Lecture 1.Pptx(2)

8/13/2019 Lecture 1.Pptx(2)

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CHAPTER 1CHAPTER 1

BASIC THERMODYNAMICSBASIC THERMODYNAMICS

CONCEPTSCONCEPTS

Anita Bt. Abu BakarSchool of Engineering

[email protected]


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Definitions of Thermodynamics

Basic Applications of Thermodynamics

System, Boundary and Surrounding

Control Volume and Control Mass

Properties, Intensive and Extensive Properties

Equilibrium and Quasi-EquilibriumState, Path, Process and Cycle

Simple Compressible Substance

Pressure and Temperature

OUTLINE

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Definitions of Thermodynamics

Thermodynamicsis the science that primarily

deals with energy

Energy => Ability to cause Change

Science that deals with heat and work and

properties of substance that bear a relation withheat and work

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1st and 2nd Laws of Thermodynamics

Thefirst law of thermodynamicsis simply an

expression of the conservation of energy principle,and it asserts thatenergyis a thermodynamic

property.

Thesecond law of thermodynamicsasserts that

energy hasqualityas well asquantity, and actual

processes occur in the direction of decreasing

quality of energy.

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Applications of Thermodynamics

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Mass (kg), Time (s), Temperature (K), Electric Current(A), Amount of Light (c), Amount of Matter (mol)

Dimensional Homogeneity~ Every term in an equation

must have the same unit for the equation to be

physically correct

Dimensions and Units

Physical quantities can be characterized byDimensions

Magnitudeassigned to dimension is calledUnitsEnglish System ~ Still in use in U.S.A.

SI System ~ Universally accepted worldwide

7 Fundamental Dimensions in SI Systsm ~Length (m),

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Dimensions and Units

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SI = International System of Units

SI and English units


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Prefixes for SI Units

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Unit Conversion

Example:

Lets convert 1 g/cm3 (SI) to lbm/ft3 (English)

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Thermodynamics Systems

Thermodynamicssystemis defined as aquantity

of matter or region in space chosen for study

The mass or region outside the system is called the

surroundings

System boundaryis the real and imaginary surface

that separates the system from the surrounding.Boundarycan be fixed or movable

May beclosedoropen

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Closed System/Control Mass

A system of fixed mass is called aclosed system,

orcontrol mass

Theclosed system boundarydoes not have to be

fixed

No mass can cross theclosed systemboundary

Energy in the form ofheatandworkcan cross theclosed system boundary

If even energy is not allowed to cross we have an

isolated system

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Closed System/Control Mass

Energy, not mass, crossesclosed-system boundaries

Closed system with moving boundary

Figure 1 Close system:

piston-and-cylinder

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Open System / Control Volume

A system that involves mass transfer across its

boundaries is called anopen system, orcontrol

volume

The boundaries of acontrol volumeis called

control boundariesand is fixed in shape and

position

Energy in the form ofheatandworkas well as

mass can cross thecontrol boundaries

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Open System / Control Volume

Mass and Energy Cross Control Volume Boundaries

Figure 2 Open system: water heater

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Form of Energy

The sum of all forms of energy of a system is calledTotalEnergy, which is considered to consist of internal, kinetic, and

potential energies.E = U +mV2

/2 + mgz Internal energyrepresents the molecular energy of a system

and may exist in sensible, latent, chemical, and nuclear forms.Represented by symbol,U.

Kinetic Energyis the energy that a system possesses as aresults of its motion relative to some reference frame.

KE =mV2/2

Potential Energyis the energy that a system possesses as aresults of its elevation in a gravitational field.PE = mgz

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Systems Internal Energy

Systems Internal Energy = Sum of Microscopic Energies

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Energy Interaction

Form of Energy not stored in a system

Occurred atSystem Boundary

In the form ofHeat TransferorWork Transferor

Mass Transfer

Forcontrol mass, if thedriving forcefor the

interaction is temperature then the interaction isheat transfer otherwise it is work transfer

Forcontrol volume~ can also involve mass

transfer

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Properties of A System

Propertiesare any measurable characteristics of a

system. eg. Pressure, temperature, volume, mass and

density.

Extensive propertiesare the mass-dependent

properties of a system. i.e. the properties that will vary

proportionally with mass of the system. E.g. volume

Intensive propertiesare the properties that are not

dependent on mass. Eg. Temperature, density. If any

Extensive Propertyis divided by the mass we would

also obtain an intensive property.

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Intensive and Extensive Properties

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State of a System

Definition - A set of properties that completely

describe the conditions or characteristics of a

system

At a given state, all the properties of a system

have fixed values

State of a system will change when the propertiesof a system change

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Thermodynamic Equilibrium

Thermodynamicsdeals withEquilibrium States

A system is said to be inthermodynamic equilibrium

if it maintains thermal, mechanical, phase, andchemical equilibrium.

