Prof' Vinod Kallur

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Prof. Vinod Kallur Chemical Engineering
Department R V College of Engineering, Bangalore
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In the earlier two sessions we discussed

• System, surroundings, and process
• Classification of systems
• State and path functions
• Intensive and extensive properties
• Equilibrium state
• Phase Rule
• Heat reservoirs, heat engines
• Reversible and irreversible processes.

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After the session you will be able to

• State first law of thermodynamics
• Understand internal, kinetic and potential energy
• Assess if the process is cyclic
• Identify flow processes

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w
What happened to work after it was input and
before it was taken out?
Internal energy
Molecular energy such as energy of vibration and
rotation etc.
Q
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First law of thermodynamics
where Q heat added to the system and W work
done by the system.
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Potential energy
Body at a height has the ability to do work and
hence should possess energy.
Spring under compressed state can also do work.
Energy possessed by a body by virtue of its
position is called potential energy.
Potential energy of a body of mass m at a height
z from a reference level is given by
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Kinetic energy
Body moving with a velocity has the ability to do
work and hence should possess energy.
Energy possessed by a body by virtue of its
velocity is called potential energy.
Kinetic energy of a body of mass m at moving with
a velocity u is given by
Kinetic energy can have absolute values but not
potential energy.
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Part increase in each energy is possible.
A system will in general, possess all forms of
energy.
If changes in potential and kinetic energy is
negligible, then
First law is thus applied to closed system.
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Cyclic process
1-2-3-1 is a cyclic process.
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Values of path functions Q and W are not zero.
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Net work done is the area under the closed loop.
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Work done by the system
Work done by the system
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Work done on the system
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Work done by the system,()
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Work done by the system,()
Work done on the system,(-)
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a
b
c
W area of the closed loop.
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For any cycle,
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10 g lead ball at 300 K is dropped from a height
of 10 m. Calculate the kinetic energy and speed
of the ball as it reaches the ground.
At 10 m height,
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As the ball goes down, potential energy decreases
and kinetic energy increases.
At the time ball reaches the ground, the
potential energy is zero and all the potential
energy gets converted into kinetic energy.
Therefore just when it touches the ground,
Kinetic energy 0.981 J
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Following equations of dynamics,
What will happen to this kinetic energy when the
ball comes to a stop?
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Heat capacity
Heat capacity of a substance is defined as the
quantity of heat to be supplied to bring about
change in temperature of unit amount of the
substance by one degree.
Since heat transfer is a path process, the value
of C depends how the process of supplying heat is
carried out.
Gases may be heated in two different ways to
increase the temperature.
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Take the sample of gas in the piston and cylinder
arrangement, fix the position of the piston and
heat.
Volume does not change. The amount of heat
supplied is called heat capacity at constant
volume denoted by CV.
Take the same again, this time the piston is
movable. Thus the pressure is held constant. The
heat supplied is called heat capacity at constant
pressure denoted by CP.
CP gt CV
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CP gt CV
Constant volume process - gas does not do any
work.
Constant pressure process gas does the work of
expansion.
T2
Additional heat is to be supplied for the work of
expansion.
T1
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At constant pressure
At constant volume
If Q is the heat supplied to any solid or liquid,
it is used up in increasing its internal energy
as long as there is no phase change.
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10 g lead ball at 300 K is dropped from a height
of 10 m. Calculate the kinetic energy and speed
of the ball as it reaches the ground and also
rise in its temperature if all kinetic energy
gets converted into internal energy. Heat
capacity of the ball material is 125.6 J/kg K.
Just before it reaches ground, potential energy
0
Kinetic energy 0.981 J
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Kinetic energy 0.981 J
Temperature increases by 0.78 K.
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Flow process
Steam is passing through the equipment.
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An element of steam is the system.
Steady state and unsteady state flow processes
Heat in, Q
z1
2
z2
Work output, Ws
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Let us recapitulate what has been done in this
session

• I law of thermodynamics
• Internal energy, kinetic energy and potential
  energy
• Cyclic process
• Heat capacity
• flow process

In the next session we will apply first law to
flow processes and solve some numerical problems
for the topics covered so far.