03 06 at Basic Concepts and Definitions

8/2/2019 03 06 at Basic Concepts and Definitions

1/33

www.pieas.edu.pk

BASIC CONCEPTS

AND DEFINITIONS

Dr. Jamil Ahmed


2/33

www.pieas.edu.pk Dr. Jamil Ahmed

2

Verily in the heavens and the earth, are Signs for those who believe.

And in the creation of yourselves and the fact that animals are scattered (through the earth), are Signs for those

of assured Faith.

And in the alternation of Night and Day, and the fact that Allah sends down Sustenance from the sky, andrevives therewith the earth after its death, and in the change of the winds,- are Signs for those that are wise.

Such are the Signs of Allah, which We rehearse to thee in Truth; then in what exposition will they believe after

(rejecting) Allah and His Signs?

Woe to each sinful dealer in Falsehoods:

He hears the Signs of Allah rehearsed to him, yet is obstinate and lofty, as if he had not heard them: then

announce to him a Penalty Grievous!

Al-Qur'an, 045.003-008 (Al-Jathiya)


3/33


3

Thermodynamics A branch of Physics

Thermodynamics = therme(heat) and dynamis(power)

A science of energy A science of energy and entropy

Deals with energy and energy transformations

Deals with the conservation of energy (quantity)during transformations

Deals with the direction of energy transformationprocesses (quality)


4/33


4

Some Application Areas


5/33


5



6/33


6



7/33www.pieas.edu.pk Dr. Jamil Ahmed

7




8




9

A Thermodynamic System



10

A Thermodynamic System comprises a device or a combination of devices containing a

quantity of matter that is being studied

To precisely define a thermodynamic system, a control volumeis chosen (a control boundary is drawn) such that it contains thedevices and the matter under investigation

Control mass with or without movable boundary



11


Open systems Control Volume



12


Types ofthermodynamic

systems

What can cross the boundary?

Heat Work Mass

Isolated System No No NoClosed System Yes Yes * No

Open System Yes Yes * Yes +

* May have moving boundaries+ May have imaginary boundaries

Control mass

Control volume



13

Macro- vs. Microscopic Approach

25 mm

25mm

1020 atoms

Monatomic gas

Atmospheric temperature and pressureTo specify using the microscopic point of view:

Position of an atom: 3 coordinates Velocity of an atom: 3 components

Description of complete system requires:6 1020 equations

A COMPUTATIONAL CHALLENGE!

Two approaches to reduce the number of simultaneous equations, so that the problembecome manageable, are:

1) STATISTICAL APPROACH: We deal with average values for all particles under

consideration. This is the approach used in kinetic theory and statistical mechanics

2) MACROSCOPIC VIEW: Here we are concerned with the gross or average effectsof many molecules. These effects can be perceived by our senses and measuredby instruments (i.e., the time-averaged influence of many molecules). Example:pressure of a gas exerted on the walls of a container. Macroscopic observations arecompletely independent of our assumptions regarding the nature of matter



14

Continuum

From the macroscopic point of view,

we are always concerned withvolumes that are very large comparedto molecular dimensions. Because weare not concerned with the behavior ofindividual molecules, we can treat thesubstance as continuous. This

concept of continuum loses validitywhen the mean-free path of themolecules approaches the order ofmagnitude of the dimensions of thevessel, e.g., in high-vacuumtechnology


15/33www.pieas.edu.pk

Properties and State of a Substance

A phase is defined as a quantity that is homogeneous throughout. For

example, phases of water are ice, water and vapor.

More than one phase can coexist, separated by phase boundaries. Ineach phase, the substance may exist at various pressures andtemperatures, or in thermodynamic terms, in various states.

The state may be identified or described in terms of a unique set ofcertain observable macroscopic properties, e.g., temperature,pressure, density, etc.

In a given state, each of the properties has only one definite value,and these properties always have the same value for a given state,regardless of how the substance arrived at the state.

