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