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 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 Cycles Path, i = j 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 All the states the system passes through during a quasi-equilibrium process may be considered equilibrium states. 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 ρ=

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