The Second Law of Thermodynamics
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Transcript of The Second Law of Thermodynamics
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The Second Law of Thermodynamics
Physics 102Professor Lee
CarknerLecture 7
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PAL #6 First Law 1 mole of gas at 300 K and 2 m3 compressed to
1 m3, constant pressure P = nRT/V = (1)(8.31)(300)/(2) = 1246.5 Pa W = PV = (P) (Vf-Vi)
Sign of work?
Volume decreased
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Engines
General engine properties: An input of heat An output of heat
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Heat and Work Over the Cycle Four parts of the cycle:
compression output of heat QC
Over the course of one cycle positive work is done and heat is transferred
Since the engine is a cycle, the change in internal energy is zero
U=(QH-QC)-W =0W = QH - QC
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Efficiency
In order for the engine to work we need a source of heat for QH
e = W/QH An efficient engine converts as much of the
input heat as possible into work
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Today’s PAL If an automobile engine outputs
149200 joules per second to the drive train and burns fuel at a rate of 746000 joules per second, what is the efficiency?
If gas is $2.00 per gallon, how much money per gallon are you wasting?
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Efficiency and Heat
e = 1 - (QC /QH) The efficiency depends on how
much of QH is transformed into W and how much is lost in QC:
Reducing the output heat means
improving the efficiency
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The Second Law of Thermodynamics
This is one way of stating the second law:It is impossible to build an engine that
converts heat completely into work
Engines get hot, they produce waste heat (QC)
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Carnot Engine In 1824 Sadi Carnot related the maximum efficiency to the
temperature of the reservoirs:eC = 1 - (TC / TH)
A hot input reservoir and a cold output reservoir make it “easier” to
move heat in and out
e < eC Another way to state the second law is:
There is a limit as to how efficient you can make your engine
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The First and Second Laws The first law of thermodynamics says:
The second law of thermodynamics
says:
The two laws imply: W < QH W QH
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Dealing With Engines Most engine problems can be solved by knowing how to
express the efficiency and relate the work and heats:W = QH - QC
e = W/QH = (QH - QC)/QH = 1 - (QC/QH)
eC = 1 - (TC/TH)
For individual parts of the cycle you can often use the ideal gas law:
PV = nRT
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P-V Diagram for Engine
The total work output per cycle
Positive work is clockwise
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Refrigerators
A refrigerator is a device that uses work to move heat from low to high temperature
The refrigerator is the device on the back of the box Your kitchen is the hot reservoir
Heat QC is input from the cold reservoir, W is input power, QH is output to the hot reservoir
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How a Refrigerator Works
The fluid is pumped into the hot chamber (coils on the back) and compressed, adding work W
Need special fluid that can evaporate and condense in the right place
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Refrigerator Cycle
Liquid
Gas
Compressor (work =W)
Expansion Valve
Heatremovedfrom fridgeby evaporation
Heat added to room bycondensation
HighPressure
Low Pressure
QC QH
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Refrigerator Performance Input equals output:
The equivalent of efficiency for a refrigerator
is the coefficient of performance COP:COP = QC / W
Unlike efficiency, COP can be greater than 1
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Today’s PAL Lets say you wanted to cool your
house on a hot day so you buy a refrigerator, plug it in and open the door.
Does the temperature of the house, increase, decrease or stay the same? Why? (assume insulated house)
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Heat Pumps
It removes QC from your house and exhausts QH to the outside
It removes QC from the outside and adds QH to your house
Heat pump COP = QH / W Want the most heat output for the work
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Refrigerators and Temperature
We can relate the coefficient of performance to the temperature:
COP = TC /(TH-TC)
This is the maximum COP for a fridge operating between these two temperatures
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Refrigerators and the Second Law
You cannot move heat from low to high temperature without the addition of work COP cannot be infinite
Heat doesn’t flow “uphill” by itself, although this would not violate the first law
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Statements of the Second Law
It is impossible for any device which
operates in a cycle to convert heat completely to work
For refrigerators:
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Next Time Read: 15.7-15.11 Homework: Ch 15, P 26, 31, 35, 37 I will also post some practice
problems Won’t count for grade