Module No. 33

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Module # 33
Thermodynamic Processes
Thermodynamics
It deals with the conversion of heat energy into mechanical
energy and vice versa. Thus, thermodynamics is the branch of
science that deals with the relation between heat and mechanical
energy. It is mainly concerned with the transformation of heat into
mechanical work and vice versa. The structure of
thermodynamics rests upon two simple laws known as the First
and Second Laws of Thermodynamics.
Alternatively, the field of Science, which deals with energies
possessed by gases and vapors, their conversion in terms of heat
and work, and their relationship with properties of system, is
called thermodynamics.
Automobile technology is based on the principle of
thermodynamics.
First Law of Thermodynamics
The first law of thermodynamics is that,
Change in internal energy of any system = Heat in flow + Work
done on the system.

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The heat and mechanical work are inter-convertible. According to
this law a definite amount of mechanical work is needed to
produce a definite amount of heat and vice versa.
If W is the amount of mechanical work converted from heat
energy Q, then
W  Q
OR
W = JQ
Where J is constant and is called Joule's mechanical equivalent
of heat. It is defined as the amount of work done to produce unit
quantity of heat.
Second Law of Thermodynamics
This law states, there is a definite limit to the amount of
mechanical energy, which can be obtained from a given quantity
of hat energy.
This law of thermodynamics has been enunciated by clausius in a
lightly different form as "it is impossible for a self-acting machine
working in a cyclic process, to transfer heat from a body at a
lower temperature to a body at a higher temperature without the
aid of on external agency”. Or, in other words, the heat cannot
flow itself from a cold body to hot body without the help of an

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external agency.
This law has also been stated by Kelvin Planck as "It is
impossible to construct an engine working on a cyclic process,
whose sole purpose is to convert heat energy into work". In other
words, no actual heat engine, working on a cyclic process, can
convert the heat energy supplied to it into mechanical work. It
means that there is a degradation of energy in the process of
producing mechanical work from heat. According to this
statement, the second law of thermodynamics is sometimes
called as law of degradation of energy.
The second law of thermodynamics was given by Kelvin.
Thermodynamic Cycle
A thermodynamic cycle consists of a series of thermodynamic
operation (processes) which take place in a certain order and the
initial conditions are restored at the end of processes.
The study of various thermodynamic cycles is very essential for
power developing system (such as petrol engine, diesel engine,
gas turbine etc.). These engines use a mixture of fuel and air for
their operations. Since the mass of fuel used as compared to
mass of air is very small, therefore, the mixture may be assumed
to obey the properties of perfect gases.

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Assumptions in Thermodynamic Cycle
The analysis of all thermodynamic cycles is based on the
following assumptions.
1 The gas in the engine cylinder is a perfect gas, i.e. it obeys
the gas laws and constant specific heats.
2 The physical constants of the gas in the engine cylinder are
same as those of air at moderate temperature.
3 All the compression and expansion processes are adiabatic
and they take place without any internal friction.
4 Heat is supplied by bringing a hot body in contact with the
cylinder at appropriate points during the process. Similarly, heat is
rejected by bringing a cold body in contact with the cylinder at
these points.
5 The cycle is considered to be a closed one and the same air
is used again and again to repeat the cycle.
6 No chemical reaction whatsoever takes place in the engine
cylinder.
Types of Thermodynamic Cycles
The following are the important types of thermodynamic cycles.
1. Carnot cycle

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2. Stirling cycle
3. Ericsson cycle
4. Joule cycle
5. Otto cycle
6. Diesel cycle
7. Dual combustion cycle.
Thermodynamic Process
The process of heating and expanding of a gas may broadly be
defined as thermodynamic process. It has been observed that as
a result of flow of energy, change takes place in various
properties of the gas such as pressure, volume, temperature,
specific energy, specific enthalpy etc. The thermodynamic
process may be performed in innumerable ways, but the following
are important from the subject point of view:
1. Constant volume process
2. Constant pressure process
3. Hyperbolic process
4. Isothermal process
5. Adiabatic process or isentropic process
6. Polytropic process

