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27 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Thermodynamics

T
hermodynamics

i s the study of the conversi on of energy i nto work and heat and i ts rel ati on to
macroscopi c

variables such as
temperature
,
vol ume

and
pressure
. Hi stori cally, ther
modynami cs devel oped out of need to i ncrease the
effi ci ency

of earl y
steam
engi nes
.
[5]



Typi cal
thermodynamic system
, showi ng i nput from a heat source (boi l er)
on the l eft and output to a heat si nk (condenser) on
the ri ght.
Work

i s extracted, i n thi s case by a seri es of pi stons.




Intro

The starti ng poi nt for most thermodynami c consi derati ons are the
l aws of thermodynami cs
, whi ch postul ate that
energy

can be
exchanged between physi cal systems as heat or
work
.
[6]

They al so post
ul ate the exi stence of a quanti ty named
entropy
, whi ch
can be defi ned for any i sol ated system that i s i n
thermodynami c equi l i brium
.
[7]

In thermodynami cs, i nteracti ons between l arge
ensembl es of objects are studi ed and categori zed. Central to thi s are the concepts of
system

and
surroundings
. A system i s
composed of parti cl es, whose average moti ons defi ne i ts properti es, whi ch i n turn are rel ated to one another through
equati ons
of state
. Properti es can be c
ombi ned to express
i nternal energy

and
thermodynami c potenti al s
, whi ch are

useful for determi ni ng
condi ti ons for
equi l i brium

and
spontaneous processes
.

The Four Laws

In thermodynami cs, there are four l aws that do not depend on the detai l s of the systems under study or how they i nteract.
Hence these l aws are very general l y val i d, can be appl i ed to systems about whi ch one knows nothi ng other than the bal ance of
energy an
d matter transfer. Exampl es of such systems i ncl ude
Ei nstei n
's predi cti on, around the turn of the 20th century, of
spontaneous emi ssi on
, and ongoi ng research i nto the thermodynami cs of
bl ack hol es

These four l aws are:



Zeroth l aw of thermodynami cs
, about
thermal equi l i brium
:

If

two
thermodynami c systems

are separatel y i n thermal equi l i bri um wi th a thi rd, they are al so i n thermal equi l i brium
wi th each other.

If we grant that al l systems a
re (tri vi ally) i n thermal equi l i brium wi th themsel ves, the Zeroth l aw i mpl i es that thermal
equi l i bri um i s an
equi valence rel ati on

on the set of
thermodynami c systems
. Thi s l aw i s taci tl y assumed i n every
measurement of temperature. Thus, i f we want to know i f two bodi es are at the same
temperature
, i t i s not necessary
to bri ng them i nto contact and to watch whether thei r observabl e properti es change wi th ti me.
[15]




Fi rst l aw of thermodynami cs
, about the
conservati on of energy
:

The

change i n the
i nternal energy

of a cl osed
thermodynami c system

i s equal to the su
m of the amount of
heat

energy
suppl i ed to or removed from the system and the
work

done on or by the system or we can say " In an i sol ated system
the heat i s constant".



Second l aw of thermodynami cs
, about
entropy
:

The total entropy of any i sol ated thermodynami c system al ways i ncreases over ti me, approachi ng a maxi mum val ue or
we can say " i n an i sol ated system, the entropy never decreases".



Thi rd l aw of thermodynami cs
, about the
absol ute zero

of
temperature
:

As a system
asymptoti cal ly

approaches absolute zero of temperature al l processes vi rtually cease and the entropy of
the system
asymptoti cal l y approaches a mi ni mum val ue; al so stated as: "the entropy of al l systems and of al l states of a
system i s zero at absol ute zero" or equi val entl y "i t i s i mpossible to reach the absol ute zero of temperature by any fi ni te
number of processes".


