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.
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.
2
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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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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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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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