Heat
Heat is that form of energy which flows from one body to
other body due to difference in temperature between the bodied. The amount of
heat contained in a body depends upon the mass of the body.
► If W work is performed and heat produced is H, then W/H =J
or, W = JH
Where J is a
constant called Mechanical Equivalent of heat. Its value is 4.186
Joule/Calorie. It
means if 4.186 joule of work is performed, 1 calorie of heat is
consumed.
Units of Heat
C.G.S Unit: Calorie =
It is amount of heat required to raise temperature of 1 g of pure water
through 1oC.
International
Calorie: It is amount of heat required to raise temperature of 1 g of pure
water from 14.5oC to 15.5oC.
FPS Unit: B.Th.U.
( British Thermal Unit) = It is amount of heat required to raise the temperature
of 1 pound of pure water through 1oF.
Relation between different units:
1 B.Th.U. = 252 calorie 1
calorie = 4.186 joule
1 Therm = 105 B.Th.U. 1 pound calorie = 453.6 calorie
Temperature: Temperature
is that physical cause which decides the direction of flow of heat from one
body to other body. Heat energy always flows from body at higher temperature to
body at lower temperature.
Measurement of
temperature:
Thermometer: The
device which measures the temperature of a body is called thermometer.
Scales of temperature
measurement:
To measure
temperature two fixed points are taken on each thermometer. One of the fixed
points is freezing point of water or ice as lower fixed point (LFP). The other
fixed point is the boiling point of water or steam as upper fixed point (UFP).
The
temperature of these fixed points, the no. of fundamental interval between the
two fixed points on different temperature scales is shown by the table given
below:
Celsius
|
Fahrenheit
|
Reaumur
|
Kelvin
|
Rankine
|
|
UFB
|
100oC
|
212oC
|
80 oC
|
373.15K
|
672
oRa
|
↑
|
↑
|
↑
|
↑
|
↑
|
↑
|
No. of
fundamental intervals
|
100
|
180
|
80
|
100
|
180
|
↓
|
↓
|
↓
|
↓
|
↓
|
↓
|
LFP
|
0
oC
|
32
oC
|
0
oR
|
273.15K
|
492
oRa
|
↓
|
↓
|
↓
|
↓
|
↓
|
↓
|
Absolute zero
|
-273.15
|
-459.6
oF
|
-218.4
oR
|
0K
|
0
oRa
|
Relation between temperatures
on different scales:
► Celsius was
initially known as centigrade.
► While
expressing temperature on Kelvin scale o (degree) is not used.
► Freezing point
of mercury is -39oC. Hence to measure temperature below this
temperature, alcohol thermometer is used. F.P. of alcohol is -115oC.
Range of different
thermometers:
Mercury Thermometer: from
30oC to 350oC
Constant volume gas
thermometer: From -200oC
to 500oC (with H2), Below
-200oC up to -268oC (with He), Above 1000oC up
to 1600oC (with N2 gas an bulb of glazed porcelain)
Platinum resistance
thermometer: from - 200oC to 1200oC
Thermocouple
thermometer: from - 200oC to 1600oC
Total Radiation Pyrometer:
When a body is at high temperature, it glows brightly and the radiation emitted by the body is directly proportional to the fourth power of absolute temperature of the body. Radiation pyrometer measures the temperature of a body by measuring the radiation emitted by the body.
►One calorie of heat is required to raise the temperature of 1 gram of
water through 1 C. Hence specific heat capacity of water is 1 cal / gram °C.
Total Radiation Pyrometer:
When a body is at high temperature, it glows brightly and the radiation emitted by the body is directly proportional to the fourth power of absolute temperature of the body. Radiation pyrometer measures the temperature of a body by measuring the radiation emitted by the body.
This thermometer is not put in contact with the body. But it can not measure
temperature below 800°C because at low temperature emission of radiation is
very small and can not be detected.
Specific Heat Capacity : Specific heat capacity of a material is the
amount of heat required to raise the temperature of unit mass of substance
through 1°. Its SI unit is joule / kilogram Kelvin (j/ kg. k)
Specific Heat Capacity of Different Materials ( in j/ kg.k)
1 calorie / gram °C = 4200 joule / kg Kelvin
Thermal Expansion
When a body is heated its length, surface area and volume increase.
