maed thermo

8/2/2019 maed thermo

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DIRECTION OF

THERMODYNAMICPROCESSES


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Thermodynamicprocesses that occur

in nature are allirreversibleprocesses.


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Irreversible Process

An irreversible process is a process that is not reversible. All real processes are irreversible. Irreversible processesoccur because of the following:

a. Frictionb. Unrestrained expansion of gasesc. Heat transfer through a finite temperature differenced. Mixing of two different substancese. I 2R losses in electrical circuitsf. Any deviation from a quasi-equlibrium process


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Irreversibility:

Once a system createsthermal energy, that system willnever by itself (spontaneously) beable to return to its previouscondition. There is an irreversibility

about any process that createsthermal energy.


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Reversible Processes

A reversible process is a quasi-equilibrium, or quasi-static,process with a more restrictive requirement. (equilibriumprocess - the system is always in thermodynamic equilibrium)

Internally reversible process

The internally reversible process is a quasi-equilibriumprocess, which, once having taken place, can be reversed andin so doing leave no change in the system . This saysnothing about what happens to the surroundingsabout the system.

Totally or externally reversible process

The externally reversible process is a quasi-equilibriumprocess, which, once having taken place, can be reversed and

in so doing leave no change in the system or surroundings.


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DISORDER

and

THERMODYNAMICPROCESSES


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heat transfer involves

changes in energy towards arandom, disorderedmolecular motion.

conversion of mechanical

energy to heat involves anincrease of randomness or

disorder.


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SECOND LAW OF

THERMODYNAMICS


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ENGINE STATEMENT of the SECONDLAW OF THERMODYNAMICS

(KELVIN PLANCK STATEMENT) It is impossible for any system to

undergo a process in which it absorbsheat from a reservoir at a singletemperature and converts the heatcompletely into mechanical work, withthe system ending in the same state inwhich it began.


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Heat engines use aportion of

the thermal energy thatflows

naturally from a high to

a lowtemperature andconvert it to

work. ====>


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A schematic representationof a heat engine. Heat is

taken in at hightemperatures, T H . Some

heat is converted to work,and the remainder isreleased at a lower

temperature, T C .


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The basis of this is the differencebetween the nature of internal energyand that of macroscopic mechanicalenergy.

Since we cannot control the motions of molecules, we cannot convert thisrandom motion COMPLETELY back toorganized motion. We can convert PART

of it, and this is what a heat enginedoes.


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The first law denies the

possibility of creating ordestroying energy; the

second law limits theavailability of energy andthe ways in which it canbe used and converted.


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REFRIGERATOR STATEMENT

of the SECOND LAW OFTHERMODYNAMICS

(CLAUSIUS STATEMENT) It is impossible for any

process to have as its soleresult the transfer of heat froma cooler to a hotter body.


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Heat will not flow spontaneouslyfrom a cold to a hot body.

Thermal energy flowsspontaneously (without external

assistance) from a highertemperature object to a lower-temperature object.

* It will not spontaneously flow theother way!


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Perpetual-Motion Machines

Any device that violates the firstor second law of thermodynamics

is called a perpetual-motionmachine. If the device violatesthe first law, it is a perpetual-motion machine of the first kind.If the device violates the secondlaw, it is a perpetual-motionmachine of the second kind.


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ENTROPY STATEMENT of the

SECOND LAW OFTHERMODYNAMICS

The entropy of an isolatedsystem can only increase(irreversible) or remain constant

(reversible process) . Its entropycannot decrease.


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When a box of hotgas & cold gas

are allowed to mix .. .

. . . eventually thefaster moleculesstriking the slowermolecules spread outsome of their energyto the slowermolecules . . .


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However, the

molecules willNOTspontaneously

re arrange. . . .leading to a wider range of re-themselves into ahot coldspeed and an intermediatetemperature in the box.


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The Second Law of Thermodynamics

The total entropy of the universeincreases if a process is spontaneous.

For a phase change at the transitiontemperature (ice melting/freezing at0 C) the system is in equilibrium. Atequilibrium there is no entropycreated. The entropy created is alwaysa positive number.


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maed thermo

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