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7. Free Expansion
8. Throttling process
Constant Volume Process
When a gas is heated at a constant volume, its temperature and
pressure will increase. Since there is no change in its volume, no
external work is done by the gas. The whole of the heat supplied
will be stored in the form of internal energy.
Constant Pressure Process
When a gas is heated at constant pressure its temperature and
volume will increase. Since there is a change in its volume, the
heat supplied is utilized in increasing the internal energy of the
gas, and for doing external work.
Constant Temperature Process
OR
Isothermal Process
When heat is supplied to a gas such that its temperature remains
constant, then such an expansion is called "Isothermal
Expansion". In this case, the whole of the heat supplied to the gas
will be used up in doing external work. Since Boyle's law assumes
constant temperature of the gas, therefore, expansion according
to Boyle's law is isothermal.

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Notes:
1. Thermodynamic processes are also applicable to the cooling
and
compression of gases. Cooling is regarded as negative heating
and compression as negative expansion.
2. In a thermodynamic process, apart from other details, we are
interested to find out the amount of work done during the process.
Low Temperature Physics
It is concerned with the study of the production and effects of very
low temperatures. It includes superconductivity and super fluidity
which take place in the very low range temperature. It is also
called cryophysics.
Brownian Motion
The phenomenon of molecular motion was first observed by
Robert Brown in 1827. He observed movement of pollen grains
suspended in water with a microscope and found that they were
constantly moving in a zig zag path. After many years, scientists
realized that this was due to collisions between the water
molecules and pollen grains. This random motion of tiny particles
is known as Brownian motion.

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This motion can also be seen in tiny graphite particles suspended
in water. These particles in water are illuminated by a strong
beam of light and are then seen with the help of a microscope.
These particles are also found to be moving in zig zag paths.
According to the molecular theory of matter, the water molecules
are always in motion and, hence, they collide with the graphite
particles and push them in some directions where they collide
with other water molecules and are pushed in some other
directions and this process continues.
This shows that molecules always remain in random motion either
vibratory or translatory.
Throttling Process
This type of expansion occurs when a gas or vapor is expanded
through an orifice or aperture of minute dimension like a valve,
which is very slightly opened, or a narrow throat. Fluid under
pressure is forced through that aperture but the velocity of fluid
flowing out is reduced to a negligible amount by the high
resistance offered by the friction between the fluid and the walls of
aperture. Hence, the kinetic energy of the fluid flowing out of
aperture is very small. In fact, the kinetic energy of the fluid has
been converted into heat, which warms up the fluid to an initial
temperature. Thus, no heat is supplied from or rejected to an

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external source and there is no external work done. Also there is
no change in temperature in case of perfect gas. In the throttling
process also,
W = 0, Q = 0 and ΔU = 0
Linear Thermal Expansion
When a metal rod is heated, its length increases. This expansion
in length is called linear thermal expansion.
Suppose the temperature of a metal rod of length L is raised by
an amount T. If L is the increase in its length, then,
L  L T
OR
L =  L T ______ [1]
Where,  is the coefficient of linear expansion. Its value depends
upon the nature of the material of the rod. Its unit is 1/°c or 1/k.
Eq. [1] can also be written as
L2 – L1 =  L1 T
OR
L2 = L1 (1 +  T)
Where, L1 and L2 are the initial and final lengths of the rod.

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From Eq. [1], we get,
 = L/L T
The coefficient of linear expansion can, therefore, be defined as
"The fractional change in length per unit change in temperature.’’
Atmosphere
The relations between various units of pressure are given below:
1 atmosphere = 14.7 lb/in2
= 760 mm of Hg
= 76 cm of Hg =1.01325 bar
Barometer
A device for measuring the atmospheric pressure is called a
barometer. OR
The instrument used for measuring the unknown pressure with
respect to the atmospheric pressure is called barometer.
There are two types of barometers. These are (1) = Mercury
Barometer (2) = Aneroid Barometer
Mercury Barometer
Atmospheric pressure is measured in a laboratory by a device
called a mercury barometer. A barometer consists of a thick
walled glass tube one meter in length which is opened at one end

