MCL140
THERMODYNAICS
Prabal Talukdar
Professor
Department of Mechanical Engineering
IIT Delhi
prabal@mech.iitd.ac.in
MECH/IITD
Thermodynamics
• Thermodynamics is science of energy
• Meaning of thermodynamics: stems from greek words:
“Therme” means heat, “dynamics” means power.
• 1st law: Energy conservation
• 2nd law: Quality and quantity of energy
• Pioneer: Kelvin, Clausis, Rankine
• Classical and Statistical thermodynamics
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History
• The first and second laws of thermodynamics
emerged simultaneously in the 1850s, primarily out
of the works of William Rankine, Rudolph Clausius,
and Lord Kelvin (formerly William Thomson).
• The term thermodynamics was first used in a
publication by Lord Kelvin in 1849.
• The first thermodynamic textbook was written in
1859 by William Rankine, a Professor at the
University of Glasgow.
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Applications
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Units
• English and metric SI unit
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System, Control Volumes
Open system – control volume
Closed system – control mass
Isolated system
A closed system with a
moving boundary.
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Control Volume
An open system (a
control volume)
with one inlet and
one exit.
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Property of a System
• Any characteristics of a system is called a property e.g. pressure,
temperature, volume, mass etc.
• Intensive: Independent of mass
• Extensive: values depends on size or extent of the system
• Density
• Specific Properties - Specific volume…..
• Specific Gravity
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State and Equilibrium
• State – In a state all the properties of a system remain same
• Equilibrium – means there is no unbalanced potential (or driving
forces) within the system
– Thermal, Mechanical, Phase, Chemical Eq.
• The state Postulate
– The state of a simple compressible system is completely specified by
two independent, intensive properties.
– T = f(P) during a phase-change process;
thus, temperature and pressure are not
sufficient to fix the state of a two-phase system.
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Continuum
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Process and Cycles
• Process: Any change that a system undergoes from one equilibrium
state to another is called a process
• Path: the series of states through which a system passes during
process is called the path of the process
• Cycle: A system is said to have undergone a cycle, if it returns to its
initial state at the end of the process
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Quasi-equilibrium process
Steady/Uniform
Steady flow devices
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Steady Flow Process
Quasi-equilibrium process
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Temperature and Zeroth Law
• Thermal Equilibrium ? – attains same temperature – contact
required
• Zeroth law helps us to determine that two bodies are in thermal
equilibrium if both have the same temperature reading even they are
NOT in contact.
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Temperature and Zeroth Law
• 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.
• Third body can be assumed as a thermometer
• The zeroth law was first formulated and labeled by R. H. Fowler in
1931
• Temperature is a thermodynamic property that determines whether
or not a system is in thermal equilibrium with another system.
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Temperature scale
All temperature scales are based on some easily reproducible states such as
the freezing and boiling points of water, which are also called the ice point
and steam point
Two point scale – Celsius and Fahrenheit scale
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Temperature scale - one point
or two points
Before 1954
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After 1954
Temperature Scales
• In thermodynamics, it is very desirable to have a temperature scale
that is independent of the properties of any substance or substances.
Such a temperature scale is called a thermodynamic temperature
scale – will discuss more in conjunction with 2nd law
• Kelvin scale (SI), Rankine scale (English)
• A temperature scale that turns out to be nearly identical to the
Kelvin scale is the ideal-gas temperature scale. The temperatures
on this scale are measured using a constant-volume gas
thermometer, which is basically a rigid vessel filled with a gas,
usually hydrogen or helium, at low pressure.
• This thermometer is based on the principle that at low pressures, the
temperature of a gas is proportional to its pressure at constant
volume. The relationship between the temperature and the pressure
of the gas in the vessel can be expressed as T = a + bP
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Ideal gas-temperature scale
• If the ice and steam points are
assigned the values 0°C and
100°C, respectively, then the
gas temperature scale will be
identical to the Celsius scale.
• In this case the value of the
constant a (which corresponds
to an absolute pressure of
zero) is determined to be
-273.15°C regardless of the
type and the amount of the
gas in the vessel of the gas
thermometer.
