The document provides an overview of thermodynamics, focusing on key concepts such as heat transfer mechanisms, thermal conductivity, and the properties of black bodies. It discusses the implications of thermal conductivity in real-world applications, specific heat, and introduces important temperature scales and their conversions. Additionally, it touches upon foundational laws in thermodynamics, including Kirchhoff's law and the Stefan-Boltzmann law.

1. Prof. Kajol Panchal
Department of Civil Engineering
Diwaliba Polytechnic,Mahuva 1
Unit -IV
Engineering Physics

2. Contents
• Introduction to thermodynamics
• Heat and Temperature
• Three modes of transmission of heat - Conduction,
Convection and Radiation
• Good and bad conductor of heat with examples
• Law of thermal conductivity
• Coefficient of thermal conductivity and its S.I. unit -
Definition and Application
• Heat capacity and Specific heat of materials
• Celsius, Fahrenheit and Kelvin temperature scales and their
conversion formula with numerical.
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5. 5

6. Modes
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7. 7

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13. • https://youtu.be/zvQZtpZnRRE
• https://youtu.be/w-R2c6qH4IU
• https://youtu.be/LuDhYzR-0SU
• https://youtu.be/Td0RU-zA4m8
Tips for better understanding…
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14. Anomalous expansion of water
• The volume of water increases instead of
decreasing when temperature of water is reduced
from 4° to 0°C.
This is called Anomalous expansion of water.
• The existence of fish and other aquatic animals is
possible because of natural convection and
Anomalous expansion of water.
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15. • In winter, as temperature decreased, upper layer
of water converts into ice and stop the heat from
below layers to atmosphere.
• So below the ice layer,thre is temperature at and
fish and other aquatic animal can survive.
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16. Tips for better understanding…
• https://youtu.be/m_sJVcyUP-Y
• https://youtu.be/WbDbH121Nv4
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17. Black body Theory
• The idea of a black body originally was introduced
by Gustav Kirchhoff in 1860.
• He supposed that bodies can be imagined which,
for infinitely small thicknesses, completely absorb
all incident rays, and neither reflect nor transmit
any.
• I shall call such bodies perfectly black or more
briefly, black bodies.
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18. Perfect black body and black body
radiation???
• A perfect blackbody is one that absorbs all
incoming light and does not reflect any.
• At room temperature, such an object would appear
to be perfectly black hence the term is blackbody.
• However, if heated to a high temperature,
a blackbody will begin to glow with
thermal radiation.
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19. • This is true for radiation of all
wavelengths and for all angles
of incidence.
• Hence the blackbody is a
perfect absorber for all incident
radiation.
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It is impossible to get 100% perfect black body in
nature on the earth.

20. Study of black body radiation
• Consider a spherical cavity as shown in diagram.
• Its inner surface is blackened and rough.
• It has a small hole.
• A radiation entering this cavity through this entrance
hole undergoes many reflections and each time it
partly absorbed and partly reflected.
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21. • When it reaches the hole again it is almost left with
no energy.
• In this sense this pin hole can be considered a
perfect black body.
• If such a cavity is uniformly heated, radiations
coming out of it can be considered to be black body
radiation.
• These radiations are called cavity radiations.
Sun can be considered as to be perfect black body.
Black body radiation depends only on the
temperature.
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22. • https://youtu.be/ktvsucLC_WY
• https://youtu.be/BbkzvJD8h-0
Tips for better understanding…
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23. Definitions …
• Absorptivity(a):On irradiating a surface, the ratio of
the radiant energy absorbed to the amount of
radiant energy incident on the surface is called
absorptivity of that surface at a given temperature.
a=
radient energy absorbed
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑒𝑛𝑒𝑟𝑔𝑦
For a completely black body a=1
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24. • Total emissive power:
The amount of radiant energy emitted per unit area
per second from a surface at a given temperature for
all possible wavelength is called the total emissive
power(w)of surface of that temperature.
• Spectral emissive power(Wf):
At a given temperature, the amount of radiant
energy emitted per second per unit surface area in a
unit frequency interval about a given frequency(f)is
called Wf.
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25. • Emissivity(e)
The ratio of total emissive power of a surface to the
total emissive power of surface of a perfectly black
body kept under the same conditions is called
emissivity of that surface.
For the substance of a completely black body, e=1
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26. Kirchhoff's law
“The value of emissivity and absorptivity are equal
for any surface.”
• From this we can understand that the surface
which is good absorber is also a good emitter and
the surface which is good reflector is also a poor
emitter.
• Due to this reason glass bottle of a thermo-flask is
kept shining.
a=e
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27. Stefan-Boltzman law
• The amount of radiant energy emitted by a surface
per unit area in unit time is directly proportional to
the forth power of its absolute temperature.
T=absolute temperature
e=emissivity of the surface
σ = Stefan-Boltzman constant
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W= 𝝈𝒆𝑻𝟒