Dr. Jamil Ahmed

15

,...,,,, 111111 suPTS



Properties of a System

The term properties of system implies that some properties have

significance for the entire system. This is true especially when thesystem is in thermodynamic equilibrium Thermal equilibrium: there is no temperature differential through the system

Mechanical equilibrium: there is no change in pressure in the system

Phase equilibrium: the mass of each phase reaches an equilibrium level and staysthere

Chemical equilibrium: the chemical composition of the system does not change withtime

Dr. Jamil Ahmed

16

Water

Vapor, P > 0

After some time

Water

Vapor, P = Pv

After long time

Water

Vacuumt= 0, P = 0

At t = 0



Intensive and Extensive Properties

Two types of thermodynamic properties: intensive and extensive.

An intensive property is independent of the mass, whereas the valueof an extensive property varies directly with the mass.

Extensive properties per unit mass are intensive properties

Dr. Jamil Ahmed

17

Intensive Extensive

Pressure Mass

Temperature Total Volume

Density


18/33

www.pieas.edu.pk

A Simple Compressible System

If we have a system where the gravitational,electrical, magnetic, motion and surface tensioneffects are absent, then this system is called a simplecompressiblesystem

Dr. Jamil Ahmed

18


19/33

www.pieas.edu.pk

The State Postulate

The state of a simple compressible systemis completelyspecified by two independent, intensive properties

Two properties are independent if one property can be varied

while the other one is held constant

The gravitational, electrical, magnetic, motion and surfacetension effects are usually negligible for most engineeringproblems. Otherwise, an additional property needs to be

specified for each effect that is significant.

Dr. Jamil Ahmed

19


20/33

www.pieas.edu.pk

Processes and Cycles When properties of a system change, we

say that a change in statehas occurred.

The series of states through which asystem passes during a process is calleda path

The path of the succession of statesthrough which the system passes iscalled the process.

To describe a process completely initialand final statesas well as the pathitfollows, and the interactions with thesurroundings should be specified

Dr. Jamil Ahmed

20

A process with identical end statesis called a cycle

Process diagrams plotted by employing thermodynamicproperties as coordinates are very useful in visualizing theprocesses.


21/33

www.pieas.edu.pk

Ideal and Non-ideal Processes

Ideal process: A process in which the

deviation from thermodynamic equilibriumis infinitesimal. All the states the systempasses through during a quasi-equilibriumprocess may be considered equilibriumstates. Also called quasi-equilibriumprocess

For non-equilibrium (non-ideal)processes, we are limited to a descriptionof the system before the process occursand after the process is completed andequilibrium is restored. We are unable tospecify each state through which the

system passes, or the rate of the process,however, we are able to describe certainoverall effects that occur during theprocess.

Dr. Jamil Ahmed

21

(b) Fast compression (non quasi-equilibrium)

20 pa

20 pa

90 pa

State 1

State 2

P

V

Non-equilibruim

process

20

90

?

(a) Slow compression (quasi-equil ibrium)

20 pa 20 pa

20 pa

20 pa 20 pa

State 1

State 2 Process path

P

V

Intermediate

states

20


22/33

www.pieas.edu.pk

Thermodynamic vs. Mechanical Cycle

Dr. Jamil Ahmed

22


23/33

www.pieas.edu.pk

Iso-processes

The prefix iso- is used to describe processes inwhich one property remains constant, e.g.,

Isothermal: constant temperature

Isobaric: constant pressure

Isochoric: constant volume

Dr. Jamil Ahmed

23


24/33


24

Units of Time, Length, Mass and ForceTime (second, s): One second isthe time required for a beam ofcesium-133 atoms to resonate

9 192 631 770 cycles in a cesiumresonator. (General Conference ofWeights and Measures CGPM,1967)

1 ft = 0.3048 m

Length(meter, m): One meter isthe length of the path traveled bylight in a vacuum during a time

interval of 1 / 299 792 458 of asecond. (CGPM, 1983)

Mass (kilogram, kg): The mass ofa certain platinum-iridium cylindermaintained under prescribedconditions at the InternationalBureau of Weights and Measures

1 lbm = 0.453 592 37 kg

1 lbf = 32.174 lbm 1 ft/s2

The SI units derived from propernouns use capital letters forsymbols; others use the lowercaseletters. The liter, with the symbol L,is an exception.

