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 System
and 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
A quasi-equilibrium process
is one in which the deviation from thermodynamic
equilibrium is infinitesimal.
Fig. 2.3
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.
2.5
Units for Mass, Length, Time, and Force
2.6 Energy

An Important Concept

Definition：
the capability to produce an effect
– From molecular view, it can be transferred
and stored.
Potential energy in the coordinate
system for a diatomic molecule
Fig. 2.4
Heat transfer to water
Fig. 2.5
2.7
Specific Volume and Density
The definition and continuum limit for
the specific volume and (specific) density

Specific volume

Density
ρ=
1
v
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 scales
oC
 Kelvin Scales
K
 Relation
K = oC +273.15