Potentials

As can be deri ved from the energy bal ance equati on (or Burks' equati on) on a thermodynami c system there exi st energeti c
quanti ti es cal led
thermodynami c potenti al s
, bei ng the quanti tati ve measure of the stored energy i n the system. The fi ve most
wel l known potenti al s are:

System models



An i mportant concept i n thermodynami cs i s the
“system”. Everythi ng i n
the uni verse except the system i s known as surroundi ngs. A system i s the regi on of the uni verse under study. A system i s
separated from the remai nder of the uni verse by a
boundary

whi ch may be i magi nary or not, but whi ch by conventi on del i mi ts a
fi ni te vol ume. The possi bl e exchanges of
work
,
heat
, or
matter

between the system and the surroundi ngs take pl ace across thi s
boundary
. Boundari es are of four types: fi xed, moveabl e, real, and i magi nary.

Basi cally, the “boundary” i s si mpl y an i magi nary dotted l i ne drawn around a vol ume of
something

when there i s goi ng to be a
change i n the
i nternal energy

of that
something
. Anythi ng that passes across the boundary that effects a change i n the i nternal
energy of the
something

needs to be accounted for i n the energ
y bal ance equati on. That
something

can be the vol umetri c
regi on surroundi ng a si ngl e atom resonati ng energy, such as
Max Pl anck

defi ned i n 1900; i t can be a body of steam or ai r i n a
steam engi ne
, such as
Sadi Carnot

defi ned i n 1824; i t can be the body
of a
tropi cal cycl one
, such as
Kerry Emanuel

theori zed i n
1986 i n the fi el d of
atmospheri c thermodynami cs
; i t coul d al so be just one
nucl i de

(i.e. a system of
quarks
) as some are
theori zi ng presentl y i n
quantum thermodynami cs
.

For an engi ne, a fi xed boundary means the pi ston i s

l ocked at i ts posi ti on; as such, a constant vol ume process occurs. In that
same engi ne, a moveabl e boundary al l ows the pi ston to move i n and out. For cl osed systems, boundari es are real whi l e for open

system boundari es are often i magi nary. There are fi ve
domi nant cl asses of systems:

1.

Isolated Systems



matter and energy may not cross the boundary

2.

Adiabatic Systems



heat must not cross the boundary

3.

Diathermic Systems

-

heat may cross boundary

4.

Closed Systems



matter may not cross the boundary

5.

Open
Systems



heat, work, and matter may cross the boundary (often cal l ed a
control vol ume

i n thi s case)

As ti me passes i n an i solated system, i nternal di fferences i n the system t
end to even out and pressures and temperatures tend
to equal i ze, as do densi ty di fferences. A system i n whi ch al l equal i zing processes have gone practi cally to compl eti on, i s
consi dered to be i n a
state

of
thermodynami c equi l i bri um
.

In thermodynami c equi l i brium, a system's properti es are, by defi ni ti on, unchangi ng
i n ti me. Systems i n equi l i brium are much
si mpl er and easi er to understand than systems whi ch are not i n equi l i bri um. Often, when anal ysi ng a thermodynami c process, i t

can be assumed that each i ntermedi ate state i n the process i s at equi l i brium. Thi s wi l l a
lso considerably si mplify the si tuati on.
Thermodynami c processes whi ch devel op so sl owl y as to al l ow each i ntermedi ate step to be an equi l i bri um state are sai d to be
reversi bl e processes
.


Internal energy


Hel mhol tz free energy


Enthal py


Gi bbs free energy


Grand potenti al


Conjugate variables

The central concept of thermodynami cs i s that of
energy
, the abi l ity to do
work
. By the
Fi rst Law
, the total energy of a system
and i ts surroundi ngs i s conserved. Ener
gy may be transferred i nto a system by heati ng, compressi on, or addi ti on of matter, and
extracted from a system by cool i ng, expansi on, or extracti on of matter. In
mechani cs
, for exampl e,

energy transfer equal s the
product of the force appl i ed to a body and the resul ti ng di spl acement.

