chapter1-basic concepts of thermodynamics -w1

8/4/2019 Chapter1-Basic Concepts of Thermodynamics -W1

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Basic Concepts ofThermodynamics

Chapter 1


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What is Thermodynamics?

Basic science that deals with energy. We all

know that energy is neither created nor

destroyed but it is transformed.

Among other things well look at Power Production

Refrigeration

Relationships between the properties of matter


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The Science of Energy


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Two ways to look at systems

MacroscopicClassical Thermodynamics

Atomic or MolecularStatistical Thermodynamics


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Dimensions and Units

Dimensions are names that characterize

physical quantities.

Units are those arbitrary magnitudes and

names assigned to dimensions, which are

adopted as standards for measurement


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Weight vs. Mass

Mass is an amount of matterWeight is a measure of how much force

is applied to the matterYour mass is the same on the earth and

on the moonYour weight is different!We often are not careful to make a

distinction between mass and weight


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F=ma

m is mass

a is acceleration

On the surface of the earth we usuallycall the acceleration g g=9.8 m/sec2

g= 32.174 ft/sec2


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Weight on the surface of the

earth

F = m g

In metric the units becomeKg m/sec2 which is the same as

N

(Nx0.2248) lbf


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Systems

Quantity of matter or a region of spaceClosed SystemOpen System

Surroundings everything thats not thesystem

BoundaryStationaryMoving


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Closed System

System

E

E


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Open System

(Control Volume)

WaterHeat

er

Matter

Matter

Energy

Energy


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Analyzing Systems

The approach is different for closed and

open systems

Energy is a lot harder to handle thanmatter, because it exists in many forms


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Energy = E

Macroscopic formsRespect to some outside reference frame

MicroscopicRelated to the molecular structure


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Macroscopic Energy

Kinetic Energy (KE)KE = mV2/2

Potential Energy (PE)PE = mgz


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Other Kinds of Macroscopic

Energy

Magnetic

Electrical

Surface TensionThese are specialized, and we dont

usually need to include them


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Microscopic Energy

Kinetic energy of individual molecules

Potential energy of individual molecules

Binding forcesChemical Energy

Nuclear Energy

Etc


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Both macroscopicand microscopicforms of energy arestatic they can be

stored in a system


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PEKEUE ++=

2

2

2mVKE =

mgzPE =

mgzmVUE ++= 2

2

2


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A unit mass basis is often

more convenient

e = E/m

u = U/m

ke = KE/m = V2

/2pe = PE/m = gz

gzVupekeue ++=++=2

2


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Dynamic Energy

When energy moves from place to

place we treat it differently

The only forms of energy that can crossa system boundary without matter

transfer are:

Heat (Q)Work (W)


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Heat

A system can not contain heat

Heat only exists as energy crossing a system

boundary

What we think of as a systems heat content

is Thermal Energy

Heat is energy transferred through a

temperature differenceAll other forms of energy transfer are work!!


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Properties of a System

IntensiveDoes not depend on the systems size

Temperature

Pressure

ExtensiveDepends on the systems size

VolumeMass Total Energy


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We often define properties in

terms of other properties

Density = m/V

Specific Volume v = V/m = 1/

We can define most extensiveproperties per unit mass

Called specific properties u = U/m is the specific internal energy


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State

At a given state, all the properties of a

system have a fixed value

If you change a property, you havechanged the state

Thermodynamics deals with

equilibrium states


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Process Diagram

P

T

State1

State2

ProcessPath

P1

T1T2

P2


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State Postulate

The state of a simple compressiblesystem is completely specified bytwo independent, intensive

properties

Remember that during a phase change,

Temperature and Pressure are notindependent


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Iso

Many times we will talk about process paths

where one property is kept constant

Isothermal Constant Temperature

Isobaric Constant Pressure

Isochoric Constant Volume


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Pressure

The force exerted by a fluid per unit

area

Only meaningful for a gas or a liquid In solids we talk about stress


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A

FP=

2m

NPa =

One Pascal isnt verymuch!!

1 atm =101,325 Pa

Units of Pressure

1 atm = 101.325 kPa = 1.01325 bar= 14.7 psi

Note: psi is lbf / in2


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Absolute vs. Gage Pressure

Absolute pressures are measured

relative to a vacuumUsually we will talk about absolute

pressure, and will use absolute pressure in

our calculations

Gage pressures are measured relative

to the surroundings


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Absolute vs. Gage Pressure

Vacuum

Surroundings

Pressure tomeasure

psia

psig

Pat

m


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Devices to measure pressure

Bourdon tube

Barometer

Manometer


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Barometer

h

A

A

FP=

mgF=

Vm=

hAV =

hA=

ghA=

gh=

Note: Thepressuremeasured by abarometer isindependent of

cross sectional


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Barometer

P=ghFor a given barometer the density and

the acceleration due to gravity are

constants, soPressure is directly proportional to

height

Pressure is often measured in mmHg1 atm = 760 mmHg


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Manometer

Used to compare pressures

gas

Atmosphericpressure

2P ghPP atm +=2

ghPPP atm == 2


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Zeroth Law of

Thermodynamics

If two bodies are in equilibrium with a

third body, they are also in equilibriumwith each other

Basis for thermometers

chapter1-basic concepts of thermodynamics -w1

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