concepts(lect1 6)
TRANSCRIPT
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Thermodynamics
Thermodynamics may be defined as the science
of energy Energy can be viewed as the ability to cause changes
therme + dynamis
First law is simply an expression of energy
principle
The second law asserts that energy has qualityas well as quantity and actual processes occur
in the direction of decreasing quality.
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A substance consists of a large number of
particles called molecules. The properties
of the substance naturally depend on the
behavior of the particles.
The macroscopic approach to study of thermodynamics
provides a direct and easy way to the solution of engineering problems. This does not require a
knowledge of behavior of individual particles. This
approach is also named CL ASSIC AL
THERMODYN A
MICS
.A more elaborate approach, based on average behavior of large
groups of
individual particles is called
STATISTICAL THERMODYNAMICS or Microscopic approach
and is rather involved
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Application areas
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Systems and control volumes
A system is defined as a quantity of matter
or region in space chosen for study.
The mass or region outside the system iscalled the surroundings.
The real or imaginary surface that
separates the system from surrounding is
called the boundary.
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SYSTEM
SURROUNDINGS
BOUND ARY
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Characteristics of a boundary
The boundary of a system can be fixed or
movable.
The boundary is a contact surface sharedby both the system and surroundings.
Mathematically, the boundary has a zero
thickness and thus it can neither contain
any mass nor occupy any volume in
space.
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Closed system
A closed system consists of fixed mass
and no mass can cross its boundary. But
energy, in form of heat or work, can crossthe boundary and the volume of a closed
system does not have to be fixed.
If , in a special case, even energy is not
allowed to cross the boundary that systemis called an isolated system.
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Open system
An open system usually encloses a devicethat involves mass flow such as acompressor, turbine or nozzle.
Flow through these devices is studied byselecting the region within the device asthe control volume.
The boundaries of a control volume arecalled a µcontrol surface¶ and they can bereal or imaginary.
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PROPERTIES OF A SYSTEM
Any Characteristic of a system is called aproperty.
Intensive properties are independent of the mass of system.
Extensive properties are those propertieswhose value depend on the size- or extent
of the system. Extensive properties per unit mass are
called specific properties.
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Thermodynamic property
It refers to the characteristic describing the
condition or state of a system
± It is a measurable characteristic describing the
system. ± It has a definite unique value when the system is in a
particular state.
± It is dependent only on the state of the system; it does
not depend on the path or route, the system follows toattain that particular state.
± Its differential is exact.
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Properties
Temperature
Pressure
Volume Colour
Composition etc
dP dZ
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Properties
A given expression is a property if its
differential is exact
dP = M dx + N dy is exact if
Partial derivative of M wrt y =
Partial derivative of N wrt x
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Properties
Which of the below listed terms areproperties?
p dv + v dp;
p dv;
v dp;
(v/T) dp + (p/T) dv subject to pv=RT
dT/T + p/T dv subject to pv = RT
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Continuum
It is convenient to disregard the atomic nature of a substance and view it as a continuous,homogeneous matter with no holes, that is, a
continuum. The continuum idealization allows us to treat
properties as point functions and to assume theproperties vary continually in space with no jumpdiscontinuities.
This idealization is valid as long as the size of the system we deal with is large relative to thespace between the molecules.
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Continuum
Oxygen
1 atm, 20 degree C
Void
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Rarefied gas flow theory
Application limited to very high vacuums or
very high elevations
For atmospheric air at elevation of 100kmthe mean free path is 0.16m!
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State postulate
The state of a simple compressible system
is completely specified by two
independent, intensive properties
T=300 K
v=0.9cum/kg
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PROCESSES AND CYCLES
Any change that a
system undergoes from
one equilibrium state to
another is called a
process, and the series
of states through which
a system passes duringa process is called the
path of the process.
State 1
State 2
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Quasistatic process
When a process proceeds in such a
manner that the system remains
infinitisimally close to equilibrium state at
all times, it is called a quasistatic or quasi-
equilibrium process. This can be viewed
as a sufficiently slow process.
QUASI-EQUILIBRIUM NON-QUASI-EQUILIBRIUM
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PROCESSES AND CYCLES
Quasi-equilibrium process is an idealized
process and is not a true representation of
an actual process.
A non-quasi-equilibrium process is
denoted by dashed line between initial
and final states
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Processes and cycles
The prefix iso- designates process for
which a particular property remains
constant
± Iso thermal process
± Iso baric process
± iso choric/iso metric process
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A system is said to have undergone a
cycle if it returns to its initial state at the
end of the process. For a cyclic process
the initial and final states are identical.
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The steady flow process
Steady: No change with time
Uniform: No change with location
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Temperature and
the zeroth law of thermodynamics
Iron
150 C
Copper
20 C
Iron
60 C
Copper
60 C
Two bodies reaching thermal equilibrium after being brought into contact inan isolated enclosure
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If two bodies are in thermal equilibrium
with a third body, they are also in thermal
equilibrium with each other.
Two bodies are in thermal equilibrium if
both have the same temperature reading
even if they are not in contact.