Conjugate vari abl es

are pai rs of
thermodynami c concepts, wi th the fi rst bei ng aki n to a "force" appl i ed to some
thermodynami c
system
, the second bei ng aki n to the resul ti ng "di spl acement," and the
product of the two equal l i ng the amount of energy
transferred. The common conjugate vari ables are:



Pressure
-
vol ume

(the
mechani cal

parameters);



Temperature
-
entropy

(th
ermal parameters);



Chemi cal potenti al
-
parti cle number

(materi al parameters).

Instrumentation

There are two types of thermodynami c i nstruments, the
meter

and the
reservoir
. A thermodynami c meter i s any devi ce whi ch
measures any parameter of a
thermodynami c system
. In some cases, the thermodynami c parameter i s actual ly defi ned i n terms
of an i deal i zed measuri ng i nstrument. For exampl e, the
zeroth l aw

states that i f two bodi es are i n thermal equi l i bri um wi th a
thi rd body, they are al so i n thermal equi l i brium wi th each other. Thi s pri nci pl e, as noted by
James Maxwel l

i n 1872, asserts that
i t i s possible to measure temperature. An i deal i zed
thermometer

i s a sampl e of an i deal gas at constant pressure. From the
i deal
gas l aw

PV=nRT
, the vol ume of such a sampl e can be used as an i ndi cator of temperature; i n thi s manner i t defi nes temperature.
Al though pressure i s defi ned mechani cal l y, a pressure
-
measuring dev
i ce, cal l ed a
barometer

may al so be constructed from a
sampl e of an i deal gas hel d at a constant temperature. A
cal ori meter

i s a devi ce whi ch i s used to measure and defi ne the
i nternal energy of a system.

A thermodynami c reservoi r i s a system whi ch i s so l arge that i t does not appreci abl y alter i ts state parameters when brought
i nto contact wi th the test system. It
i s used to i mpose a parti cul ar value of a state parameter upon the system. For exampl e, a
pressure reservoi r i s a system at a parti cul ar pressure, whi ch i mposes that pressure upon any test system that i t i s mechani c
ally
connected to. The Earth's atmosphere

i s often used as a pressure reservoi r.

It i s i mportant that these two types of i nstruments are di sti nct. A meter does not perform i ts task accuratel y i f i t behaves
l i ke a
reservoi r of the state vari abl e i t i s tryi ng to measure. If, for exampl e, a
thermometer were to act as a temperature reservoi r i t
woul d al ter the temperature of the system bei ng measured, and the readi ng woul d be i ncorrect. Ideal meters have no effect on
the state vari abl es of the system they are measuri ng.

States & processes

When a system i s at equi l i bri um under a gi ven set of condi ti ons, i t i s said to be i n a defi ni te
state
. The
thermodynamic state

of
the system can be descri bed by a number of
i ntensi ve vari ables

and
extensi ve vari abl es
. The properti es of the system can be
descri bed by an
equati on of state

whi ch speci fi es the rel ati onshi p between these vari ables. State may be thought of as the
i nstantaneous quanti tati ve descri pti on of a system wi th a set number of vari abl es hel d constant.

A
thermodynamic process

may be d
efi ned as the energeti c evol uti on of a thermodynami c system proceedi ng from an i ni ti al
state to a fi nal state. Typi cal l y, each thermodynami c process i s di sti nguished from other processes, i n energeti c character,
accordi ng to what parameters, as temperature
, pressure, or vol ume, etc., are hel d fi xed. Furthermore, i t i s useful to group these
processes i nto pai rs, i n whi ch each vari able hel d constant i s one member of a
conjugate

pai r. The seven most common
thermodynami c processes are shown bel ow:

1.

An
i sobaric process

occurs at constant
pressure
.

2.

An
i sochoric process
, or
isometric/isovolumetric process
, occurs at constant
vol ume
.

3.

An
i sothermal process

occurs at a constant
temperature
.

4.

An
adi abati c process

occurs wi thout l oss or gai n of energy by
heat
.

5.

An
i sentropi c process

(reversi ble adi abatic process) occurs at a constant
entropy
.

6.

An
i senthal pi c process

occurs at a constant
enthal py
.

7.

A
steady state

process occ
urs wi thout a change i n the
i nternal energy

of a
system
.