Concepts(Lect1 6)

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Thermodynamics

Thermodynamics may be defined as the science

of energy Energy can be viewed as the ability to cause changes

therme + dynamis

First law is simply an expression of energy

principle

The second law asserts that energy has qualityas well as quantity and actual processes occur

in the direction of decreasing quality.



A substance consists of a large number of

particles called molecules. The properties

of the substance naturally depend on the

behavior of the particles.

The macroscopic approach to study of thermodynamics

provides a direct and easy way to the solution of engineering problems. This does not require a

knowledge of behavior of individual particles. This

approach is also named CL ASSIC AL

THERMODYN A

MICS

.A more elaborate approach, based on average behavior of large

groups of

individual particles is called

STATISTICAL THERMODYNAMICS or Microscopic approach

and is rather involved



Application areas



Systems and control volumes

A system is defined as a quantity of matter

or region in space chosen for study.

The mass or region outside the system iscalled the surroundings.

The real or imaginary surface that

separates the system from surrounding is

called the boundary.



SYSTEM

SURROUNDINGS

BOUND ARY



Characteristics of a boundary

The boundary of a system can be fixed or

movable.

The boundary is a contact surface sharedby both the system and surroundings.

Mathematically, the boundary has a zero

thickness and thus it can neither contain

any mass nor occupy any volume in

space.



Closed system

A closed system consists of fixed mass

and no mass can cross its boundary. But

energy, in form of heat or work, can crossthe boundary and the volume of a closed

system does not have to be fixed.

If , in a special case, even energy is not

allowed to cross the boundary that systemis called an isolated system.





Open system

An open system usually encloses a devicethat involves mass flow such as acompressor, turbine or nozzle.

Flow through these devices is studied byselecting the region within the device asthe control volume.

The boundaries of a control volume arecalled a µcontrol surface¶ and they can bereal or imaginary.







PROPERTIES OF A SYSTEM

Any Characteristic of a system is called aproperty.

Intensive properties are independent of the mass of system.

Extensive properties are those propertieswhose value depend on the size- or extent

of the system. Extensive properties per unit mass are

called specific properties.





Thermodynamic property

It refers to the characteristic describing the

condition or state of a system

± It is a measurable characteristic describing the

system. ± It has a definite unique value when the system is in a

particular state.

± It is dependent only on the state of the system; it does

not depend on the path or route, the system follows toattain that particular state.

± Its differential is exact.



Properties

Temperature

Pressure

Volume Colour

Composition etc

dP dZ



Properties

A given expression is a property if its

differential is exact

dP = M dx + N dy is exact if

Partial derivative of M wrt y =

Partial derivative of N wrt x



Properties

Which of the below listed terms areproperties?

p dv + v dp;

p dv;

v dp;

(v/T) dp + (p/T) dv subject to pv=RT

dT/T + p/T dv subject to pv = RT



Continuum

It is convenient to disregard the atomic nature of a substance and view it as a continuous,homogeneous matter with no holes, that is, a

continuum. The continuum idealization allows us to treat

properties as point functions and to assume theproperties vary continually in space with no jumpdiscontinuities.

This idealization is valid as long as the size of the system we deal with is large relative to thespace between the molecules.



Continuum

Oxygen

1 atm, 20 degree C

Void



Rarefied gas flow theory

Application limited to very high vacuums or

very high elevations

For atmospheric air at elevation of 100kmthe mean free path is 0.16m!







State postulate

The state of a simple compressible system

is completely specified by two

independent, intensive properties

T=300 K

v=0.9cum/kg



PROCESSES AND CYCLES

Any change that a

system undergoes from

one equilibrium state to

another is called a

process, and the series

of states through which

a system passes duringa process is called the

path of the process.

State 1

State 2



Quasistatic process

When a process proceeds in such a

manner that the system remains

infinitisimally close to equilibrium state at

all times, it is called a quasistatic or quasi-

equilibrium process. This can be viewed

as a sufficiently slow process.

QUASI-EQUILIBRIUM NON-QUASI-EQUILIBRIUM



PROCESSES AND CYCLES

Quasi-equilibrium process is an idealized

process and is not a true representation of

an actual process.

A non-quasi-equilibrium process is

denoted by dashed line between initial

and final states



Processes and cycles

The prefix iso- designates process for

which a particular property remains

constant

± Iso thermal process

± Iso baric process

± iso choric/iso metric process



A system is said to have undergone a

cycle if it returns to its initial state at the

end of the process. For a cyclic process

the initial and final states are identical.



The steady flow process

Steady: No change with time

Uniform: No change with location



Temperature and

the zeroth law of thermodynamics

Iron

150 C

Copper

20 C

Iron

60 C

Copper

60 C

Two bodies reaching thermal equilibrium after being brought into contact inan isolated enclosure



If two bodies are in thermal equilibrium

with a third body, they are also in thermal

equilibrium with each other.