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and closed from the other side. This glass tube is filled with
mercury. The open end is firmly covered with a thumb and then
carefully inverted in a vessel containing mercury. When the open
end is completely immersed in mercury, the thumb is removed.
Some of the mercury from the mercury column in the tube drops
in the vessel leaving a space at the closed end. This space
contains no air and is nearly a vacuum. If the mercury column in
the tube is measured, it is found to be approximately 760 mm
above the surface of mercury in the vessel. This length of 760 mm
always remains the same even if tubes of different diameters are
taken. The column of the mercury (the mercury column in the
tube) does not fall as the atmospheric pressure exerted on the
surface of mercury in the vessel is balanced by the pressure of
the mercury column in the tube. The length of the mercury column
in the tube is referred to as the atmospheric pressure since it is a
convenient measurement and is directly proportional to the
pressure. At sea level, the normal atmospheric pressure is 760
mm of mercury. It is often used as unit of pressure and is
abbreviated as 1 atm. Therefore, 1 atm. = 760 mm (mercury).
The fact that the atmosphere exerts pressure has been put into
use in several devices such as siphons, pumps and syringes. We
live at the bottom of a deep sea of air called the atmosphere. The
air, which is a mixture of hydrogen, oxygen and many other gases

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in our atmosphere, exerts pressure. The density of air varies from
sea level to different altitudes. It is more dense at sea level, its
density decreases with increase of height above sea level.
At sea level, the pressure of air is about 100000 Pa. We do not
normally feel atmospheric pressure as the pressure inside our
bodies is almost the same as that outside.
Pressure Law
The pressure of a fixed mass of gas at constant volume is
proportional to its thermodynamic temperature.
p
----------- = constant
T
Where p is the pressure and T is the absolute temperature.
"The pressure of a given mass of a gas is directly proportional to
its absolute temperature, provided the volume of the gas is kept
constant". This is known as pressure law and can be expressed
as:
P  T
OR
P/T = a constant

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Graphical representation of this law is shown in the figure. It is
clear that as the temperature is decreased, the pressure of the
gas also decreases. At -273°C pressure becomes zero, because
at this temperature all molecular motion ceases and, therefore,
the gas does not exert any pressure on the walls of the container.
P
Fig: (1) Temperature Pressure Graph
Pressure
Force per unit area is called pressure.
Mathematically:
Pressure = Force / Area
P = F / A
OR
Pressure is defined as the force acting normally per unit area.

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Thrust
Pressure = -------------
Area
The SI unit of pressure is 1 newton per metre2
(N/m2
). It is called
the Pascal (Pa). High pressure is usually expressed in kilo pascal.
(1 k Pa = 1000 Pa)
Pressure in Liquids
Water contained in a glass has weight. As the weight is a force
which acts downward, therefore, the water exerts a pressure on
the bottom of the glass. This pressure is equal to the force per
unit area. The pressure at any point in a liquid depends on the
density and the depth in the liquid. Let us calculate the pressure
at the bottom of a liquid. Imagine a circle of area A at the bottom
of the liquid. This area lies at a depth h below the surface of the
liquid. Now the liquid which exerts pressure on this area is
contained in a cylinder whose bottom has an area A and whose
height is h. The volume of the liquid in this imaginary cylinder is
Ah. If  is the density of the liquid, then mass of the liquid above
the area A will be Ah and its weight will be Ahg (where g is the
acceleration due to gravity).

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Therefore, the pressure of the liquid on the area A is given by
Force Ahg
Pressure = ----------- = --------- = hg
Area A
It is evident that the pressure at a point inside the liquid depends
on the height of the liquid above this point and the density of the
liquid. Persons who dive into a swimming pool experience an
increase in pressure as they descend below the surface of water.
If a beaker is filled with water and another similar beaker is filled
with a liquid whose density is higher than the density of water,
such as saline water, then pressure at a point in saline water will
be greater than the pressure at a point in ordinary water although
both the points are taken at the same level. Liquids also exert a
pressure on the vertical walls of the container. Internal stresses
are set up in the liquids by external forces, and these allow the
pressure in a liquid to be transmitted in all directions. The solid
surface needed to contain a liquid must exert a force on that
liquid. This force is equal and opposite to the force exerted by the
liquid on the containing surface.
Pressure in Gases
It is hard to believe that air possesses mass. However, when our
body feels the blow in a wind storm, then it is the mass of air

