Bell Ringer

transcript

Bell RingerName: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 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• 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

• -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• +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

Masses

Cup + Water

Cup + Ice + Water

Optional Ice + Water

Temperatures

Water only

Water + Ice

• Calculating total energy for process• Energy gained by ice = Energy lost by water

lostgained QQ T

owaterlost

1gainedchangeafterchangeduring QQQ __

ficechangeduring HmQ _

TcmQ icechangeafter _

gainedoicefice QTCg

calmHm

Chapter 23 Homework

Extra credit:

Use for the phase change and for the change as water, then add them together.

at phase change after phase change

icef mHQ TmcQ

calmHQ icef 800010080

calgTmcQ oo

o0301100

calCCg

calgTmcQ o

o3000301100

calcalcalQtotal 1100030008000

Thermodynamics

• Thermodynamics - the study of heat movement

• Thermodynamics concentrates on the macroscopic (big) world, rather than the microscopic (tiny) world

• Thermodynamics provides us with theories for operating heat engines

Absolute Zero

• No upper limit on how quickly molecules can move because of kinetic energy

• There is a definite lower limit called absolute zero. The kinetic energy approaches zero.

Absolute Zero

• Which is larger, a Celsius degree or a Kelvin degree?

Absolute Zero

• Which is larger, a Celsius degree or a Kelvin degree?

– Neither, they are both the same.

Absolute ZeroKelvins Celsius

Absolute Zero 0 -273

Dry Ice 195 -78

Water Freezes 273 0

Water Boils 373 100

Iron Melts 1811 1538

Incandescent Bulb 2500 2227

Sun’s Surface 6000 5727

Lightening 28000 27727

Center of Sun 20000000

Hydrogen Bomb 100000000

Thermodynamics History

• 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

• If objects only get warm because of heat transferred from another object, how do my hands get warm if I rub them together?

• 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

• 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.

• 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

• James Joule used the paddle-wheel apparatus to compare heat energy with mechanical energy

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

• 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

• 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

• 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

• Our book restates the first law of thermodynamics as:

Heat added = increase in + external work

internal energy

• 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

• 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

• 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

• 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

• 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

• In the automobile engine, the gases in the piston chamber expand and contract so quickly that most of the heat stays in the chamber

• We have a repeating cycle of:– 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

• 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

• Many weather changes are driven by adiabatic processes where:

– Change in air temp ~ Change in air pressure

• It requires large masses of air for the effect to be adiabatic

– 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

• Why does air moving quickly down the mountainside feel warm?

Adiabatic Processes

• 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.

• We’ve been talking about this for a couple weeks now and seen many examples.

• We’ve only seen energy transfer from a warm object to a cold one

• The heat pump is an example of energy flowing in the other direction, but work is required to accomplish the task.

Heat Engines

• Work can be changed completely into heat– Rub your hands together

• Changing heat completely into work can never occur

– The best we can do is convert some heat into mechanical energy

Heat Engines

• 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

• 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

• 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

• Before the 2nd law was understood, it was thought that low friction devices might convert nearly all energy to useful work.

• 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

• The relationship is summarized by the following equation:

– when all temperatures are measured in Kelvins

coldhot

TTefficiencyIdeal

Disorder

• 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

• The 2nd law of thermodynamics can be stated another way:

– Natural systems tend to proceed toward a sate of greater disorder

Disorder

• What happens to a stack of pennies if somebody bumps the table they rest on?

Disorder

• 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

• In other words, they become more disordered

Entropy

• Entropy is the measurement of the amount of disorder

– When disorder increases, entropy increases

• Organized structures become disorganized as time passes

– If a house is not maintained on a regular basis, it weakens and may eventually collapse

Entropy

• 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

• 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)

Bell Ringer

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