Introduction to thermodynamics

Introduction to Thermodynamics
Power point presentation 1
By
M.Veeramanikandan,
Assistant Professor,
Department of Mechanical Engineering,
Sri Ramakrishna Institute of technology,
Coimbatore.
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Thermodynamics
Thermodynamics is the science that deals with heat and work and those
properties of substance that bear a relation to heat and work.
Thermodynamics is derived from two words: ‘Thermo’ which means ‘Heat
energy’ and ‘Dynamics’ which means ‘conversion’ or ‘transformation’
Thermodynamics is the study of the patterns of energy change. Most of this
course will be concerned with understanding the patterns of energy change.
More specifically, thermodynamics deals with (a) energy conversion and (b)
the direction of change.
Basis of thermodynamics is experimental observation. In that sense it is an
empirical science. The principles of thermodynamics are summarized in the form
of four laws known as zeroth, first, second, and the third laws of
thermodynamics.
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 The zeroth law of thermodynamics deals with thermal equilibrium and
provides a means of measuring temperature.
 The first law of thermodynamics deals with the conservation of energy and
introduces the concept of internal energy.
 The second law of thermodynamics dictates the limits on the conversion of
heat into work and provides the yard stick to measure the performance of various
processes. It also tells whether a particular process is feasible or not and specifies
the direction in which a process will proceed. As a consequence it also introduces
the concept of entropy.
 The third law defines the absolute zero of entropy.
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Applications of Engineering
Thermodynamics
Engineering application of thermodynamic principles is the design of various
systems using fluid properties to cause energy transformation
Applications in design of heat engines, refrigeration machines, air conditioning
systems
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Scope of Thermodynamics
It is limited to macroscopic properties of matter i.e. properties of large number of
particles of systems
It considers the initial and final states of a system and not the mechanism of the
process
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Concept of continuum
The concept of continuum is a kind of idealization of the continuous
description of matter where the properties of the matter are considered as
continuous functions of space variables. Although any matter is composed of
several molecules, the concept of continuum assumes a continuous distribution of
mass within the matter or system with no empty space, instead of the actual
conglomeration of separate molecules.
Describing a fluid flow quantitatively makes it necessary to assume that flow
variables (pressure, velocity etc.) and fluid properties vary continuously from one
point to another. Mathematical descriptions of flow on this basis have proved to be
reliable and treatment of fluid medium as a continuum has firmly become established.
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Comparison of microscopic and
macroscopic approach
Microscopic approach uses the statistical considerations and probability
theory, where we deal with “average” for all particles under consideration. This is the
approach used in the disciplines known as kinetic theory and statistical mechanics.
In the Macroscopic point of view, of classical thermodynamics, one is
concerned with the time-averaged influence of many molecules that can be perceived
by the senses and measured by the instruments.
The pressure exerted by a gas is an example of Macroscopic Approach. It
results from the change in momentum of the molecules, as they collide with the wall.
Here we are not concerned with the actions of individual molecules but with the time-
averaged force on a given area that can be measured by a pressure gage.
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Path and Point Function
• Path function It depend on path followed
during a process as well as end states. (i.e. Work
&heat)
• Point function It depend on the state only. (i.e.
V, P &T)
• From diagram, since the area under each curve
represents the work of each process, the amount
of work involved in each case is not a function
of the end states of process, and it depends on
the path the system follows in going from state 1
to state 2.For this reason work is called path
function and thermodynamic properties are
called point function.
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Basic Properties
Pressure is the force acting on the given area, p = F / A .
Unit is bar or Pa or N/m2 or (bar =105pa)
Volume is the space occupied by the given mass, v = Area x Length . Unit- m3 or cc(cm3)
Temperature is the measure of hot or cold condition, Unit is K(Kelvin) or 0C (Celsius)
Density Mass contained in the given volume, ρ= mass/volume. Unit- kg/m3
Specific Volume The volume occupied by the given mass, v = 1/ρ. Unit – m3 /kg
Internal Energy Energy of molecules due to its temperature, U = m Cv T (kJ)
Enthalpy Total energy of system and it’s a sum of internal energy (u) and flow energy (pv)
i.e. h = u + p v (kJ/kg)
Entropy A measure of degradation of energy (or ) A degree of measure of available or
unavailable energy. dS = dQ / T (kJ/kg-K)
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Intensive and Extensive property
Intensive property: Whose value is independent of the size or extent i.e. mass of the
system. e.g., pressure (p) and temperature (T).
Extensive property: Whose value depends on the size or extent i.e. mass of the
system. e.g., Volume, Mass (V, M). If mass is increased, the value of extensive
property also increases. e.g., volume (V), internal energy (U), enthalpy (H), entropy