28. Law of thermal conductivity
• Thermal conductivity refers to the ability of a given
material to conduct/transfer heat.
• It is generally denoted by the symbol ‘k’ but can
also be denoted by ‘λ’ and ‘κ’.
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29. • Consider a metal slab of area of cross-section A and
thickness d. Let the end faces (ABCD & EFGH) be maintained
at temperatures θ1 and θ2 (θ1 > θ2).
• The amount of heat conducted (Q) from the hotter to colder
face is,
1. directly proportional to the area of cross-section (A)
2. directly proportional to the time for which heat flows (t)
3. directly proportional to the temperature difference between the
faces (θ1 – θ2)
4. inversely proportional to the distance between the faces (d) This
is called the law of thermal conductivity.
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30. • According to the law
• K is a constant of proportionality called coefficient
of thermal conductivity.
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Q ꝏ
𝑨𝒕(𝞗𝟏−𝞗𝟐)
𝒅
Q =
𝑲𝑨𝒕(𝞗𝟏−𝞗𝟐)
𝒅

31. • Unit of K
C.G.S = Cal/cm° CS
M.K.S = K-Cal/m°CS
SI = Watt/mk
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32. Application of thermal conductivity
• Heat conduction is applied in cooking with metal
pot e.g. Aluminum pots.
• Ironing of clothes with pressing iron.
• Welding of two iron metals together.
• The handles of the cooking utensils are made of
materials like plastic and sometimes wood which
cannot conduct heat when carried by the cook.
• Woollen clothes which prevent body from losing
heat to the surrounding are worn during cold.
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33. Good and bad conductor of heat
• The substances through which heat is conducted
rapidly is called good conductor of heat.
EX All metals
• The substances through which heat is conducted
very slowly is called bad conductor of heat.
EX All non metals(glass wool, wood, thermo Cole)
So used as heat insulators.
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34. Heat capacity and Specific heat of
materials
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35. • Heat energy: It is the transfer of energy between two
objects or an object and its surroundings due to
temperature difference.
• Calorie:It is the required heat energy to raise temp of 1
g of pure water from 14.5° c to 15.5° c.
• Kilo calorie: The amount of heat energy to raise temp
of one kilogram of pure water from 14.5° c to 15.5° c.
1 kilo calorie = 1000 calorie
• Heat capacity: The ratio of heat Q supplied to a body to
a change in its temperature ∆T.
SI unit JK-1or Cal/K.
(Joule/Kelvin) or (Calorie/Kelvin)
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Hc=
𝑸
∆T

36. • Specific heat: The quantity of heat required per
unit mass for unit change in temperature of a body
is called the specific heat of the material of the
body.
Unit cal/ g-1 K-1 or J kg-1 K-1
(Calorie/Gram.Kelvin) or (Joule/Kilogram.Kelvin)
C =
𝑄
∆T
𝑚
=
𝑄
𝑚∆T
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Q = m C ∆T
Specific heat = (𝐻𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦)
/𝑀𝑎𝑠𝑠

37. Specific heat of gases
• Molar specific heat : The amount of heat required
to change the temperature of one mole of a gas by
1 K (1°C) is called molar specific heat of the gas.
• Specific heat a constant volume (Cv):The amount
of heat required to change the temperature of 1
mole of gas by 1 K , keeping its volume constant, is
called the specific heat Cv of the gas at constant
volume.
• Specific heat a constant pressure (Cp):The amount
of heat required to change the temperature of 1
mole of gas by 1 K,keeping its pressure constant,is
called the specific heat Cp of the gas at constant
pressure.
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38. • Relation between Cp and Cv:
R is universal gas constant.
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Cp-Cv=R

39. Celsius Scale
In this scale melting point of ice is taken as 0° C and
boiling point of water is taken as 100 ° C.
 Relation between kelvin scale and Celsius scale:
Where Tc= Temperature on Celsius scale
T = Temperature on kelvin scale
• Temperature of triple point of water on Celsius
scales (melting point of ice)
Tc = 273.16 – 273.15 = 0.01 ° C ̃= 0 ° C
T = 100 + 273.15 = 373.15 K
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Tc = T – 273.15

40. Fahrenheit scale
In this scale freezing point of water(ie melting point
of ice) is taken 32 °F and Boling point of water is
taken 212 ° F.
 Relation between Fahrenheit scale and Celsius
scale:
Where Tc= Temperature on Celsius scale
Tf = Temperature on Fahrenheit scale
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Tf =
𝟗
𝟓
𝑻𝒄 + 𝟑𝟐

41. Examples…
• Coefficient of thermal conductivity
• Conversion of temperature scale
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