Let us consider a system of gas contained in a cylinder. The system
initially is in equilibrium state, represented by the properties P1, V1, T1.
The weight on the piston just balances the upward force exerted by
the gas. If the weights are removed one by one very slowly from the
top of the piston, at any instant of the upward travel of the piston,
considering gas system is isolated, every state passed through by the
system will be an equilibrium state. Such a process, which is locus of
all the equilibrium points passed through by the system, is known as
quasi-static or quasi-equilibrium process. A quasi-static process is thus
a succession of equilibrium states and represented by a continuous
line.
If the weight is single and is removed, there will be an unbalanced force
between the system and the surroundings, and under gas pressure, the
piston will move up till it hits the stops. The system again comes to an
equilibrium state, being described by the properties P2, V2, T2. Thus, the
intermediate states passed through by the system are non-
equilibrium states which cannot be described by thermodynamic
properties. such process is called non quasi-equilibrium process or
irreversible process denoted by a dashed line between initial and final
states.
When a process or processes are performed on a system in such a way
that the final state is identical with the initial state, it is the known as a
thermodynamic cycle or cyclic process. In Figure, 1 –A –2 and 2 – B – 1
are processes whereas 1 –A –2 – B – 1 is a thermodynamic cycle.
----> Some Common Properties:
a) Pressure:
It is defined as the normal force exerted per unit area.
With the restriction that area over which the force is applied must not smaller than some
minimum value a , the mathematical definition of the local pressure is vast.
The actual pressure at a given position is called absolute pressure and it is measured relative
to absolute vacuum i.e. absolute zero pressure. The difference between absolute pressure
and local atmospheric pressure is called gauge pressure.
Pressure Variation with depth
For fluids whose density changes significantly with elevation, a
relation for the variation of pressure with elevation can be
obtained by
-
The negative sign is due to our taking the positive z direction to be
upward so that dP is negative when dz is positive since pressure
decreases in an upward direction. When the variation of density
with elevation is known, the pressure difference between points 1
and 2 can be determined by integrating.
Manometer
A manometer is a device to measure the pressure difference
based on the principle of variation of pressure with depth.
! -
"
where, is the density of the manometer liquid and L is the
pressure head.
b) Specific Volume:
The specific volume (v) of a substance is defined as the volume
per unit mass and is measured in m3
/kg. Thus, mathematical
definition of specific volume is
v= V/m
Hence, specific volume is the reciprocal of the density, i.e.,
V= 1/row
c) Temperature
It is an intensive thermodynamic property, which determines the degree of hotness or the
level of heat intensity of a body. A body is said to be at a high temperature or hot, if it shows
high level of heat intensity in it. Similarly, a body is said to be at a low temperature or cold, if
it shows a low level of heat intensity.
The instrument used to measure the temperature is called thermometer. For most
Temperature Scales:
Temperature Scales enable us to use a common basic for temperature measurements. All
temperature scales are based on some easily reproducible states such as the freezing and
boiling point of water.
The two scales (Kelvin and Rankin scale) most useful in thermodynamics are so called
absolutes scales. The absolute scale for SI is the Kelvin scale. This scale is a one point scale
based on the second law of thermodynamics. The single point is the triple point of water,
where ice, liquid water, and water vapor coexist in a closed system in the absence of air.
Rankin scale is related to Kelvin scale by
1.8o
R=1K
Two other commonly used scales are the Fahrenheit scale and the Celsius scale.
----> Equality of Temperature and Zeroth Law of thermodynamics
The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a
third body, they are also in thermal equilibrium with each other. It serves as a basis for the
validity of temperature measurement.
PROPERTIES OF COMMON SUBSTANCE
3.1 Pure Substance and State Postulate
Pure implies substances composed of a single chemical species. Hence a pure substance is a
system which is
(a) homogeneous in chemical composition,
(b) homogeneous in chemical aggregation, and
(c) invariable in chemical composition.
Homogeneous in chemical composition means that the composition of each part of the system
is the same as the compositions of every other part. Composition means relative proportion of
the chemical elements into which the sample can be analyzed. It does not matter how these
elements are combine.
In Figure 3.1 for example, system (i), comprising steam and water, is homogeneous in
composition, since chemical analysis would reveal that hydrogen and oxygen atoms are
present in the ratio 2:1 whether the sample be taken from the steam or from the water. The
same is true for system (ii), containing uncombined hydrogen and oxygen gas in the atomic
ratio 2:1 in the upper part, and water in the lower part. System (iii) however is not
homogeneous in composition; for the hydrogen and oxygen are present in the ratio 1:1 in the
upper part, but in the ratio 2:1 (as water) in the lower part.
Homogeneous in chemical compostion means that the chemical elements must be combined
chemically in the same way in all parts of the system. Consideration of Figure 3.1 shows that
system (i) satisfies this condition also; for steam and water consists of identical
molecules. System (ii) on the other hand is not homogeneous in chemical aggregation
since, in the upper part of the system, the hydrogen and oxygen are not combined
chemically (individual atoms of H and O are not uniquely associated), whereas in the lower part
of the system the he hydrogen and oxygen are combined in the form of water.
Invariable chemical aggregation means that the state of the chemical combination of the
system does not change with time. Thus a mixture of hydrogen and oxygen, which changed
into steam during the time that the system was under consideration, would not be a pure substance.
initially is in equilibrium state, represented by the properties P1, V1, T1.
