding and performing work
The gas then gives up energy and cools
Adiabatic Processes
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A while back we talked about blowing air on
your hand with your mouth wide open
– Its warm
• If you narrow your mouth opening and blow air
again
– Its cooler
– Adiabatic expansion is occurring and
causing the cooling
Adiabatic Processes
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Many weather changes are driven by adiabatic
processes where:
– Change in air temp ~ Change in air pressure
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It requires large masses of air for the effect to
be adiabatic
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The changes occur around the edges of the masses
Chinook winds are an example where cold air
moves down slope, compresses and warms
Adiabatic Processes
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Why does air moving quickly down the
mountainside feel warm?
Adiabatic Processes
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Why does air moving quickly down the
mountainside feel warm?
– Cold air moves down slope, compresses
and then warms
– Chinook winds are an example where
Second Law of Thermodynamics
Heat will never of itself flow from a cold object
to a warm object.
Second Law of Thermodynamics
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We’ve been talking about this for a couple
weeks now and seen many examples.
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We’ve only seen energy transfer from a warm
object to a cold one
Second Law of Thermodynamics
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The heat pump is an example of energy
flowing in the other direction, but work is
required to accomplish the task.
Heat Engines
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Work can be changed completely into heat
– Rub your hands together
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Changing heat completely into work can never
occur
– The best we can do is convert some heat
into mechanical energy
Heat Engines
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The heat engine is any device that converts
internal energy into mechanical work
– Steam engine
– Automobile engine
– Jet engine
The mechanical work can only be obtained
when heat flows from high temperature to low
temperature
Heat Engines
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When heat engines are discussed, reservoirs
of high and low temperature are frequently
mentioned
– Heat absorbed from high temperature
reservoir
– Some converted into mechanical work
– Remaining heat expelled to low temperature
reservoir
Heat Engines
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In the gasoline engine:
– Fuel burned in the high temperature
reservoir or combustion chamber
– Mechanical work is done on the pistons
– Remaining heat expelled as exhaust fumes
Heat Engines
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Before the 2nd law was understood, it was
thought that low friction devices might convert
nearly all energy to useful work.
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In 1824, Sadi Carnot analyzed the cycles of
compression and expansion and discovered
that the heat converted to useful work depends
on the temperature difference between the
high and low temperature reservoirs.
Heat Engines
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The relationship is summarized by the
following equation:
Thot  Tcold
Ideal _ efficiency 
Thot
– when all temperatures are measured in
Kelvins
Disorder
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The 1st law of thermodynamics states that
energy can’t be created or destroyed
The 2nd law of thermodynamics adds that
whenever energy transforms, some of it
degenerates into waste
The waste energy is unavailable and lost
Disorder
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The 2nd law of thermodynamics can be stated
another way:
– Natural systems tend to proceed toward a
sate of greater disorder
Disorder
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What happens to a stack of pennies if
somebody bumps the table they rest on?
Disorder
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What happens to a stack of pennies if
somebody bumps the table they rest on?
– They fall and most likely will not all land with
the same side up
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In other words, they become more disordered
Entropy
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Entropy is the measurement of the amount of
disorder
– When disorder increases, entropy increases
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Organized structures become disorganized as
time passes
– If a house is not maintained on a regular
basis, it weakens and may eventually
collapse
Entropy
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All living organisms extract energy from their
surroundings in order to become more
complex (organized)
– They do so at the expense of increased
disorder to their surroundings
– In the end, all organisms die and become
disordered
Entropy
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The laws of thermodynamics can put another
way:
– You can’t win (energy in is always less than
energy out), you can’t break even and you
can’t get out (entropy is increasing
everywhere)