, Thermodynamics
[From Basic to Advance]
Introduction to Thermodynamics
1. Basic Idea about a Thermodynamic System:
A thermodynamic system is a certain portion of the universe selected (under interest) for
the purpose of investigation and is thus distinct, is essentially macroscopic.
The system may be a gas such as air, a vapour such as steam, a vapour in contact with its
liquid such as liquid ammonia and ammonia vapour, a mixture such as air and gasoline
vapour etc. In addition to these systems relevant to engineering in particular, there may
be such thermodynamic systems as a stretched wire, electric capacitors, thermocouples,
magnetic materials, surface films and electric cells which are of more relevance in physics.
Plainly, a thermodynamic system is perceptible by our senses.
A system may also be homogeneous or heterogeneous where each component can exist in
different phases. A gas enclosed in a cylinder fitted with a friction-less gas-tight piston is a
simple homogeneous system, but a phenol-water mixture is an example of a complex
heterogeneous system. Whatever be the system, it is always finite.
2. Concept of Surroundings with respect to System and Boundary of the
System:
By surroundings of a system is the remaining part of the universe. It is meant everything
outside it that can influence the behavior of
the system. The boundary of a system is the
envelope that encloses the system and
separates it from its surroundings.
The boundary of a system may or may not
allow it to interact with its surroundings. The
boundary that does not allow any exchange of
matter and energy between the system and its
surroundings is called an isolating boundary. A system bounded by an isolating boundary
is termed an isolated system and is not important thermodynamically.
,A system with a boundary that permits exchange of matter and energy between the
system and the surroundings is called an open system. The system, however, is said to be
a closed system if its boundary allows exchange of energy, but prevents exchange of
matter between the system and the
surroundings. A closed system, however,
is not an isolated system.
The exchange of energy between the
system and the surroundings may take
place either thermally or by doing work
on the system. A system is said to be
dynamically insulated if any exchange of
energy in the form of work between the
system and the surrounding is impossible. The surface or boundary that prevents thermal
interaction with surroundings is called adiabatic, and the system is thermally isolated. If,
however, heat exchange can occur through the boundary, it is said to be diathermic. A
system with a diathermic boundary will necessarily be in thermal contact with the
surroundings.
3. State of a System and Thermodynamic Variables: Extensive and Intensive
Variables:
In thermodynamics, the state of a system at any instant is represented by its condition at
that instant, the condition being completely specified by a set of measurable quantities,
called variables of state or thermodynamic variables. For example, to a stretched wire
correspond the thermodynamic variables tension and length; for a surface film the
variables are the surface tension
and the area; for a capacitor
they are e.m.f and the charge;
for a hydrostatic system the
variables are pressure and
volume; and so on. The
thermodynamic variables are
also termed thermodynamic
coordinates. A particularly
simple condition of state of a
system is the equilibrium state
in which the variables specifying
the state are time-independent, that is, do not change with time and are reproducible.
The thermodynamic variables may be of two different kinds: intensive and extensive.
Intensive variables of a system in a given state are those which hare independent of its
, mass or the number of particles. Extensive variables, however, are proportional to the
mass or, to the number of particles in the system. Tension, surface tension, e.m.f. etc. are
intensive variables; length, area, charge etc. are extensive variables. Pressure and
temperature are intensive, while volume is an extensive variable.
More clearly, suppose 𝐲 be a macroscopic parameter characterizing the state of a
homogeneous system and if the system be divided, by a partition, into two parts or sub-
systems having 𝐲 −values 𝐲𝟏 𝐚𝐧𝐝 𝐲𝟐 respectively, then
(𝐢) 𝐲 𝐢𝐬 𝐞𝐱𝐭𝐞𝐧𝐬𝐢𝐯𝐞, 𝐢𝐟 𝐲 = 𝐲𝟏 + 𝐲𝟐 , 𝐚𝐧𝐝 (𝐢𝐢) 𝐲 𝐢𝐬 𝐢𝐧𝐭𝐞𝐧𝐬𝐢𝐯𝐞, 𝐢𝐟 𝐲 = 𝐲𝟏 = 𝐲𝟐 .
𝐲 𝛛𝐲
Again if 𝐱, 𝐲 be two arbitrary intensive variables, then 𝐱𝐲, 𝐱, , 𝐱 + 𝐲 - all are also
𝛛𝐱
𝐘
intensive. If 𝐗, 𝐘 are two arbitrary extensive variables, then 𝐗 + 𝐘 is also extensive, but 𝐗,
𝛛𝐘
are intensive.
𝛛𝐗
𝐗 𝛛𝐗
Similarly if 𝐱 is intensive and 𝐗 extensive then 𝐱𝐗, , will be extensive. Extensive
𝐱 𝛛𝐱
variables when referred to their specific values, that is; the value per unit mass of the
system are known as specific variables. Interestingly, a specific variable corresponding to
any extensive variable becomes, by definition, mass-independent and is thus an intensive
variable.
4. Thermodynamic Equilibrium:
An absence of unbalanced force in the interior of a system, or between the system and
the surroundings, implies mechanical equilibrium. The absence of any spontaneous
change of internal structure (by way of diffusion or chemical reaction or both) in a system
in mechanical equilibrium implies chemical equilibrium.
For a system in mechanical and
chemical equilibrium, thermal
equilibrium is said to be attained
when there is no spontaneous
change in the coordinates of the
system when it is separated from
the surroundings by a diathermic
wall. It is obvious that in thermal
equilibrium, the temperature is the
same all throughout the system
and is identical with that of the
surroundings.