(S), etc.
Specific property: It is a special case of an intensive property. It is the value of an
extensive property per unit mass of system. e.g: specific volume, density (v, ρ).
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System and their types
Thermodynamic system
A thermodynamic system is defined as the quantity of matter or a region in
space upon which attention is concentrated in the analysis of a problem. Here quantity
of matter may be gas, solid or liquid, magnetic field, electric field or even photons.
Surroundings/Environment
Everything external to the system is called the surroundings or environment.
The system is separated from the surroundings by the system boundary. The boundary
may be fixed or flexible.
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System classified into three types,
1.Closed system
2.Open system
3.Isolated system
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Closed system-in which no mass is permitted to cross the system boundary i.e. we would
always consider a system of constant mass. We won’t permit heat and work to enter or leave
but not mass.
No mass entry or exit
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Open system-in which we permit mass to cross the system boundary in either
direction (from the system to surroundings or vice versa).In analysing open systems,
we typically look at a specified region of space, and observe what happens at the
boundaries of that region.
Most of the engineering devices are open system.
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Isolated System-in which there is no interaction between system and the
surroundings. It is of fixed mass and energy, and hence there is no mass and energy
transfer across the system boundary.
Example: Universe
No Heat and Mass Entry or Exit
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Control volume & Control surface
 Control volume: Certain fixed
region or volume in space
surrounding system.
Control surface: The surface of
control volume.
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Thermodynamic Equilibrium
Thermodynamic Equilibrium No change in the state of the system. It consists of
following,
1. Mechanical Equilibrium No unbalanced forces in the system
2. Thermal Equilibrium No temperature differences in the system
3. Chemical Equilibrium No chemical reaction the system. (Stable chemically)
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State, path, Process
State: A condition/ moment of system at a given
instant.
One or more property will describe the state.
E.g. T for hot or cold state
Change of state: Any operation in which one or
more properties changes of sys.
Path of system: The succession of state passed
through change of state.(or) The consecutive traces of
the changes of state.
Process: The path is completely specified the change
of state during an energy transfer. A process can have
many different paths. A series of change of state
such that initial state identical with the final state.
Cyclic Process The process having same initial and
final states.
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Quasi-static Process
Quasi-static Process: The very slow process
consists of many equilibrium states. Ideal
process.
A quasi-static process is one in which
 The deviation from thermodynamic
equilibrium is infinitesimal.
 All states of the system passes
through are equilibrium states.
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If we remove the weights slowly one by one the
pressure of the gas will displace the piston
gradually. It is quasi-static.
On the other hand if we remove all the weights at
once the piston will be kicked up by the gas
pressure.(This is unrestrained expansion) but we
don’t consider that the work is done -because it is
not in a sustained manner
In both cases the systems have undergone a change
of state.
Another eg: if a person climbs down a ladder from
roof to ground, it is a quasi-static process. On the
other hand if he jumps then it is not a quasi-static
process.
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Reversible & Irreversible process
Reversible Process The process having the same
path (traces of states) travel while reversed. Both
the system and the surroundings may be restored to
their initial states, without effect in the rest of
universe.
In a reversible process the system changes in such a
way that the system and surroundings can be put
back in their original states by exactly reversing the
process.
Changes are infinitesimally small in a reversible
process.
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Irreversible Process Almost all the real or actual processes are irreversible. (Non-
equilibrium). Energy Loss occurs during the real processes.
Irreversible processes cannot be undone by exactly reversing the change to the
system.
All Spontaneous processes are irreversible.
All Real processes are irreversible.
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Heat & Work
Thermodynamic definition of work:
Positive work is done by a system when the sole effect external to the system could
be reduced to the rise of a weight.
Thermodynamic definition of heat:
It is the energy in transition between the system and the surroundings by virtue of the
difference in temperature.
Work is the form of energy useful in displacement of a body
dW= F.dL= p. A dL= p.dV
Heat is the form of energy transferred due to temperature gradient between
two bodies (Joules)
difference
e
Temperatur
Specific
Mass
Heat 


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Sign Conventions
• Work done by the system is +ve
• Obviously work done on the system is –ve
• Heat given to the system is +ve
• Obviously Heat rejected by the system is -ve
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Introduction to thermodynamics

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