applied thermodynamics(lecture 3)
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APPLIED THERMODYNAMICSEL 325 (3+0)
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Topic 3Laws
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1. LAW OF CONSERVATION OF MASS
dtdmmm
mmm
systemoutin
systemoutin
/
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Conservation of Mass for General Control Volume
The conservation of mass principle for the open system or control volume isexpressed as
or
Steady state ?Change of Any quantity with time = 0
outin
systemoutin
mm
dtdmmm
0/
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Reversible & Irreversible Process
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A reversible process is one the direction can be reversed by an infinitesimal change ofvariable.
If a block of material (at T) is in contact with surrounding at (TT), then ‘heat will flow’into the surrounding. Now if the temperature of the surrounding is increased to (T+T),then the direction of heat flow will be reversed.
Irreversible processes
generally non-equilibrium processes
have a preferred direction (towards increasing “).
T
Heat flowdirection
T+T
T
Heat flowdirection
TT
Reversible process
Reversible process
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3. Equilibrium:In thermodynamics equilibrium is a term used to determine
whether there is a process taking place in a system. Ifthere is no changes in states of a system, then it is said tobe in equilibrium
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4. THE ZEROTH LAW OF THERMODYNAMICS
If two bodies are separately in thermal equilibrium with a thirdbody then they must be in thermal equilibrium with each other.
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The zeroth law of thermodynamics is an observation. When two objects areseparately in thermodynamic equilibrium with a third object, they are inequilibrium with each other. Three objects as shown on the slide. Object #1and object #2 are in physical contact and in thermal equilibrium. Object #2 isalso in thermal equilibrium with object #3. There is initially no physical contactbetween object #1 and object #3. But, if object #1 and object #3 are broughtinto contact, it is observed that they are in thermal equilibrium.
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The Zeroth Law of Thermodynamics
If object A is in thermal equilibrium withobject C, and object B is separately inthermal equilibrium with object C, thenobjects A and B will be in thermalequilibrium if they are placed in thermalcontact.
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5. FIRST LAW OF THERMODYNAMICS
•Energy can be neither created nor destroyed but only transformed
•In thermodynamics cycles , if work is transferred during the cyclethen, since there is no final change in the properties of the workingsubstance, the energy to provide a work must have been transferred asheat and must exactly equal to work.
Thus, for a cycle, there is no net property change:
Net heat transfer = Net work transfer
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First law of thermodynamics:
Energy In = Energy Out
For open system: W = QFor closed system which does not execute acycle: Q = W + U (If heat and work transfers
are not equal)
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ENTROPY
It is the measure of the disorder in the system.
As the change in entropy can be described as the heat addedper unit temperature
ΔS = Q/Twhere S is the change in entropy,
Q is the heat flow into or out of a system, and T is theabsolute temperature in degrees Kelvin (K).
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Entropy:a state variable whose change is definedfor a reversible process at T where Q is theheat absorbed.
Entropy: a measure of the amount of energy whichis unavailable to do work.
Entropy: a measure of the disorder of a system.
Entropy: a measure of the multiplicity of a system.
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THE SECOND LAW OF THERMODYNAMICS
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5. THE SECOND LAW OF THERMODYNAMICS5.1.CLAUSIUS STATEMENT
The second law of thermodynamics describes the flow ofenergy in nature in processes which are irreversible.The second law of thermodynamics may be expressed inmany specific ways.
Second Law and Refrigerator
It is not possible for heat to flow from a colderbody to a warmer body without any work havingbeen done to accomplish this flow. Energy willnot flow spontaneously from a low temperatureobject to a higher temperature object.
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5.2. KELVIN-PLANCK STATEMENT
Second Law and Heat Engine
It is impossible to extract an amount of heat froma hot reservoir and use it all to do work. Someamount of heat must be exhausted to a coldreservoir.
It means that the efficiency of a heat engine cycleis never 100%.
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Thus ,
Net work transfer = Net heat Transfer
But generally,
Net work transfer is less than Net heat transfer.
This means that some heat transfer must be rejected and is lost.
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HEAT ENGINE
Qin is the heat flow from the hot reservoir to the engineQout is the heat flow from the engine to the coldreservoir.The work done by the heat engine is the differencebetween Qin and Qout.Heat engine efficiency:
in out
in
Q Q
Q
output
heat engineinput
w
q
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5.1. PERPETUAL MOTION OF THEFIRST KIND
An engine which could provide work done without heattransfer would run forever; in other words, it would haveperpetual motion. IMPOSSIBLE!
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WHAT IS “PERPETUAL MOTION”?“Perpetual Motion”
• Describes HYPOTHETICAL MACHINES thatoperate or produce useful work indefinitely &,more generally, hypothetical machines that
producemore work or energy than they consume,whether they might operate indefinitely or not.
• There is undisputed scientific consensus that
Perpetual motion would violateeither the 1st Law or the 2nd Lawof Thermodynamics, OR BOTH!
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“Perpetual Motion”• Describes a theoretical machine that, without anylosses due to friction or other forms ofdissipation of energy, would continue to operateindefinitely at the same rate without any externalenergy being applied to it.
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OVERBALANCED WHEEL
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