The increase in length, area and volume with the increase in temperature are measured
in terms of coefficient of linear expansion or linear expansivity (α),
coefficient of superficial expansion or superficial expansivity (β) and coefficient
of cubical expansion or cubical expansivity (γ).
Relation between α, β and γ.
α : β : γ = 1 : 2 : 3 or β = 2 α and γ = 3 α
Anomalous expansion of water : Almost every liquid expands with the increase in temperature. But when
temperature of water is increased from 0°C to 4°C, its volume decreases. If the temperature is' increased
above 4°C, its volume starts increasing. Clearly density of water is maximum at
4°C.
Transmission of Head : The transfer of heat from one place to other
place is called transmission of heat. There are three modes of heat transfer-
(i)
Conduction,
(ii)
Convection
(iii)
Radiation.
Conduction : In this process, heat is transferred from one place to
other place by the successive vibrations of the particles of the medium without
bodily movement of the particles of the medium. In solids, heat transfer takes
place by conduction.
Convection : In this process, heat is transferred by the actual
movement of particles of the movement from one place to other place. Due to
movement of particles, a current of particles set up which is called convection
current.
In liquids and gases, heat transfer takes place by convection.
► Earth’s atmosphere is heated by convection.
Radiation : In this method transfer of heat
takes place with the speed of light without affecting the intervening medium.
Newton's law of cooling : The rate of loss of heat by a body is
directly proportional to the difference in temperature between the body and the
surrounding.
Kirchhoff’s law: According to Kirchhoff's law, the ratio of emissive power
to absorptive power is same for all surfaces at the same temperature and is
equal to emissive power of black body at that temperature.
Kirchhoff’s law signifies that good absorbers are good emitter.
If a shining metal ball with some black spot on its on its
surface is heated to its high temperature and seen in dark, the shining ball
becomes dull but the black spots shines brilliantly because black spot absorbs
radiation during heating and emit in dark.
Stefan's law : The radiant energy
emitted by a black body per unit area per unit time (i.e. emissive power) is
directly proportional to the fourth power of its absolute temperature.
i.e. E α T4
or,
E = αT4
`where α is a constant called Stefan’s constant.
Change of State
Any material can remain in any of its three states (solid,
liquid and gas). To change the substance from one state to other state is
called change of state. For this either substance is heated or heat is
extracted from the substance. Change of state takes place at a fixed temp.
Fusion : The process by which a
substance is changed from solid state to liquid state is called fusion. Fusion
takes place at a fixed temperature called melting point (M.P.)
Freezing :The process by which a
substance is changed from liquid state to solid state is called freezing.
Freezing takes at a fixed temperature called freezing point (F.P.)
For a substance M.P =
F.P.
►M.P. of a substance changes with
the change in pressure. Melting point of substances which contracts in the
process of fusion (as ice) decreases with the increase in pressure. Melting
point of substances which expands in the process of fusion (as wax) increases
with the increase in pressure.
►With the addition of impurity (as
salt in ice), melting point of substance decreases.
Vapourisation : The process by
which a substance is changed from liquid state to vapour state is called
vapourisation .
Vapourisation takes place by two methods : (i) Evaporation
& (ii) Boiling or Ebullition
Evaporation : The process of
vapourisation which takes place only from the exposed surface of liquid and
that at all temperatures is called evapourisation.
Evaporation causes cooling. This is why water in a earth pot
cooled in summer.
Boiling : The process of
vapourisation which takes at a fixed temperature and from whole part of liquid
is called boiling.
The temperature at which boiling takes place is called
boiling point.
Latent Heat of Water
Condensation :
The process by which a substance is changed from vapour state to liquid state is called condensation.
► Boiling point of a liquid
increases with the increase in pressure.
► Boiling point of a liquid
increases with the addition of impurity.
Latent heat or heat of transformation
The amount
of heat required to change the state of unit mass of substance at constant
temperature is called latent heat.