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which gives a push to our body. We can weigh the mass of air by
weighing a balloon inflated with air, then, allowing its air to escape
and weighing it again. We will note that there will be a loss in
weight when the air has escaped from the balloon. All gases exert
force due to their weight and hence exert pressure. The kinetic
theory enables us to account for the pressure a gas exerts on the
walls of its container. When a moving molecule strikes the walls of
its container, a force is exerted on the walls during the impact.
The continuous bombardment of molecules striking the walls
accounts for the pressure of the gas.
Vacuum Pressure
Vacuum pressure means pressure below atmospheric pressure. If
vacuum pressure is given, absolute pressure may be obtained
from the following relation.
Absolute pressure =Atmospheric pressure - vacuum pressure.
Absolute Pressure
All the pressure gauges read the difference between the actual
pressure in any system and the atmospheric pressure. The
reading of the pressure gauge is known as gauge pressure, while
the actual pressure is called absolute pressure.
Mathematically:

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Absolute pressure = Gauge pressure + Atmosphere pressure.
Atmospheric Pressure
We live at the bottom of a deep sea of air called the atmosphere.
The air, which is a mixture of nitrogen, oxygen and many other
gases in our atmosphere, exerts pressure. The density of air
changes from sea level to different altitudes. It is most dense at
sea level and its density decreases with increase of height above
the sea level. The density of air at sea level is about 1.2g per liter.
At an altitude of 2000 m, it is about 1g per liter and at 10000 m; it
is about 0.4 g per liter.
The atmosphere, because of its weight exerts pressure on the
surface of the earth and on everything on the earth including
ourselves. This pressure is called atmosphere pressure. At sea
level, the pressure of air is about 100,000 Pa. We do not normally
feel atmospheric pressure as the pressure inside our bodies is
almost the same as that outside.
The existence of atmospheric pressure was first demonstrated by
a German scientist Von Guericke. He took two hollow metallic
hemispheres which were made to fit with each other tightly. Air
inside the hemispheres was removed through a small hole by
means of an air pump. It was found that the hemispheres could
not be pulled apart when the air had been removed. The force

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keeping the hemispheres together was the air pressure acting
from outside. The hemispheres were so tightly held that it took
two teams each of eight horses to separate them. This
experiment was first performed in the city of Magdeburg.
Therefore, the experiment is called the Magdeburg hemisphere
experiment.
Fig: Magdeburg Hemisphere Apparatus
Pascal (1623-1662)
Pascal gave us law of fluid pressure.
Pascal's Law
Liquids transmit pressure equally in all directions. This is known
as Pascal's Law.
Thermometer
The device (or instrument) which is used for the exact or
quantitative measurement of temperature is called as
thermometer. Common thermometers use the expansion of liquid
to show change of temperature.

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Principle of Thermometer
When an object is heated, then, it undergoes various changes
e.g. change in length of the object, change in color or increase in
the resistance. Any one of these changes may be utilized to
construct a thermometer.
Construction of a Mercury Thermometer
An ordinary mercury thermometer is made up of a long glass
capillary tube having a uniform bore. One end of the tube is
closed while the other end is provided with a glass bulb filled with
mercury as a thermometric substance.
Working of the Mercury Thermometer
As bulb of the thermometer is placed in contact with a hot body,
then, the mercury expands and rises higher in the capillary tube.
This, height of the mercury in the capillary tube can be used as a
measurement of the temperature of the body.
Thermometric Substance
The substance (mercury, alcohol etc.) used in thermometer is
called thermometric substance.

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Mercury is usually used as thermometric substance because of its
following qualities.
(1) Pure mercury can be easily obtained i.e. it is easily available.
(2) It remains liquid over a fairly wide range of temperature i.e.
does not change its physical state within a wide range of
temperature variation.
(3) Over a wide range of temperature, its expansion is quite a
linear function of the temperature and its expansion is directly
proportional to the temperature.
(4) Being practically non-volatile, it is affected very little by its
vapor pressure.
(5) It has a low specific heat so that it does not remove much of
the heat of the body whose temperature is to be measured.
(6) On account of its high conductivity, it quickly assumes the
temperature of the body with which it is in contact.
(7) Being an opaque and shining liquid, it can be easily seen
through glass.
(8) It does not wet the glass walls of tube of thermometer and
does not stick to the glass walls.