Thermal Equilibrium =>Temperatureis the same

throughout the system

Mechanical Equilibrium=>Pressureis the samethroughout the system

Phase Equilibrium=>No phase changeprocess in the

system

Chemical Equilibrium=>No chemical reactions 21


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Process, Path and Cycle

Process- Any change that a system undergoes

from one equilibrium state to another is called a

process.

Path- The series of state through which a system

passes during a process is calledapath

Cycle- A process with identical end states is

called acycle.

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State, Path, Process and Cycle

Compressed Process P-V Diagram

Each Point Along the Path is in

Quasi-Equilibrium State

If the Process returns to its initial

State then we have a Cycle

If the Outgoing and ReturningPaths are Different ~ Net work is

Produced (+ve or -ve)23


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Pressure Pressure is defined as force per unit area

The SI unit of pressure is Nm-2, also known as Pascal (Pa)

Theabsolute, gage, and vacuum pressures arerelated by

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

Gage pressure: The difference between the absolute pressure and the localatmospheric pressure. Most pressure-measuring devices are calibrated to read

zero in the atmosphere, and so they indicate gage pressure.

Vacuum pressures: Pressures below atmospheric pressure.


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Figure 2: In

stacked-up fluid

layers, the

pressure change

across a fluid

layer of density

and height h

is gh.

Figure 1:The basic manometer.

It is commonly used to measure small and moderate pressure differences. A

manometer contains one or more fluids such as mercury, water, alcohol, or oil.

1. Manometer,

Pressure Measurements


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Atmospheric pressure is measured by a device called abarometer; thus, theatmospheric pressure is often referred to as the baromet ric pressure.

A frequently used pressure unit is thest andard at mosphere, which is defined

as the pressure produced by a column of mercury 760 mm in height at 0C

(Hg = 13,595 kg/m3) under standard gravitational acceleration (g = 9.807

m/s

2

).

The basic barometer.

Pressure Measurements

1. Bar ometer


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A vacuum gauge connected to a chamber reads 40 kPa at a location where the

atmospheric pressure is 100 kPa. Determine the absolute pressure in thechamber.

Pabs

= Patm

- Pvac

= 100 - 40

= 60 kPa

Example 1: Absolute Pressure of a Vacuum Chamber

Solution


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Example 2 : Measuring Pressure with a Manometer

A manometer is used to measure the pressure in a tank. The fluid used has a

specific gravity of 0.85, and the manometer column height is 55 cm, as shown

in figure. If the local atmospheric pressure is 96 kPa, determine the absolutepressure within the tank.

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OH kg/m850)0kg/m(0.85)(100)SG( 2

ghPP atm

= 96

kPa

850

kg/m3

9.81

m/s20.5

5

m

1 N 1 kPa

1

kg.m

/s2

1000

N/m2

= 100.6 kPa

Solution:


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Temperature and Zeroth Law of Thermodynamics

Temperatureis a measure ofhotnessorcoldness

Thezeroth law of thermodynamicsstates that twobodies are inthermal equilibriumif both have the

same temperature readingeven if they are not in

contact.

Basis for validity of Temperature Measurement More fundamental than 1st and 2nd Laws of

Thermodynamics

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Temperature Scale

Temperature scales used in the SI system areCelsiusandKelvin.

The absolute Temperature Scale in SI isKelvinand is related to

Celsius by

And

Temperature scale used in the English system areFahrenheitand

Rankine. The absolute temperature scale isRankineand relatedtoFahrenheitby

And T(R)= T(oF) 30


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Temperature Scale Comparison

1 K = 1oC = 1.8 R = 1.8oF 31

Boiling point of pure water at

standard atmospheric pressure

Freezing point of water

saturated with air at standard

atmospheric pressure

Lower limit of temperature

Relations among temperature

scales


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Self Exercises

1.A manometer containing oil ( = 850 kg/m3) is attached to a

tank filled with air. If the oil-level difference between the two

columns is 36 cm and the atmospheric pressure is 98 kPa,

determine the absolute pressure of the air in the tank.

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AIR

Patm= 98 kPa

0.36 m


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2. The water in a tank is pressurized by air, and the pressure is

measured by a multi-fluid manometer, as shown in the figure.

Determine thegauge pressureof air in the tank at point 1,

P1,gau ifh1 = 0.2 m, h2 = 0.3 m and h3 = 0.46 m.

Given that; Densities of water, oil and mercury to be 1000

kg/m3, 850 kg/m3 and 13,600 kg/m3, respectively. Patm =

101.325 kPa. Acceleration of gravity,g = 9.81 ms-2

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Air

Water

Oil

Mercury

h1

h2

h3

Patm

1

2


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3. The absolute pressure of the below system is measured to

be 80 kPa. Determine the differential height, h of the

mercury column. SG for water is 1.


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4. A MULTIFLUID container is connected to a U-tube, as

shown in figure. For the given specific gravities and fluid

column heights, determine the gage pressure at A. also

determine the height of a mercury column that would createthe same pressure at A.


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ENDEND

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Lecture 1.Pptx(2)