# Some Basic Concepts and Definitions of Thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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Some Basic Concepts and Definitions of Thermodynamics

System, Surroundings, and Boundary

That part of the physical world, be it a quantity of matter or a region in space, whose
behavior
is the subject of study is called a system.
Any other part of the univ
erse that can interact
with the system is its surroundings. The system is separated from its surroundings by its boundary.

Properties and States

A thermodynamic system is characterized by
quantities

known
as
thermodynamic properties.
To describe a system,
we give the values of its properties. These properties

fall into two general
categories: the extensive and the intensive. Extensive properties

are directly proportional to the
amount of matter in the system. They are additive. Intensive properties are
inde
pendent

of the
quantity of matter in the system. They are not additive.

A system is said to be at a given state when all its
properties are fixed. A system is

said to
have undergone a process when it changes from one state to another . A cyclic process, or

simply a
cycle, is said to have occurred when the initial

state and the final state are i
denti
c
al.

Thermodynamic Equilibrium, Thermal Equilibrium, and Temperature

Thermodynamic

equilibrium is a primitive concept. It may be defined as that state of a
syste
m in which all its properties are no longer time dependent. In principle, this is possible only
when a system is prevented from having any interaction with its surroundings.
Thermodynamic

equilibrium may also be expressed in terms
of
a
tendency

to spontane
ous change. A system will
have been in thermodynamic
equilibriums

if it spontaneously returns to its original state after
being subjected to a small disturbance.

Thermal equilibrium is

a

necessary
condition

for thermodynamic equilibrium. From this
concept,

we deduce the existence of the property

temperature,

which is unique to
thermodynamics. When two bodies are in contact with each other through a heat conducting wall,
thermal equilibrium can exist only when there is equality of temperature; otherwise, t
here will be
thermal interaction.

Heat Reservoir and Work Reservoir

A heat reservoir is any object or system that can serve as a heat sink or heat source for
another system
. It is defined as a system the temperature of which remains constant when
subjected

to a heat interaction. This is possible only if the energy capacity of the heat reservoir is
very, very large compared to the finite amount of energy crossing
its

boundary
, Furthermore, this
definition implies that a heat
reservoir

must
remain

in a give
n

equilibrium state at all times. That
is ,from consideration of the second law of thermodynamics, all processes
occurring

in a heat
reservoir are reversible process
es.
.

To keep track of the amount of work produced by a thermodynamic system, we introduce the

concept of a work reservoir defined as an energy
-
storage system in which every unit of energy
crossing its boundary is energy that can do work for us.

Thermodynamic

Processes

When a collection of matter experiences a change from one equilibrium state to a
nother
equilibrium state, it is said to have
undergone

a process. Since an
engineering

device or system in
thermodynamics is simply a creation of man making use of various kinds of processes for the
purpose

of carrying out controlled interaction among ener
gy and matter, it is essential that we
develop a logical structure an
d

methodology for the evaluation of changes in the equilibrium states
of matter. A necessary part of this methodology is that we must be able to recognize what process
actually takes plac
e.

We shall encounter

many processes in our
course

of study. The special features of certain

process
es

may be recognized from the names given to them. For examples, an isothermal process
is a constant temperature process, an isobaric process is
a
constant
temperature
process, an
iso
baric

process is a constant
-
pressure,and an isometric process is aconstant
-
volume

process. On
the other hand, the significance of some processes may be recognized only if we fully understand
the definitions involved. Examples of

this kind are adiabatic process, cyclic pr
ocess, quasi
-
static
process, an
d reversible process.

If a process is carried out in such a manner that at every instant the system departs only
infinitesimally from an equilibrium state, the process
is called quas
i
-
static(sometime
s

called
quasi
-
equilibrium).
For such a process, the path followed by the system may b
e represented by a
succession of

equilibrium states
. If there are finite departures from equilibrium, the process is
non
-
quasi
-
static. A quasi
-
static pro
cess is reversible, or more correctly, internally reversible.

A quasi
-
static process is an ideal process. It is approximately realized by making the change
very slowly. All

real

processes are not quasi
-
static because they take place with finite differences

of pressure, temperature, and so on, between
system

and surroundings.

A process is reversible if , after it has been carried out, it is possible by any means
whatsoever to restore the
system

and the surroundings involved in the interaction to exactly the
same states they were in before the process. This implies that, if a process is reversible
, it is
possible to undo it in such a manner that there will be no trace anywhere of the fact that the
process
occurred
. Thus

a reversible process must be internally
reversible
as well as externally
reversible. It is perfection from the thermodynamic point of view. More elaborate discussion of
this ideal process will be given in connection with our study of the property entropy. Real
processes are all irreversible proc
esses, but some are less irreversible than others. An important
part of our study of
engineering

thermodynamics
is to recognize the factors that contribute to
irreversibility
so that we may select or create the best possible processes for a given problem.

Irreversibility is encountered when there is friction of any kind, be it mechanical friction,
fluid friction, or electrical resistance. Frictional effects are known as dissipative effects in which
the work
-
producing ability of the system and surrounding i
nvolved has been de
creased due to the
irreversible process. We shall see that the effect of irreversibility may be quantified through the
use of the
property

entropy.

Word
s

and Expressions

1.

thermodynamics n.

2.

behavior n.

3.

surroundings(pl)n.

，外界

4.

property n.

5.

characterize v.

6.

category n.

7.

identical a.

8.

primitive a.n.

9.

spontaneous a.

10.

disturbance n.

11.

unique a.

12.

heat
reservo
ir n.

13.

work reservoir n.

14.

serve as.

15.

imply vt.

16.

reversible process

17.

track v.n.

18.

a collection of

19.

undergo vt.

20.

logical a.

21.

methodology n.

22.

recognize v.

23.

isothermal a.

24.

isobaric a.

25.

isometric a.

26.

only if

27.

28.

quasi
-
static a.

29.

30.

succession n.

31.

whatsoe
ver=whatever pron.a.

32.

restore v.

33.

undo vt.

34.

trace n.

35.

elaborate a.v.

36.

entropy n.

37.

irreversibility n.

38.

dissipative a.