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Chapter 2
Some Concepts Definitions
Definition of Thermodynamics
The science of energy and entropy
The science that deals with heat and work and those properties of substances that bear a relation to heat and work
Basis of thermodynamics
Experimental observations (findings)
Formalize into certain basic laws(zeroth, first, second, and third laws)
Objective of Student
To gain both
– a thorough understanding of the fundamentals, and
– an ability to apply these fundamentals to thermodynamic problems
2.1 Thermodynamic Systemand Control Volume
Thermodynamic System– It contains matter and device(s) in a control
surface.– Its boundaries may be closed (fixed mass/
control mass) or open ( involving a flow of mass), and may be movable or stationary.
– It may have flows of energy in term of heat transfer and work across it.
Isolated system –
No mass, heat, or work across the boundary.
Control Mass
Control Volume
2.2 Macroscopic vs Microscopic Point of View
Macroscopic Point of View- concerned with the Gross or Average Effects of many molecules– Continuum
Microscopic Point of View - Classical and Statistical approach
2.3 Properties and State of a Substance Definition of a Phase
– Phase is a quantity of matter that is homogeneous throughout.
Phase of a Substance– Substance may exist in a different state which involves
observable and macroscopic properties
– It may be regarded as a function of some statei, therefore Phasej = f (statei) = f(Pi, Ti , Densityi,…, etc. )
• Factors, like Pi, Ti , Densityi,…, etc., are all properties of some statei.
Each property of a substance in a given state has only one(same) definite value.
– Regardless of how the substance arrive at the state (path).
Thermodynamic Equilibrium
Equilibrium is a state when system properties are invariable.
Thermodynamic Properties
Intensive Property
– independent of mass, like P, T, and density
Extensive Property
– varies directly with mass, like total volume
Extensive properties per unit mass are intensive(specific) properties.
2.4 Processes and Cycles
Processes– Path of different states
– Statei (Pi,Ti ,Di,.) to Statej (Pj,Ti ,Dj,.) , i≠j
Cycles– Path, i = j
Quasi-equilibrium process
Fig. 2.3
A quasi-equilibrium process
is one in which the deviation from thermodynamic equilibrium is infinitesimal.
Quasi-equilibrium process
1. All the states the system passes through during a quasi-equilibrium process may be considered equilibrium states.
2. Many actual processes closely approach a quasi-equilibrium process and may be so treated with essentially no error.
2.5Units for Mass, Length, Time, and Force
An Important Concept
Definition :
the capability to produce an effect– From molecular view, it can be transferred
and stored.
2.6 Energy
Potential energy in the coordinate system for a diatomic molecule
Fig. 2.4
Heat transfer to water
Fig. 2.5
2.7Specific Volume and Density
The definition and continuum limit for the specific volume and (specific) density Specific volume
Density
v1ρ=
Density of common substances
2.8 Pressure
Fig. 2.8
Pressure Units
Pascal ( Pa )
1 Pa = 1 N/m2
bar
1 bar = 1x105 Pa = 0.1 MPa
atm
1 atm = 101325 Pa
Terms used in Pressure Measurement
Pressure measurement - using a column of fluid
2.9 Equality of Temperature
Two bodies have equal temperature
if, when they are in thermal communication, no
change in any observable property occurs.
2.10 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.
2.11 Temperature Scales
Celsius scalesoC
Kelvin ScalesK
RelationK = oC +273.15