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Concepts and Definitions
Thermodynamics
The science of energy and entropy
the science that deals with heat and work and the
properties of substances that bear a relation toheat and work
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Concepts and Definitions
Stems from the greek word therme (heat) and
dynamis (power)
basis is experimental observation and formalized
into basic laws which are the First, Second, Third,and Zeroth laws of thermodynamics
the word thermodynamics was first used in a
publication by Lord Kelvin in 1849
The first textbook was written in 1859 by William
Rankine, at the University of Glasgow
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Applications of Thermodynamics
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The Thermodynamic System
A system is defined as a quantity of matter or
a region in space chosen for study. The mass
or region outside the system is called the
surroundings. The real or imaginary surfacethat separates the system from its
surroundings is called the boundary.
The extent of the system in space at any giventime is defined by the system boundary
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The Thermodynamic System
The envelope that represents the systemboundary which encloses the thermodynamicsystem is also known as the system control
surface The boundary can be fixed or movable
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The Thermodynamic System
Types of System
1) Closed System
2) Open System
3) Isolated System
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Closed System
Also known as control mass(amount of matter
inside control remains constant with time)
consists of a fixed amount of mass, and no
mass can cross its boundary. That is, no mass
can enter or leave a closed system, but energy,
in the form of heat or work, can cross the
boundary
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Closed System
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Open System
is a properly selected region in space. It
usually encloses a device that involves mass
flow such as a compressor, turbine, or nozzle.
Both mass and energy(in the form of work
and/or heat) can cross the boundary.
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Open System
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Open System
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Open System
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Isolated System
A system that is not influenced in any way by
the surroundings or environment no mass
and energy flow across the system boundary
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Macroscopic vs. Microscopic
Microscopic Point of View
System behavior is described by describing the
behavior of each molecule which comprise the
system Governing equations are written for each
molecule, e.g., equations for position, velocity,
etc.
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Macroscopic vs. Microscopic
Macroscopic Point of View The gross/average effects or time-averaged
influence of many molecules is used to describesystem behavior
Uses measurable parameters, e.g., pressure,temperature, etc. System volume should be very large compared
with molecular dimensions(System shouldcontain many molecules)
System is treated as continuous, disregardingthe action of individual molecules
Is completely independent of the assumptionregarding the nature of matter
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Properties and State of a Substance
Phase
A quantity of matter that is homogenous
throughout; solid, liquid, gas
When more than one phase is present, eachphase is separated by phase boundaries
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State of a Substance
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StateIndentified or described by certain
observable, macroscopic properties
called properties
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Property of a Substance
Any quantity that depends only on the state of
the system
Independent of the path by which the state is
arrived at.
*Given a state , each property has only one
definite value
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Property of a Substance
Some familiar properties are pressure P,temperature T, volume V, and mass m. The listcan be extended to include less familiar ones
such as viscosity, thermal conductivity, modulusof elasticity, thermal expansion coefficient,electric resistivity,and even velocity andelevation.
A property of a system has significance for theentire system only when the system is inequilibrium.
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Property of a Substance
2 General Class of Properties
Intensive Properties
independent of mass
Examples: Pressure, Temperature, Color, Odor,Ductility etc.
Extensive Properties
dependent of mass
Examples: Mass, Weight, Volume, Length
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Processes and Cycles
Occurs when a change in property occurs
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 during a process is
called the path of the process
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Processes
Non-Quasi-Equilibrium Process
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Processes
Quasi-Equilibrium Process(ideal process)
When a process proceeds in such a manner that
the system remains infinitesimally close to an
equilibrium state at all times.
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Processes
The prefix iso- is often used to designate a
process which a particular property remains
constant.
Isothermal-Constant Temperature
Isobaric(Isopiestic)-Constant Pressure
Isochoric(Isometric)-Constant Volume
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Cycle
a series of processes, one after the other, such
that the initial and final states are the same
initial and final system compositions are
similar.