The weight on the piston just balances the upward force exerted by
the gas. If the weights are removed one by one very slowly from the
top of the piston, at any instant of the upward travel of the piston,
considering gas system is isolated, every state passed through by the
system will be an equilibrium state. Such a process, which is locus of
all the equilibrium points passed through by the system, is known as
quasi-static or quasi-equilibrium process. A quasi-static process is thus
a succession of equilibrium states and represented by a continuous
line.
If the weight is single and is removed, there will be an unbalanced force
between the system and the surroundings, and under gas pressure, the
piston will move up till it hits the stops. The system again comes to an
equilibrium state, being described by the properties P2, V2, T2. Thus, the
intermediate states passed through by the system are non-
equilibrium states which cannot be described by thermodynamic
properties. such process is called non quasi-equilibrium process or
irreversible process denoted by a dashed line between initial and final
states.
When a process or processes are performed on a system in such a way
that the final state is identical with the initial state, it is the known as a
thermodynamic cycle or cyclic process. In Figure, 1 –A –2 and 2 – B – 1
are processes whereas 1 –A –2 – B – 1 is a thermodynamic cycle.
----> Some Common Properties:
a) Pressure:
It is defined as the normal force exerted per unit area.
With the restriction that area over which the force is applied must not smaller than some
minimum value a , the mathematical definition of the local pressure is vast.
The actual pressure at a given position is called absolute pressure and it is measured relative
to absolute vacuum i.e. absolute zero pressure. The difference between absolute pressure
and local atmospheric pressure is called gauge pressure.
Pressure Variation with depth
For fluids whose density changes significantly with elevation, a
relation for the variation of pressure with elevation can be
obtained by
-
The negative sign is due to our taking the positive z direction to be
upward so that dP is negative when dz is positive since pressure
decreases in an upward direction. When the variation of density
with elevation is known, the pressure difference between points 1
and 2 can be determined by integrating.
Manometer
A manometer is a device to measure the pressure difference
based on the principle of variation of pressure with depth.
! -
"
where, is the density of the manometer liquid and L is the
pressure head.
b) Specific Volume:
The specific volume (v) of a substance is defined as the volume
per unit mass and is measured in m3
/kg. Thus, mathematical
definition of specific volume is
v= V/m
Hence, specific volume is the reciprocal of the density, i.e.,
V= 1/row
c) Temperature
It is an intensive thermodynamic property, which determines the degree of hotness or the
level of heat intensity of a body. A body is said to be at a high temperature or hot, if it shows
high level of heat intensity in it. Similarly, a body is said to be at a low temperature or cold, if
it shows a low level of heat intensity.
The instrument used to measure the temperature is called thermometer. For most
Temperature Scales:
Temperature Scales enable us to use a common basic for temperature measurements. All
temperature scales are based on some easily reproducible states such as the freezing and
boiling point of water.
The two scales (Kelvin and Rankin scale) most useful in thermodynamics are so called
absolutes scales. The absolute scale for SI is the Kelvin scale. This scale is a one point scale
based on the second law of thermodynamics. The single point is the triple point of water,
where ice, liquid water, and water vapor coexist in a closed system in the absence of air.
Rankin scale is related to Kelvin scale by
1.8o
R=1K
Two other commonly used scales are the Fahrenheit scale and the Celsius scale.
----> Equality of Temperature and Zeroth Law of thermodynamics
The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a
third body, they are also in thermal equilibrium with each other. It serves as a basis for the
validity of temperature measurement.
PROPERTIES OF COMMON SUBSTANCE
3.1 Pure Substance and State Postulate
Pure implies substances composed of a single chemical species. Hence a pure substance is a
system which is
(a) homogeneous in chemical composition,
(b) homogeneous in chemical aggregation, and
(c) invariable in chemical composition.
Homogeneous in chemical composition means that the composition of each part of the system
is the same as the compositions of every other part. Composition means relative proportion of
the chemical elements into which the sample can be analyzed. It does not matter how these
elements are combine.
In Figure 3.1 for example, system (i), comprising steam and water, is homogeneous in
composition, since chemical analysis would reveal that hydrogen and oxygen atoms are
present in the ratio 2:1 whether the sample be taken from the steam or from the water. The
same is true for system (ii), containing uncombined hydrogen and oxygen gas in the atomic
ratio 2:1 in the upper part, and water in the lower part. System (iii) however is not
homogeneous in composition; for the hydrogen and oxygen are present in the ratio 1:1 in the
upper part, but in the ratio 2:1 (as water) in the lower part.
Homogeneous in chemical compostion means that the chemical elements must be combined
chemically in the same way in all parts of the system. Consideration of Figure 3.1 shows that
system (i) satisfies this condition also; for steam and water consists of identical
molecules. System (ii) on the other hand is not homogeneous in chemical aggregation
since, in the upper part of the system, the hydrogen and oxygen are not combined
chemically (individual atoms of H and O are not uniquely associated), whereas in the lower part
of the system the he hydrogen and oxygen are combined in the form of water.
Invariable chemical aggregation means that the state of the chemical combination of the
system does not change with time. Thus a mixture of hydrogen and oxygen, which changed
into steam during the time that the system was under consideration, would not be a pure substance.