Basic Concepts of Thermodynamics

ObjectiveObjective

• Definition of Thermodynamic System

• Concepts of State and its properties

• Pressure, Temperature and Specific Volume

• Quasi-static or Quasi-equilibrium Process

and Reversible Process

• Thermodynamic Cycle

Thermodynamic System

System—A quantity of matter or

a region in space bounded by an

arbitrary surfaces for study.

Surrounding—The mass or regi

on outside the system.

Boundary—The surfaces separ

ate the system from its surroundi

ngs.

The boundary can be real or imaginary, fixed or variable.

–It is critically important to define your system before

attempting to solve a thermodynamic problem.

Thermodynamic System

Closed system—No mass can cross its boundary. It consists of a fixed amount of mass. (also known as control mass)

Open system —A selected region in space. Both mass and energy can cross its boundary. (also known as control volume)

Thermodynamic System

–Simple Thermodynamic System

Adiabatic System —No heat cross its boundary.

SurroundingsHeat

One Type Work

Isolated System— Nether mass nor energy can cross its boundary.

Thermodynamic System

Single Composition System —Consists of only one kind

of substance.

Multi-Composition System —Consists of tow or more

kinds of substances.

Single Phase System —Consists of only one phase.

Multi-phases System —Consists of tow or more

phases.Homogeneous System —The composition and property are identical all over the system.

Thermodynamic State

Thermodynamic state of a system –the condition of the system as characterized by the values of its properties.

Property –any characteristic of a system

• Basic Property

–can be directly measured

• Derived Property

–indirectly calculated from basic ones

Pressure

ForcePressure=

Area

[ ]2

lbfpsi

in=

[ ]2

NPa

m=

[ ]torr mmHg=

[ ] 5bar 10 Pa=

Common Use Units:

English

SI

Other

Pressure Measurement

Gage and Absolute Pressure Scales

We commonly use three different kinds pressure scales (not

units).

Absolute — is relative to full vacuum (absolute zero).

Gage (Gauge) — is relative to the ambient pressure.

Used for pressure above the ambient pressure.

Vacuum — is relative to the ambient pressure.

Used for pressure below the ambient pressure.

g a b

v b a

p p p

p p p

= -

= -

Gage and Absolute Pressure Scales

0ap =

bp

vp

ap

bp

Absolute

Vacuum

bp

ap

gp

Absolute

Vacuum

0.1MPa 14.5psiap = =

Temperature

• Two objects in thermal equilibrium are at the same

temperature.

•Zeroth law — if two bodies are in thermal equilibrium

with a third body, they are also in thermal

equilibrium with each other. Thermometers — measure the temperature

Celsius (Centigrade) scale and Fahrenheit scale —Two points scale — Dependent of physical properties of substances

Temperature Scale

ice point

steam point

0℃ F32O

100℃ F212O

Celsius Fahrenheit

o o( F) 1.8 ( C) 32T T= +

Temperature Scale•Thermodynamic temperature scale— based on the principle that at low pressure, the temperature of gas is proportional to its pressure at constant volume.— One point scale— independent of property of substance

HydrogenHelium

Temperature Scale

so that: T = a + b P

For gases at low pressures (ideal gasses)

For Celcius scale, a=-273.15

For Fahrenheit scale,a=-459.67

Temperature Scale

Four temperature scales are

in common use:

–Rankine

–Fahrenheit

–Kelvin

–Celsius

UnitAbsolute Relative

SI Kelvin Celsius

English Rankine Fahrenheitdiffer in:- dependent / independent of property of substance- location of the zero points- size of the degree unit

Temperature Scale Comparisons

o

o

o o

o

o

o

(K) ( C) 273.15

(R) ( F) 459.67

( F) 1.8 ( C) 32

(R) 1.8 (K)

(K) ( C)

(R) ( F)

(R) 1.8 ( C)

T t

T t

t t

T T

T t

T t

T t

= +

= +

= +

=

D =D

D =D

D = D

Specific Volume

Specific Volume 3/ (m )v V m=

31/ (kg/m )vr =

Density

vv

Characteristic of State Property

Mathematical

1

2A

C

B

integral2

2 11

0

f df f f

df

D = = -

=

ò

ò

differential

2 2

( ) ( )y x

f fdf dx dy

x y

f f

x y y x

¶ ¶= +

¶ ¶

¶ ¶=

¶ ¶ ¶ ¶

intensive and extensive

Characteristic of State Property

–Intensive property: a property that is independent of the mass of the system.

• Example –temperature, pressure, density, …

–Extensive property: a property whose value is proportional to the mass of the system.

• Example – volume, total enthalpy, energy ,…

–Specific property: An extensive property per unit mass. Specific properties are intensive.

• Example –specific volume (= volume/mass)

Equilibrium

•Sufficient and Necessary Conditions for Equilibrium: –Thermal: the temperature is the same throughout the system. –Mechanical: the pressure is the same. –Phase: there is no driving force for the total mass in each phase to change. –Chemical: there is no driving force for chemical composition to change.

Stable equilibrium state —a state in which the system is not capable of spontaneous change to another state without a finite change in the surroundings.

—There are no driving forces to carry out a change.

State Equation

We need only specify a certain number of properties to

fix the state of system. The exact number depends on compositions and phases

of system?

State postulate(相律）

2f a b= - +f

a

b

-

-

-

number of independent properties

number of different compositions of system

number of different phases of system

State equation ( , , ) 0f p v T =

Property Diagram

1p

2p

1 Initial state

2 Final state • Any point illustrates the state of system.• The state must be Equilibrium.

Thermodynamic Process

Thermodynamic Processes—transformations from one equilibrium state to another. Path—The series of states through which the system passes.

1P

2P

1 Initial state

2 Final state

path

Quasi-Equilibrium Process

1P

2P

1 Initial state

2 Final state

P

V1V2V

System remains infinitesimally close to an equilibrium state at all times.

Process occurs slow enough to keep the properties identical throughout the system.The interval of process change takes much longer time than system spontaneously adjusts to a new state after the previous equilibrium state was destroyed.

Quasi-equilibrium Process

To connect states with line must be QE process.

Reversible Process

Reversible process: the reverse process could be

performed so that the system and surroundings can be

restored to their initial states with no change in the

system or surroundings.

Sufficient and necessary conditions:•Quasi-equilibrium process• no dissipation in process

-friction, non-elastic deformation, resistance…

Thermodynamic Cycle

Cycle –a collection of processes that ultimately lead to the system being returned to the original state.

P

V

•clockwise

heat work

•Anti-clockwise

work heat