F = m a1 N = 1 kg 1 m/s2

gravitational constant, g= 9.806 65 m/s2

= 32.174 ft/s2


25/33


25

Units of Time, Length, Mass and Force

Dimensions SI EES Conversion

Length meter (m) foot/ feet (ft) 1 ft = 0.3048 m

Time second (s) second (s)

Mass kilogram (kg) pound-mass (lbm) 1 lbm = 0.45359 kg

Force newton (N) pound-force (lbf) 1 lbf = 4.448 N

Unit Force 1 kg 1 m/s2 32.174 lbm 1 ft/s21 lbm 32.174 ft/s2

Temperature degree Celsius (C)degree Fahrenheit

(F)C = (5/9) (F-32)

Absolute Temp.Kelvin (K)

K = C + 273.15Rankine (R)

R = F + 459.67K = (5/9) R


26/33


26

Energy

A fundamental concept

Energy is defined as the capability to produce an effect

Energy can be stored within a system and can be transferredfrom one system to another (e.g., heat)

From a molecular point of view, energy is stored as:a) Intermolecularpotential energy (forces between molecules)

b) Molecular kinetic energy (translational velocity of individual molecules)c) Intramolecular energy (energy within the individual molecules)

Electronic energy Nuclear energy Rotational energy Vibrational energy

From the macroscopic point of view, we are

concerned only with the energy that istransferred as heat, causing change inproperties (temperature, pressure) and thechange in total energy that the water containsat any instant.


27/33


27

Specific Volume and Density Specific Volume: Volume per unit mass, v

Density: Mass per unit volume, = 1/v

Specific volume vary with location in the gravitational field. As altitudeincreases, specific volume increases.

Specific gravity: the ratio of the density of a substance to the densityof some standard substance at specified temperature (usually water at4 oC)


28/33


28

Pressure

The term pressure is applicable for liquids and gases. For solids, the

term stress is applicable.

Pressure is defined as the normal component of force per unit area.

UNITS

1 Pa = 1 N/m2

1 bar = 105 Pa = 0.1 MPa

1 atm = 101 325 Pa = 14.696 lbf/in2


29/33


29

Gas Pressure and External Forces

The pressure exerted by the gas on all its boundaries is thesame (when the gas is in equilibrium state).

This pressure is fixed (determined) by the external forceacting on the piston, since the internal (PA) and the externalforces (Fext) must be balanced for the piston to remainstationary.

Alternately, if heat is supplied to the gas from the outside, thegas pressure will increase, but the piston will move outwardsto establish a force balance at a new equilibrium state.


30/33


30

Absolute and Gauge Pressure

Pressure and vacuum gauges read the difference between the absolute pressureand the atmospheric pressure existing at the gauge.

Pressure below, and slightly above atmospheric, and pressure differences (e.g.,across an orifice) are measured using manometers containing water, alcohol,mercury, oil and other fluids.

atmabsatmvac

atmatmabsgage

PPPP

PPPP

belowpressurefor

abovepressurefor


31/33


32

Equality of TemperatureTemperature is difficult to define exactly.

Sense of hotness or coldness, when we touch an object.

When a hot and a cold body are brought in contact, hot body gets colder and the cold body getshotter, and after sometime they both appear to attain a same level of hotness or coldness.

Our sense of hotness or coldness is very UNRELIABLE.

Because of these difficulties of defining temperature, we define equality of temperature. We say

that two bodies have equality of temperature if, when they are in thermal communication,no change in any observable property occurs.

t = 0 t = t

Electrical resistance

Physical dimensions

Height of Hg column


32/33


33

The Zeroth Law of ThermodynamicsThe Zeroth Law of Thermodynamics states that when two bodies have equality of temperature with a thirdbody, they in turn have equality of temperature with each other.

No change in thethermometer reading Equality of temperature exists between

the two blocks.

If the thermometer is according to astandard measuring scale, we canmeasure the temperature.

Note the thermometerreading

Therefore, both blocks arein thermal equilibrium withthe thermometer


33/33

34

Temperature Scales

The temperature of the Ice point is defined as the temperature of a mixture of ice and water that isin equilibrium with saturated air at a pressure of 1 atm.

The temperature of the Steam point is defined as the temperature of water and steam that is inequilibrium at a pressure of 1 atm.

Ice point Steam PointFahrenheit 32 212Celsius 0 100

Absolute Scale: K = oC + 273.15

Rankine Scale: R = oF + 459.67

Documents

03 06 at Basic Concepts and Definitions