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• An absolute gas
temperature scale can be
achieved by assigning a
value of zero to the
constant a . In that case
Eq. reduces to T = bP,
• and thus we need to
specify the temperature at
only one point to define an
absolute gas temperature
scale.
Kelvin scale
Rankine scale
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Constant Volume Gas
Thermometer
T = bp
T
P
=
Tref Pref
T
P
=
273.16 Pt .p.w .
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Pressure
Normal force exerted by a fluid per unit area is called pressure
Variation of pressure with depth
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zontal direction is that the pressure applied to a confined fluid increases the
pressure throughout by the same amount. This is called Pascal’s law, after
Blaise Pascal (1623–1662). Pascal also knew that the force applied by a
fluid is proportional to the surface area. He realized that two hydraulic
cylinders of different areas could be connected, and the larger could be used
Pressure
Patm
Water
h
A
B
C
D
E
PA = PB = PC = PD = PE = PF = PG = Patm + rgh
Mercury
PH ≠ PI
H
F
G
I
FIGURE 1–43
The pressure is the same at all points on a horizontal plane in a given fluid regardless of geometry, provided that the
points are interconnected by the same fluid.
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ometer
to aof manageable
heavy
F1 equilibrium,
Fit
F2 fluids
A2
id column
height h is in level,
static
andsuch
2 is as
S anticipated.
!
S
!
P1 ! P2 are
(1–2
pressure
differences
ere.
Then
the
pressure
at
point
2
is
determined
directly
F2
A2
A1
A2
F1
A1
!
(1–22)
erF1shown
in
Fig.
1–45
that is used to measure the
A
1
area
ratio A2/A1 is
called
the ideal
mechanical
advantage of th
the gravitational
effects
of
gases
are
negligible,
the
P
# rgh
(1–23)
2 ! Patm
ulic lift.advantage
Using
a hydraulic
car jack with a piston
area ratio of A2/A1
anical
of
the
tank and at position 1 has the same value. Furthera in
person
lift athat
1000-kg
car by applying a force of ju
yr example,
of area
the fluid
the cross-sectional
/Acan
!Note
ston
ratio
oftheA2tube.
1
luid does not vary in the horizontal direction within
no
effect
on N).
the
differential
gf
(! 981
y applying
a force
of justheight h, and thus the pres! P1.
nt
is the same
the pressure
at point
P2large
uid.2 However,
the as
diameter
of the tube
should1,be
olumn
of height htois ensure
in static
equilibrium,
and it is
a few
millimeters)
that
the
surface
tension
■
0pillary
THE
MANOMETER
Then
the
at point 2 is determined directly
risepressure
is negligible.
Manometer
otice from Eq. 1–18 that an elevation change of "z in a fluid at re
eponds
of "z
in"P/rg,
at restsuggests that a fluid column
which
P2to
!
Paatmfluid
# rgh
(1–23) can be used
Pressure
withtoaAManometer
dMeasuring
column
can
be used
ure
pressure
differences.
device based on this principle is called
the fluid
initthe
tube.
Note
that to
themeasure
cross-sectional
this
principle
is
called
a
meter,
and
is
commonly
used
and moderate pre
d to measure the pressure in a tank. The fluid used small
has
fect onand
themoderate
differential
height
h, consists
and thusofthea pressmall
presdifferences.
A
manometer
mainly
0.85, and the
manometer
column
height
is 55 cm, asglass or plastic U-tub
However,
the
diameter
ofsuch
theis tube
should
be
largealcohol, or oil. T
aIf glass
oror
plastic
U-tube
theone
local
atmospheric
pressure
96 mercury,
kPa,
determine
ning
more
fluids
as
water,
w
millimeters)
to oil.
ensure
that
the surfacelevel,
tension
ehe
within
the
water,
or
To to
sizealcohol,
of tank.
the manometer
a manageable
heavy fluids such
ry
rise
is negligible.
evel,
heavy
fluids
such
as
ry are
used
if large
pressure
differences are anticipated.
anticipated.
nsider
the manometer shown in Fig. 1–45 that is used to measure th
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at is used to measure the
Measurement of Pressure
Pgage = Pabsolute - Patm
Pvac = Patm - Pabsolute
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Multifluid Manometer
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