by nitin oke for safe hands

8/14/2019 By Nitin Oke for Safe Hands

1/31

RadiationByNitin Oke

For Safe Hands


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Flow of heat

Net Heat flow is from body with more heattemperature to body at less heat temperature

Flow of heat can take place in three ways Conduction

Conviction

Radiation


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Facts about flow-

Heat flow

ConvictionConduction Radiation

No Motion ofparticle

Slowest

Mostefficient

Motion ofparticle

Moderate

Moderateefficient

Electromagnetic wave

Fastest

Leastefficient


4/31

Facts about Radiation-

Heat flow

ConvictionConduction Radiation

No Motion ofparticle

Slowest

Mostefficient

Motion ofparticle

Moderate

Moderateefficient

Electromagnetic wave

Fastest

Leastefficient

Co

smic rays

Gamma rays

UV rays

Visible rays

IR rays

Micro waves

Radio waves


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More about Electromagneticspectrum


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Properties of heat radiation:

It exhibits the phenomena of reflection,refraction, interference, diffraction andpolarization.

Heat radiations can travel through vacuum andother transparent media.

Heat radiations do not affect the medium throughwhich they pass.

Heat radiation consists mostly of infrared rays,which are electromagnetic waves whose wavelengthrange from 8 x 10 7m to 4 x 10 4m.


8/31

Facts about Radiation-

Heat radiation is with wave length rangingbetween 10-6m to 10-3m in reality thewavelength of heat radiation ranges from8 x 10 7 m to 4 x 10 4 m where as the wave

length of visible light ranges from 4 x 10 7mto 8 x 10 7m. These waves are invisible to human

Frequency ranges from 1011

Hz to 1014

Hz


9/31

QTQR

What means a, r, t

If Q amount of hest isincident on surface.

QR is reflected

QA is absorbed

QT is transmitted

By law of conservation of

energy Q = QR+QA+ QT

Q

QA

Q

Qt,

Q

Qr

Q

Qa TRA === ,


10/31

Classification based on a, r and t

If t = 1 then substances are called asdiathermanous.

Examples of substances which are transparent toheat radiation are Quartz, glass, Rock salt, dry air, O2 , H2, NaCl,

CCl4, CHCl3 If t = 0 then substances are called as

athermanous. Examples of substances which are transparent toheat radiation are Water, Wood, C6H6, R-OH, Cu, Iron


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Facts about Radiation- To detect these waves Crooks radiometer or Boys

radiometer are used. Energy of radiation can be measured by Bolometer Heat Radiations spectrum was studied graphically by

Langley ( Not black body spectrum )

Spectrum of Black body was studied at differenttemperatures by Lummer and Prigsheim The black body used was constructed by Fery Wien found relation between temperature and maximum

corresponding wavelength.

Stefan and Boltzman related area under the curvemeans total heat and T4

The graph was explained by Max Plank using Quantumtheory.


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Observations of graph When radiations of Black body were studied at

different temperatures by Lummer andPrigsheim the observations were as follows The graph is different at different temperature As temperature increases the graph shifts up

The graphs maxima shifts backward as temperatureincreases. The area under the curve, means total energy per

unit area per unit time means emissive power isproportional to T4 (Stefans law)

The Emax is proportional to T5

The wavelength corresponding to Emax is inverselyproportional to T. max.T = b The value of b is0.2892 x 10-2mK ( Wiens displacement law).


16/31

Prevost theory of heat exchange

Every body continuously radiates heatenergy at all temperatures except absolutezero.

The amount of radiant energy emitted perunit time depends only on absolutetemperature of body and NOT on

surrounding temperature.


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Hot

Heat exchange is as --

Less Hot

Hot

More Hot


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Kirchhoffs Law of radiation

The coefficient of absorption is same as coefficient ofemission.

Theoretical proof of Kirchhoffs law


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Ritchies Experiment

aAEb 1xAE

aAEb = 1.A.E

a =E/Eb = e


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Ritchies Experiment


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Stefans Law and its applications

The radiant energy emitted by perfectlyblack body per unit area per unit time isdirectly proportional to forth power of

absolute temperature. The constant of proportionality is called as

Stefans constant and denoted by .

The value of is 5.67 x 10-8 J/m2.s.K4 (W/m2K4)


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Generalization of Stefans Law

Using Kirchhoffs law Stefans Law can be generalizedas Emissive power: The amount of heat radiation emitted

by a body per unit time per unit area is called asemissive power of the body.

If above body is black body then it is left hand side ofStefans law.

Coefficient of emission or emissivity of a body is ratioof emissive power of a body and perfectly black body

at same temperature. Denoted by e. e = E/Eb As a = e Hence E = a. Eb = a ..T4

For ordinary bodies Stefans Law will be

Q = a(A.t..T4

)


25/31

Newtons law of heat

The rate of loss of heat by a body is directlyproportional to the excess temperature of thebody over the surrounding.

Please note that the law was stated quite earlierthan Stefans law and Provost's theory.

)(

dt

dQ0

Newtons law of cooling

)k.(

dt

dQ0=

dt

dm.s.

dt

dQbut

=

)k(dt

dm.s. 0= )K(dt

d0=


26/31

Newtons law as approximation of Stefans Law

Latter on when new theory was developed the Newtonslaw was obtained as approximation of Stefans BoltzmanLaw.

Assuming T = T0 + x and using binomial expansion and the

fact that T0 > > > x we get

Obviously we need to use T-T0 = - 0

)k.(dt

dQ0=


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Solar constant

The solar constant is the amount of radiant energy receivedper second per unit area by a perfect black body placed onthe Earth with its surface perpendicular to the direction ofradiation from the Sun.

Solar constant is different for different planets as theirdistance is different from Sun.

Value of Solar constant is 1.388 x 103W/m2

Instrument used to measure Solar constant is

Pyro heliometre Simplest of all is Angstrongs Compensation pyro heliometre


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Temperature of Sun Using Wiens Law

If is wavelength of radiation for

which Solar radiation is maximum( 4900 x 10-10m) then

002892.0T. =maxlawWien'sUsing

K590210x4900

002892.0T

10==


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Note the following

If two bodies are of surface area A

1

and A2

coefficient of absorption a1 and a2 attemperature T1 and T2 then rate of emission

of heat radiation is

222

111

2

1

TAa

TAa

dt

dQ

dt

dQ

=

If they are of same material then a1 = a2

22

11TA

TA

=If they are of same material then a

1

= a2

and are at same temperature T1 = T2

2

2

2

1

2

1

r

r

A

A== If they are of same material then a1 = a2

and are at same temperature T1 = T2 and

spherical in shape.


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Note the following If two bodies of mass m1 & m2 ,surface area

A1 & A2 coefficient of absorption a1 and a2 attemperature T1 & T2 specific heats s1 and s2and densities 1& 2 then rate of cooling is

11222

22111

22

222

11

111

2

1

smTAa

smTAa

sm

TAa

sm

TAa

dt

d

dt

d

=

=

If they are of same materialthen a1 = a2 and s1 = s2

122

211mTA

mTA

=

In addition if are at same

temperature T1 = T2 andspherical

1

2

1

3

1

2

2

2

3

2

2

1

12

21

r

r

)r

3

4(r

)r3

4(r

mA

mA=

==

by nitin oke for safe hands

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