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Units for Mass, Length, Time, and Force
SI Units
Time second ( s )
Length meter ( m )
Masskilogram ( kg )
Forcenewton ( N )F = m a
1 (N) = 1 (kg) x 1 (m/s2)= 1 (kg-m/s2)
English Units
Time second ( s )
Length foot ( ft )
Mass pound mass ( lbm )
Force pound force ( lbf )F = m a
1 (lbf) = 1 (lbm) x 32.174 (ft/s2)= 32.174 (lbm-ft/s2)
For locations where theacceleration is different from
32.174 ft/s2 , the force or weight is F = m a / gc
Where gc = 32.174 lbm-ft/lbf-s2
Weight - is correctly used only asa force
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SI and English Units
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Mass v.s. Weight
The mass of a body remains the
same regardless of its location in
the universe. Its weight, however,
changes with a change in
gravitational acceleration
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Density and Specific Volume
Density is mass per unit volume
Specific Volume is the reciprocal of density
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Specific Gravity and Specific Weight
specific gravity, or relative density, and is defined as the ratio
of the density of a substance to the density of some standard
substance at a specified temperature.
Substances with SG of less than 1 are lighter than water, thus
they would float on water
The weight of a unit volume is called specific weight
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Pressure
Of a liquid or gas is defined as the normal component offorce per unit area
where
A = a differential area of a systemA= smallest area over which the fluid can be consideredas a continuumFn = component of force normal to A
Typical units,SI: 1 Pascal (Pa) = 1 Newton / m2 (N/m2)
English: pound-force / ft2 (lbf/ft2), pound-force / in2(lbf/in2)Others: 1 bar = 105 Pa = 0.1 Mpa
atm = 101,325 Pa = 14.696 lbf/in2
'
limn
A A
FP
A
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Fluid pressure in relation to a movable boundary
Under equilibrium conditions,pressure P exerted by the gas onall its boundaries is the same
With no heat transfer, the pressureis fixed by the external force Fextacting on the piston ; also, Fext =
Pressure x Piston Area (from FBDof piston)
Heating/cooling of the gas tends toincrease/decrease pressure andmove piston to the right/left suchthat Pressure x Piston Area = Fext
is satisfied.
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Pressure
Pressure is typically measured or indicated relative toeither of two references which are
Atmospheric Pressure typically sea level pressure atstandard conditions; measured by a barometerGauge pressure - indicates how much actual pressureis above atmospheric pressure; measured by apressure gauge
Vacuum pressure - indicates how much actualpressure is below atmospheric pressure; measured bya vacuum gauge
Absolute Zero Pressure zero pressure or perfectvacuum; measured by an absolute pressure gauge orcalculated from gauge/vacuum pressure
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Pressure
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Pressure Measurement
Using Dial Gauges
Consider the following
a. If Pi > Po ,
Pi = Po + Pg If Po = Patm , Pi, abs = Patm + Pg
b. If Pi < Po ,
Pi = Po - Pvac If Po = Patm , Pi, abs = Patm - Pvac
Tube
side
Dial side
Pd
= pressure
reading
= Pg or Pvac
Po = pressure outside Compartment= ambient pressure
Pi
= pressure
inside
compartment
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Pressure Measurement Example
A manometer is used to measure the pressure in atank. The fluid used has a specific gravity of 0.85, and
the manometer column height is 55 cm, as shown in
the figure. If the local atmospheric pressure is 96 kPa,
determine the absolute pressure within the tank.
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Pressure Measurement Example
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Equality of Temperature
Two bodies have equality of temperature if, when
they are in thermal equilibrium, no change in any
observable property occurs.
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The Zeroth Law of Thermodynamics
When two bodies have equality of temperature with
a third body, they in turn have equality of
temperature with each other.
"IfA is in thermal equilibrium with B and ifB is in
thermal equilibrium with C, thenA is in thermal
equilibrium with C."