When all the conditions stated above are fulfilled by a system, it is said to be in
thermodynamic equilibrium and there will be no proneness of either the system or the
[From Basic to Advance]
Introduction to Thermodynamics
1. Basic Idea about a Thermodynamic System:
A thermodynamic system is a certain portion of the universe selected (under interest) for
the purpose of investigation and is thus distinct, is essentially macroscopic.
The system may be a gas such as air, a vapour such as steam, a vapour in contact with its
liquid such as liquid ammonia and ammonia vapour, a mixture such as air and gasoline
vapour etc. In addition to these systems relevant to engineering in particular, there may
be such thermodynamic systems as a stretched wire, electric capacitors, thermocouples,
magnetic materials, surface films and electric cells which are of more relevance in physics.
Plainly, a thermodynamic system is perceptible by our senses.
A system may also be homogeneous or heterogeneous where each component can exist in
different phases. A gas enclosed in a cylinder fitted with a friction-less gas-tight piston is a
simple homogeneous system, but a phenol-water mixture is an example of a complex
heterogeneous system. Whatever be the system, it is always finite.
2. Concept of Surroundings with respect to System and Boundary of the
System:
By surroundings of a system is the remaining part of the universe. It is meant everything
outside it that can influence the behavior of
the system. The boundary of a system is the
envelope that encloses the system and
separates it from its surroundings.
The boundary of a system may or may not
allow it to interact with its surroundings. The
boundary that does not allow any exchange of
matter and energy between the system and its
surroundings is called an isolating boundary. A system bounded by an isolating boundary
is termed an isolated system and is not important thermodynamically.
,A system with a boundary that permits exchange of matter and energy between the
system and the surroundings is called an open system. The system, however, is said to be
a closed system if its boundary allows exchange of energy, but prevents exchange of
matter between the system and the
surroundings. A closed system, however,
is not an isolated system.
The exchange of energy between the
system and the surroundings may take
place either thermally or by doing work
on the system. A system is said to be
dynamically insulated if any exchange of
energy in the form of work between the
system and the surrounding is impossible. The surface or boundary that prevents thermal
interaction with surroundings is called adiabatic, and the system is thermally isolated. If,
however, heat exchange can occur through the boundary, it is said to be diathermic. A
system with a diathermic boundary will necessarily be in thermal contact with the
surroundings.
3. State of a System and Thermodynamic Variables: Extensive and Intensive
Variables:
In thermodynamics, the state of a system at any instant is represented by its condition at
that instant, the condition being completely specified by a set of measurable quantities,
called variables of state or thermodynamic variables. For example, to a stretched wire
correspond the thermodynamic variables tension and length; for a surface film the
variables are the surface tension
and the area; for a capacitor
they are e.m.f and the charge;
for a hydrostatic system the
variables are pressure and
volume; and so on. The
thermodynamic variables are
also termed thermodynamic
coordinates. A particularly
simple condition of state of a
system is the equilibrium state
in which the variables specifying
the state are time-independent, that is, do not change with time and are reproducible.
The thermodynamic variables may be of two different kinds: intensive and extensive.
Intensive variables of a system in a given state are those which hare independent of its
, mass or the number of particles. Extensive variables, however, are proportional to the
mass or, to the number of particles in the system. Tension, surface tension, e.m.f. etc. are
intensive variables; length, area, charge etc. are extensive variables. Pressure and
temperature are intensive, while volume is an extensive variable.
More clearly, suppose 𝐲 be a macroscopic parameter characterizing the state of a
homogeneous system and if the system be divided, by a partition, into two parts or sub-
systems having 𝐲 −values 𝐲𝟏 𝐚𝐧𝐝 𝐲𝟐 respectively, then
(𝐢) 𝐲 𝐢𝐬 𝐞𝐱𝐭𝐞𝐧𝐬𝐢𝐯𝐞, 𝐢𝐟 𝐲 = 𝐲𝟏 + 𝐲𝟐 , 𝐚𝐧𝐝 (𝐢𝐢) 𝐲 𝐢𝐬 𝐢𝐧𝐭𝐞𝐧𝐬𝐢𝐯𝐞, 𝐢𝐟 𝐲 = 𝐲𝟏 = 𝐲𝟐 .
𝐲 𝛛𝐲
Again if 𝐱, 𝐲 be two arbitrary intensive variables, then 𝐱𝐲, 𝐱, , 𝐱 + 𝐲 - all are also
𝛛𝐱
𝐘
intensive. If 𝐗, 𝐘 are two arbitrary extensive variables, then 𝐗 + 𝐘 is also extensive, but 𝐗,
𝛛𝐘
are intensive.
𝛛𝐗
𝐗 𝛛𝐗
Similarly if 𝐱 is intensive and 𝐗 extensive then 𝐱𝐗, , will be extensive. Extensive
𝐱 𝛛𝐱
variables when referred to their specific values, that is; the value per unit mass of the
system are known as specific variables. Interestingly, a specific variable corresponding to
any extensive variable becomes, by definition, mass-independent and is thus an intensive
variable.
4. Thermodynamic Equilibrium:
An absence of unbalanced force in the interior of a system, or between the system and
the surroundings, implies mechanical equilibrium. The absence of any spontaneous
change of internal structure (by way of diffusion or chemical reaction or both) in a system
in mechanical equilibrium implies chemical equilibrium.
For a system in mechanical and
chemical equilibrium, thermal
equilibrium is said to be attained
when there is no spontaneous
change in the coordinates of the
system when it is separated from
the surroundings by a diathermic
wall. It is obvious that in thermal
equilibrium, the temperature is the
same all throughout the system
and is identical with that of the
surroundings.
When all the conditions stated above are fulfilled by a system, it is said to be in
thermodynamic equilibrium and there will be no proneness of either the system or the