If Q heat is required to change the state of a substance of
mass m at constant temperature and L is the latent heat,
then Q = mL.
S.I. unit of
latent heat is joule / kilogram.
Any material
has two types of latent heat.
(i) Latent heat of
fusion : It is the amount of heat energy required to convert unit mass a
substance from solid state to liquid state at its melting point. It is also the
amount of heat released by unit mass of liquid when changed into solid at its
freezing point.
(ii) Latent heat of vaporization
: It is the amount of heat required to change unit mass of a substance from
liquid state to vapor state at its boiling point. It is also the amount of heat
released when unit mass of a vapour is changed into liquid
Latent
Heat
|
In
Cal/g
|
J/kg
|
of
fusion
|
80.00
|
336.00 x103
|
of
vapourisation
|
540.00
|
2256 x103
|
Sublimation : Sublimation is the process
of conversion of a solid directly into vapour.
►
Sublimation takes place when boiling point is less than melting point.
►
Sublimation is shown by camphor or ice in vacuum.
Hoar Frost : Hoar
frost is just the reverse process of sublimation i.e. it the process of direct
conversion of vapour into solid.
► Steam produces more
severe burn than water at same temperature because internal energy of steam is
more than that of water at same temperature.
Relative Humidity
: Relative humidity is defined as the ratio of water vapour present in a given
volume of atmosphere to the amount of water vapour required to saturate the
same volume at same temperature. The ratio is multiplied by 100 to express the
relative humidity in percentage.
►
Relative humidity is measured by Hygrometer.
►Relative
humiditymincreases with the increase of temperature.
Air conditioning
: For healthy and favourable atmosphere of human-being, the conditions are as
follows
(i) Temperature :
From 23°C to 25°C.
(ii) Relative
humidity : From 60% to 65%.
(iii) Speed of air
: from 0.75 meter/minute to 2.5 meter/minute.
Thermodynamics
First law of thermodynamics:
Heat energy given to a system is used in the following two ways :
(i)
In increasing the temperature and hence
internal energy of the system.
(ii)
In
doing work by the system.
If ΔQ = heat energy given to the system
ΔU = Increase in the internal energy of the system.
ΔW = Work done by the system
Then, ΔQ = ΔU + ΔW is the mathematical statement of first
law of the thermodynamics.
► First law of
thermodynamics is equivalent to principle of conservation of energy.
Isothermal Process:
If the changes are taking place in a system in such a way that temperature of
the system remains constant throughout the change, then the process is said to
be an isothermal.
Adiabatic Process:
lf the changes are taking place in a system in such a way that there is no
exchange of heat energy between the system and the surrounding, then the process
is said to be an adiabatic process.
► lf carbon dioxide is
suddenly expanded, it is changed into dry ice. This is an example of adiabatic
process.
Second Law of Thermodynamics:
The first law of thermodynamics guarantees that in a thermodynamic process,
energy will be conserved. But this law does not tell whether a given process in
which energy is conserved will take place or not. The second law of
thermodynamics gives the answer.
Through this law can be stated in many forms, the following
two forms are worth mentioning :
Kelvin’s statement:
Whole of the heat can never be converted into Work.
Clausius statement:
Heat by itself can not flow from a colder body to a hotter body.
Heat Engine :
Heat energy is a device which converts heat energy into tnechanical work
continuousiy through a cyclic process. Every heat engine basically consists of
the three parts :
(i)
Source (a hot body)
(ii)
Sink (a
cold body)
(iii)
A working substance.
Heat engine may be devided into two types :
(i) Internal
Combustion Engine : In this engine, heat is produced in the engine itself. For example : Otto engine or petrol engine
(efficiency = 52% ), Diesel engine (efficiency = 64%)
(ii)
External Cumbustion Engine : In this
engine heat is produced outside the engine. Steam engine is an example of
external combustion engine. (efficiency = 20%)
Refrigerator or Heat Pump: A refrigerator is an apparatus which transfers heat energy from cold to a hot body at the expanse of energy supplied by an external agent. The working substance here is called refrigerant.
In actual refrigerator , vapors of freon (CCI2F2) acts as refrigerant.