R H Fowler (1931)
Temperature and
the zeroth law of thermodynamics
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TEMPER ATURE
IS ASSOCIATED WITH THE
ABILITY TO DISTINGUISH HOT
FROM COLD ZEROTH LAW IS THE BASIS OF
TEMEPRATURE MEASUREMENT
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The working fluid
Characteristic
varying
Reference body
Thermometric
substanceThermometric
property
Temperature
THERMOMETER
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ME ASUREMENT OF TEMPER ATURE
Note
X ± Thermometric property
(X): Temperature at X
METHODS
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Temperature on linear scale
(X)= aX, where a is an arbitrary
constant
(X2)= [ (X1)/X1].X2
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Temperature measurement methods
(X1)/ (X) = X1/X
(X2)/ (X) = X2/X
[ (X1)- (X2)] / [ (X)] = [X1-X2] / X
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Temperature measurement methods
(X)=[{ (X1)- (X2)} / (X1-X2)]X
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Temperature scales
Scale Ice point Steam point
Degree Celsius 0 100Degree Farhrenheit 32 212
Kelvin 273.15 373.15
Rankine 491.67 671.67
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Try this
Same numeric value is obtained
in degree Centigrade and degree
Fahrenheit scale for a particular bath. In Kelvin scale and degree
Rankine scale what would be the
value of this temperature.
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Try this
A new temperature scale in degree N
is designed with ice point 100 degree
N and steam point 400 degree N.What shall be the reading
corresponding to 150 degree C? At
what temperature both the Celsiusand the new temperature scale
reading would be same?
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Temperature Scales
Temperature scales enable us to
use a common basis for
temperature measurements. These are based on easily
reproducible states called fixed
point.
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Measurement of Temperature
Before 1954 two fixed points were used
±Ice point :Ice- Air saturated water
equilibrium at 1 atm.
±Steam point: Pure water-Pure
steam at 1 atm.
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Temperature measurement
Limitations with 2 fixed point method
1. Difficulty of achieving equilibriumbetween ice and air-saturated water
2. Extreme sensitiveness of the steam point
to the change in pressure
Only one fixed point then!
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Temperature measurement
methods (after 1954)
t = a Xt
a = t / Xt = 273.16/ Xt
= (273.16/Xt) X = (273.16) (X / Xt)
t = Triple point of water = 273.16 K
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Types of thermometers
Constant
volume gasthermometer
= (273.16) (p/pt)
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Types of thermometers
Constant
pressure gasthermometer
= (273.16) (V/V t)
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Types of thermometers
Electrical
resistancethermometer
= (273.16) (R/R t)
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Types of thermometers
Thermocouple = (273.16) (e/et)
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Types of thermometers
Liquid-in-
glassthermometer
= (273.16) (L/Lt)
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Fact: Gas is chosen as
the standardthermometric
substance
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Pressure is defined as a normal
force exerted by a fluid per unit area
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Do U compute the number of
molecules or count the collisionwith wall or use software program
to find the net rate of change of
momentum to get the pressure?
I use a
barometer or
manometer
to determine
it.
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SI unit of pressure, i.e , Pascal is too
small. The pressure exerted by 102 gmobject on 1sqm table.
In practice we use kPa and MPa
Other units are 1 bar = 0.1 Mpa
1 atm = 1.01325 bar
1 atm =14.696 psi
1 kgf/cm2
=9.807X104
Pa
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Pressure effects
Snow shoes are large
Sharp knives need lower effort
Discomfort for a man with excessiveweight while standing
Standing on one foot only!
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Absolute, gage and vacuum pressure
P
atm
P
atm
P
atm
P
gage
P
abs
P
abs
P vac
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Facts
Pressure is not a vector!
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Density
Compute the density of a substance, 2 kg of which occupies 0.4 cubic metre.
Density = Mass/Volume
Just a minute!
Will the density be different if we
consider 1 kg of mass
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Equation of state of an ideal gas
pv = RT
pV=mRT pV0=MoRT
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Gas constants
Air (28.97)
H2(2)
N2 (28)
O2 (32)
He (4) CO2 (44)
0.287
4.124
0.297
0.260
2.077 0.189
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M0 R = 8.314 kJ/kg mol K
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Try this!
A pressure bottle stores 5 cubic
metre of an inert gas at a
pressure of 10 MPa andtemperature 300K. Make
calculations for the mass, density
and specific volume of the gas.Molecular mass is 28.
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Ans: m = 561.3 kg
Density = 112.26 kg/cubic m
Specific volume = 0.0089 cubic
m/kg
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Try this!
Two spheres each of capacity 2 cum, are
connected by a pipe with a valve inserted
in between. Sphere 1 contains Oxygen at
50 kPa and 320K. Sphere 2 containsOxygen at 45 kPa and 290K. The valve is
opened and the entire system is allowed to
come to equilibrium. If temperature at thisequilibrium is 300 K, determine final
pressure neglecting volume of the pipe.
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Ans 45.94kPa