Two bodies are in thermal equilibrium if

both have the same temperature reading

even if they are not in contact.

R H Fowler (1931)

Temperature and

the zeroth law of thermodynamics



TEMPER ATURE

IS ASSOCIATED WITH THE

ABILITY TO DISTINGUISH HOT

FROM COLD ZEROTH LAW IS THE BASIS OF

TEMEPRATURE MEASUREMENT



The working fluid

Characteristic

varying

Reference body

Thermometric

substanceThermometric

property

Temperature

THERMOMETER



ME ASUREMENT OF TEMPER ATURE

Note

X ± Thermometric property

(X): Temperature at X

METHODS



Temperature on linear scale

(X)= aX, where a is an arbitrary

constant

(X2)= [ (X1)/X1].X2



Temperature measurement methods

(X1)/ (X) = X1/X

(X2)/ (X) = X2/X

[ (X1)- (X2)] / [ (X)] = [X1-X2] / X



Temperature measurement methods

(X)=[{ (X1)- (X2)} / (X1-X2)]X



Temperature scales

Scale Ice point Steam point

Degree Celsius 0 100Degree Farhrenheit 32 212

Kelvin 273.15 373.15

Rankine 491.67 671.67



Try this

Same numeric value is obtained

in degree Centigrade and degree

Fahrenheit scale for a particular bath. In Kelvin scale and degree

Rankine scale what would be the

value of this temperature.



Try this

A new temperature scale in degree N

is designed with ice point 100 degree

N and steam point 400 degree N.What shall be the reading

corresponding to 150 degree C? At

what temperature both the Celsiusand the new temperature scale

reading would be same?



Temperature Scales

Temperature scales enable us to

use a common basis for

temperature measurements. These are based on easily

reproducible states called fixed

point.



Measurement of Temperature

Before 1954 two fixed points were used

±Ice point :Ice- Air saturated water

equilibrium at 1 atm.

±Steam point: Pure water-Pure

steam at 1 atm.



Temperature measurement

Limitations with 2 fixed point method

1. Difficulty of achieving equilibriumbetween ice and air-saturated water

2. Extreme sensitiveness of the steam point

to the change in pressure

Only one fixed point then!



Temperature measurement

methods (after 1954)

t = a Xt

a = t / Xt = 273.16/ Xt

= (273.16/Xt) X = (273.16) (X / Xt)

t = Triple point of water = 273.16 K



Types of thermometers

Constant

volume gasthermometer

= (273.16) (p/pt)




Constant

pressure gasthermometer

= (273.16) (V/V t)




Electrical

resistancethermometer

= (273.16) (R/R t)




Thermocouple = (273.16) (e/et)




Liquid-in-

glassthermometer

= (273.16) (L/Lt)



Fact: Gas is chosen as

the standardthermometric

substance



Pressure is defined as a normal

force exerted by a fluid per unit area



Do U compute the number of

molecules or count the collisionwith wall or use software program

to find the net rate of change of

momentum to get the pressure?

I use a

barometer or

manometer

to determine

it.



SI unit of pressure, i.e , Pascal is too

small. The pressure exerted by 102 gmobject on 1sqm table.

In practice we use kPa and MPa

Other units are 1 bar = 0.1 Mpa

1 atm = 1.01325 bar

1 atm =14.696 psi

1 kgf/cm2

=9.807X104

Pa



Pressure effects

Snow shoes are large

Sharp knives need lower effort

Discomfort for a man with excessiveweight while standing

Standing on one foot only!



Absolute, gage and vacuum pressure

P

atm

P

atm

P

atm

P

gage

P

abs

P

abs

P vac



Facts

Pressure is not a vector!



Density

Compute the density of a substance, 2 kg of which occupies 0.4 cubic metre.

Density = Mass/Volume

Just a minute!

Will the density be different if we

consider 1 kg of mass



Equation of state of an ideal gas

pv = RT

pV=mRT pV0=MoRT



Gas constants

Air (28.97)

H2(2)

N2 (28)

O2 (32)

He (4) CO2 (44)

0.287

4.124

0.297

0.260

2.077 0.189



M0 R = 8.314 kJ/kg mol K



Try this!

A pressure bottle stores 5 cubic

metre of an inert gas at a

pressure of 10 MPa andtemperature 300K. Make

calculations for the mass, density

and specific volume of the gas.Molecular mass is 28.



Ans: m = 561.3 kg

Density = 112.26 kg/cubic m

Specific volume = 0.0089 cubic

m/kg



Try this!

Two spheres each of capacity 2 cum, are

connected by a pipe with a valve inserted

in between. Sphere 1 contains Oxygen at

50 kPa and 320K. Sphere 2 containsOxygen at 45 kPa and 290K. The valve is

opened and the entire system is allowed to

come to equilibrium. If temperature at thisequilibrium is 300 K, determine final

pressure neglecting volume of the pipe.



Ans 45.94kPa

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Concepts(Lect1 6)