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Thermometry
The branch of physics which deals with the measurement of
temperature is called as thermometry. The basic principle of
thermometry is that two bodies in contact and in thermal
equilibrium have the same temperature.
Thermos Flask
Thermos flask is a pot designed to prevent heat loss from the fluid
inside it due to all of the three modes of heat transfer (i.e.
conduction, convection and radiation). It also prevents the heat
present outside the flask from getting into the flask. Any hot or
cold drink contained in the thermos flask remains hot or cold for a
relatively long time.
Thermostat
Thermostats are devices which control temperature in a certain
region.
Kinetic Theory
The molecules of all the substances are not at rest. In liquids and
gases, the molecules move with different velocities and have
kinetic energies of translation. In solids, the molecules vibrate and
rotate and thus have vibrational and rotational energies. When the
energies of the molecules increase, they can transfer their

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energies to other bodies by collision. The energy transferred is
called the heat.
Kinetic Theory of Matter
OR
Molecular Theory of Matter
The kinetic theory of matter says that anything that moves does a
certain amount of work and possesses some energy.
British Thermal Unit (B.T.U)
It is defined as the amount of heat required to raise the
temperature of one pound of water by 1°F.
1 B.T.U = 252 calorie
1 calorie = 4.18 J
1 K Cal = 1000 calorie
1 B.T.U = 252 Calorie = 1055.06 J
Dew Point
The dew point is defined as the temperature at which the water
vapor present in the air is just sufficient to saturate it.

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Evaporation
The phenomenon of changing liquids into vapors without boiling is
called evaporation.
For example, when wet clothes are hung on the cord, they get
dry. Water present in clothes changes into vapors and disappears
into atmosphere.
Factors Upon Which Evaporation Depends
Fig: Evaporation depends upon Surface Area
Experiments have shown that evaporation of liquids depends on
the following factors.
1. Nature of Liquid:
The liquids having low boiling points evaporate more easily e.g.
Alcohol, ether, etc.
2. Temperature of Liquid:

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Evaporation process increases with the increase of temperature,
e.g., under hot iron wet clothes dry out quickly as the water
evaporates quickly.
3. Surface Area of the Liquid:
The more the surface area of liquid, the more will be the rate of
evaporation.
4. Dryness of Air:
The dryer the air, the quicker is the rate of evaporation. In the
rainy season, the clothes take much longer to dry.
5. Wind Speed:
The higher the wind speed, the greater is the rate of evaporation.
6. Air Pressure on the Surface of Liquid:
If the pressure on the surface of liquid is lowered, then, its rate of
evaporation is increased.
Cooling Through Evaporation
According to the kinetic theory, the molecules of a liquid move
freely. Some of the molecules have more kinetic energy while
some have less kinetic energy. Some of the molecules having
more kinetic energy and moving towards the surface overcome
the forces of attraction.

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When the molecules having more kinetic energy leave a liquid
surface, the average kinetic energy of the molecules left behind
becomes low and causes decrease of temperature of liquid. The
liquid, thus, cools down.
Example
If a person sits under a fan with wet clothes, his body feels
cooling because under the fan air, the water in the clothes
evaporates quickly. Here, each gram of water on evaporation
takes away 4.2J of heat from the body. Rapid evaporation thus
cools the body rapidly.
Experiment
Place some spirit in a beaker. Put the beaker on a wooden block
on which some water has been spilled. A thin layer of water is
formed between the beaker and block. By blowing on the spirit
through a glass tube, bubbles will start appearing in spirit, i.e. the
rate of evaporation of spirit increases.
As the molecules of spirit escape from the surface, the
temperature of spirit will fall. More and more molecules of spirit
turn into vapor and at one stage the temperature of spirit
becomes less than 0°C. This cools the water under the beaker to
0°C and it freezes.

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Fig: Cooling produced by Evaporation

Module No. 33

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Module No. 33