Chap 1 Intro to Thermodynamic

8/13/2019 Chap 1 Intro to Thermodynamic

1/21

Chapter 1

INTRODUCTION ANDBASIC CONCEPTS


2/21

2

Objectives

Review the metric SI and the English unit systems.

Explain the basic concepts of thermodynamics such

as system, state, state postulate, equilibrium,

process, and cycle.

Review concepts of temperature, temperature scales,

pressure, and absolute and gage pressure.


3/21

3

THERMODYNAMICS AND ENERGY

Thermodynamics:The science of

energy. Energy:The ability to cause changes.

The name thermodynamicsstems fromthe Greek words therme (heat) anddynamis (power).

Conservation of energy principle:During an interaction, energy can changefrom one form to another but the totalamount of energy remains constant.

Energy cannot be created or destroyed.

Thermodynamics gives the criteria for aprocess to take place; it does NOT dealwith the rates of chemical and physicalproperties. Energy cannot be created

or destroyed; it can only

change forms (the first law).


4/21

4

SYSTEMS AND CONTROL VOLUMES System:A quantity of matter or a region

in space chosen for study.

Surroundings:The mass or regionoutside the system

Boundary:The real or imaginary surfacethat separates the system from itssurroundings.

The boundary of a system can be fixed ormovable.

Systems may be considered to be closedor open.

Closed system

(Control mass):A fixed amountof mass, and nomass can crossits boundary.


5/21

5

Open system(control volume):A properlyselected region in space.

It usually encloses a device that involvesmass flow such as a compressor, turbine, or

nozzle.

Both mass and energy can cross theboundary of a control volume.

Control surface:The boundaries of a controlvolume. It can be real or imaginary.

Isolated system: No mass and energyexchanged

An open system (a

control volume) with one

inlet and one exit.


6/21

6

PROPERTIES

OF A SYSTEM

Property:Any characteristic of asystem.

Some familiar properties arepressure P, temperature T, volumeV, and mass m.

Properties are considered to beeither intensive or extensive.

Intensive properties:Those thatare independent of the mass of asystem, such as temperature,pressure, and density.

Extensive properties:Thosewhose values depend on the sizeor extentof the system.

Specific properties: Extensiveproperties per unit mass.

Criterion to differentiate intensive

and extensive properties.


7/21

7

STATE AND EQUILIBRIUM

Thermodynamics deals withequilibrium states.

Equilibrium:A state of balance.

In an equilibrium state there are nounbalanced potentials (or drivingforces) within the system.

Thermal equilibrium:If thetemperature is the same throughoutthe entire system.

Mechanical equilibrium:If there isno change in pressure at any pointof the system with time.

Phase equilibrium:If a systeminvolves two phases and when themass of each phase reaches anequilibrium level and stays there.

Chemical equilibrium:If thechemical composition of a systemdoes not change with time, that is,no chemical reactions occur. A closed system reaching thermal

equilibrium.

A system at two different states.


8/21

Systems of Units

8

Systems of

Units

StandardInternationalSystem

AmericanEngineeringSystem

CentimetreGram SecondSystem


9/21

Systems of units

9

System/

Dimension

Length

Mass

Moles

Time

Temperature

SI

meter, m

kilogram, kg

gram-mole,

mole

second, s

Kelvin, k

American

foot, ft

pound-mass,lbm

pound mole,

lb-mole

second, s

Rankine, R

CGS

centimeter, cm

gram, g

gram-mole,

mole

second, s

Kelvin, K


10/21

Multiple SI Units

10


11/21

Derived SI Units

11


12/21

12

Temperature Scales

All temperature scales are based on some easily reproducible statessuch as the freezing and boiling points of water: the ice point and thesteam point.

Ice point:A mixture of ice and water that is in equilibrium with airsaturated with vapor at 1 atm pressure (0C or 32F).

Steam point:A mixture of liquid water and water vapor (with no air) inequilibrium at 1 atm pressure (100C or 212F).

The reference temperature in the original Kelvin scale was the icepo in t,273.15 K, which is the temperature at which water freezes (or

ice melts).

The reference point was changed to a much more precisely

reproducible point, the t r ip le poin tof water (the state at which all

three phases of water coexist in equilibrium), which is assigned thevalue 273.16 K.


13/21

Temperature Scales

13


14/21

Example 1

Water boils at 212oF at one atmosphere

pressure. At what temperature does water boil

in oC.

14


15/21

Force (F)

Definition-the force acting on a body is proportional tothe product of the mass and the acceleration in the

direction of the force.

Newtons Law: F=ma

SI unit- newton (N)

1 N-the force applied to a mass of 1 kg to produce an

acceleration of 1 ms-2

English unit-pound force (lbf)

1 lbfthe force applied to a mass of 1 pound mass (lbm)

to produce an acceleration of 32.174 ft/s215


16/21

16

PRESSURE

The normal stress (or pressure) on the

feet of a chubby person is much greaterthan on the feet of a slim person.

Pressure:A normal force exerted

by a fluid per unit area

68 kg 136 kg

Afeet=300cm2

0.23 kgf/cm2 0.46 kgf/cm2

P=68/300=0.23 kgf/cm2

F

A

P=F/A

SI unit: [P]=[F]/[A]=N/m2=1 Pa

English unit: [P]=[F]/[A]=lbf/in2

=1 psi


17/21

PRESSURE Additional units

1 torr- the pressure equivalent to a column of Hg of 1 mmat T=0oC in a standard gravitational field

1 atm- the approximate average pressure exerted by the

earths atmosphere at sea level 1 psig Pgauge=PabsolutePbarometric

1 bar =105Pa = 0.986923 atm

1 mmHg =1 torr =133.322 Pa

1atm = 760 torr

1 bar =750.061 torr =14.5 psia17


18/21

18

Absolute pressure:The actual pressure at a given position. It ismeasured relative to absolute vacuum (i.e., absolute zero pressure).

Gage pressure:The difference between the absolute pressure andthe local atmospheric pressure. Most pressure-measuring devices are

calibrated to read zero in the atmosphere, and so they indicate gagepressure.

Vacuum pressures:Pressures below atmospheric pressure.

Throughout

this text, the

pressure P

will denote

absolute

pressure

unless

specifiedotherwise.


19/21

Example 2

An object at sea level has a mass of 400 kg

a) Find the weight of this object on earth

b) Find the weight of this object on the moonwhere the local gravitational acceleration is

one-sixth that of earth.

19


20/21

Example 3

A vacuum gage connected to a tank reads 30

kPa at a loation where the atmospheric

pressure is 98 kPa. What is the absolute

pressure in the tank?

20


21/21

21

Summary

Importance of dimensions and units

Some SI and English units Systems and control volumes

Properties of a system

TemperatureTemperature scales

Pressure

Chap 1 Intro to Thermodynamic

Documents

Transcript of Chap 1 Intro to Thermodynamic