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PHYSICS 231 Lecture 31: Engines and fridges. Remco Zegers Question hours: Thursday 12:00-13:00 & 17:15-18:15 Helproom. Metabolism. U=Q+W. Work done (negative). Heat transfer: Negative body temperature < room temperature. Change in internal energy: Must be increased: Food!. t. t. - PowerPoint PPT Presentation
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PHY 2311
PHYSICS 231Lecture 31: Engines and fridges
Remco ZegersQuestion hours: Thursday 12:00-13:00 & 17:15-
18:15Helproom
PHY 2312
Metabolism
U=Q+W
Change in internal energy: Must be increased: Food!
Heat transfer: Negativebody temperature < room temperature
Work done (negative)
PHY 2313
Metabolic rate
U = Q + Wt t t
Metabolic rate: rate in which food and oxygen are transformed into internal energy (to balance losses dueto heat loss and work done).
|W/t| Body’s efficiency: |U/t|
PHY 2314
Body’s efficiency
U/t~oxygen use ratecan be measured
W/t can be measured
PHY 2315
Types of processes
A: Isovolumetric V=0B: Adiabatic Q=0C: Isothermal T=0D: Isobaric P=0
PV/T=constant
First law of thermo-Dynamics:U=Q+W
PHY 2316
Isovolumetric processes (line A)
V=0W=0 (area under the curve is zero)U=Q (Use U=W+Q, with W=0)In case of ideal gas:U=3/2nRT•if P then T (PV/T=constant)
so U=negative Q=negative(Heat is extracted from the gas)
•if P then T (PV/T=constant)so U=positive Q=positive
(Heat is added to the gas)v
p
PHY 2317
Adiabatic process (line B)
Q=0No heat is added/extracted from thesystem.U=W (Use U=W+Q, with Q=0)In case of ideal gas:U=3/2nRT•if T
U=negative W=negative(The gas has done work)
•if TU=positive W=positive
(Work is done on the gas)
v
p
PHY 2318
isothermal processes
v
pT=0The temperature is not changedQ=-W (Use U=W+Q, with U=0)•if V
W=positive Q=negative(Work is done on the gas and energy extracted)
•if VW=negative Q=positive
(Work is done by the gas and energy added)
PHY 2319
isobaric process
v
p
P=0Use U=W+QIn case of ideal gas:W=-PV & U=3/2nRT•if V then T (PV/T=constant)
W: positive (work done on gas)U: negative Q: negative (heat extracted)
•if V then T (PV/T=constant)W: negative (work done by gas)
U: positive Q: positive (heat added)
PHY 23110
Cyclic processes
The system returns to itsoriginal state. Therefore,the internal energy mustbe the same after completionof the cycle (U=0)
PHY 23111
Cyclic Process, step by step 1Process A-B.Negative work is done on the gas:(the gas is doing positive work).
W=-Area under P-V diagram
=-[(50-10)*10-3]*[(1.0-0.0)*105]-½[(50-10)*10-3]*[(5.0-1.0)]*105==4000+8000W=-12000 J
U=3/2nRT=3/2(PBVB-PAVA)= = 1.5*[(1E+5)(50E-03)-(5E+5)(10E-03)]=0The internal energy has not changedU=Q+W so Q=U-W=12000 J: Heat that was added to thesystem was used to do the work!
PHY 23112
Cyclic process, step by step 2
Process B-CW=-Area under P-V diagram
=-[(10-50)*10-3*(1.0-0.0)*105]=W=4000 JWork was done on the gas
U=3/2nRT=3/2(PcVc-PbVb)= =1.5[(1E+5)(10E-3)-(1E+5)(50E-3)]=-6000 JThe internal energy has decreased by 6000 JU=Q+W so Q=U-W=-6000-4000 J=-10000 J10000 J of energy has been transferred out of the system.
PHY 23113
Cyclic process, step by step 3
Process C-AW=-Area under P-V diagramW=0 JNo work was done on/by the gas.
U=3/2nRT=3/2(PcVc-PbVb)= =1.5[(5E+5)(10E-3)-(1E+5)(10E-3)]=+6000 JThe internal energy has increased by 6000 JU=Q+W so Q=U-W=6000-0 J=6000 J6000 J of energy has been transferred into the system.
PHY 23114
Summary of the processQuantityProcess
Work(W) Heat(Q) U
A-B -12000 J 12000 J 0
B-C 4000 J -10000 J -6000
C-A 0 J 6000 J 6000
SUM -8000 J 8000 J 0
-AREA
A-B B-C C-A
PHY 23115
What did we do?
The gas performed net work (8000 J)while heat was supplied (8000 J):We have built an engine!
What if the process was done inthe reverse way?Net work was performed on thegas and heat extracted from the gas.We have built a heat pump!(A fridge)
PHY 23116
More general engine
heat reservoir Th
cold reservoir Tc
engine work
Qh
Qc
W=|Qh|-|Qc|efficiency: W/|Qh|e=1-|Qh|/|Qc|
W
The efficiency is determinedby how much of the heat yousupply to the engine is turnedinto work instead of being lostas waste.
turns water to steam
the steam moves the piston work is done
the steam is condensed
PHY 23117
Reverse direction: the fridge
heat reservoir Th
cold reservoir Tc
engine work
Qh
Qc
W
heat is expelled to outside
a piston compresses the coolantwork is done
the fridge is cooled
PHY 23118
The 2nd law of thermodynamics
1st law: U=Q+W In a cyclic process (U=0) Q=W: we cannot do more workthan the amount of energy (heat) that we put inside
2nd law: It is impossible to construct an engine that, operating in a cycle produces no other effect than theabsorption of energy from a reservoir and the performanceof an equal amount of work: we cannot get 100% efficiency
What is the most efficient engine we can makegiven a heat and a cold reservoir?
PHY 23119
Carnot engineAB isothermal expansion
DA adiabatic compression
W-, T-
Thot
CD isothermal compressionTcold
W+, Q-
BC adiabatic expansion
W-, T-
W-, Q+
PHY 23120
Carnot cycle
Work done by engine: Weng
Weng=Qhot-Qcold
efficiency: 1-Tcold/Thot
inverse Carnot cycle
A heat engine or a fridge!By doing work we cantransport heat
PHY 23121
Next lecture
Entropy and examples