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Nature of Heat:
Heat is the transfer of energy (every in transit) from one body to another due to the
temperature difference between the two bodies. When the body absorbs the heat, its
internal energy increases and when it losses heat, its internal energy decreases.
Temperature:
Definition:
“Temperature is the degree of hotness or coolness of a body “.
OR
“The measurement of average translational kinetic energy of the molecules is called
temperature “.
Thermometer:
Definition:
An instrument which is used for the measurement of temperature is called the
nanometer. Any property of a substance which changes uniformly with temperature
can be used to measure the temperature.
Temperature scales:
The three scales commonly used to measure the temperature are:
i. Centigrade or Celsius scale.
ii. Kelvin’s or absolute scale.
iii. Fahrenhelt scale.
1. Centigrade or Celsius scale, in the melting point of ice is taken as 0oC and boiling
point of water is taken as 100oC . The interval between them is divided into 100
equal parts. Each part is called one degree centigrade which is written as 1oC.
2 The lowest temperature which can be reached is –273oC. This temperature is
taken 0oC on Kelvin’s scale. The size of the degree on the scale is also same as
that on centigrade scale.
3 In Fahrenheit scale, the melting point of ice is taken as 32oC and boiling point of
water is taken as 212o, and the internal given then is divided into 180 parts. Each
part is called one degree. Fahrenheit which is written as 1oF.









Inter Conversion of Scale:
The conversion of temperature in centigrade (Tc) into the temperature in Kelvin (Tk) is
performed by the relation.
Tk = Tc + 273
OR Tc = Tk - 273
Temperature in Fahrenheit (Tf) is converted into temperature in centigrade (Tc) by the formula.
Tf = 9 / 5 (Tc) + 32
Tc = 5 / 9 (Tf – 32 )
Ordinary Liquid in Glass Thermometer:
It consist of a glass slem with a capillary tube having a small bulb at one end. The bulb
end part of the capillary tube is filled with liquid (mercury or alcohol). The upper end of
the capillary tube is sealed to prevent the liquid from evaporation. Air is reserved from
the upper end before sealing , when the bulb is sealed , the mercury expands and rise in
the capillary tube. The stem is calibrated in degrees to measure the temperature. The
thermometer used in the laboratory has a range from – 10oc to 110oC.



Clinical thermometer:
Clinical thermometer is used to measure the temperature of human body. The normal
body temperature is about 98.4oF. The bulb of the thermometer is placed under the
tongue to measure the temperature of a body. The clinical thermometer has a narrow
bend or construction in its capillary tube bore near the bulb to prevent the mercury
level from falling when the bulb is removed from the patient’s mouth.


Maximum and Minimum Thermometer:
The purpose of this thermometer is to read
maximum and minimum temperature reached
over a period of time. It consist of a bulb A
continuously alcohol is connected through a
U shaped tube from B to C (containing mercury)
to a second bulb D nearly full with alcohol. Above D,
the space is evacuated.
Left limb of U-shaped tube is calibrated according
to the expansion of alcohol in AB. The right limb of
U-tube is also calibrated. So that both mercury
surfaces point to the same member. If mercury in AB
contracts, the pressure in CD well drive the mercury back.
On each mercy surface, there is a small iron index provided with a spring. Due to fall
in temperature the alcohol in A contracts and the left index moves up and is left there.
Due to rise in temperature, the alcohol in A expands and the right index moves and is
left there. The lower end of left index gives minimum temperature and the lower end
of right index gives maximum temperature over a period of time. The indices are
brought in contact with mercury by using small bouquet.
Transfer of Heat:
There are three ways by which heat is transferred from one place to another. They are
i. Conduction.
ii. Convection.
iii. Radiation.
Conduction:
Definition:
“The process in which heat is transferred from one place to another by the vibration
and collision of molecules or atoms, called conduction.”
Explanation:
When the heat is provided to one end of the heat conductor. Its temperature rise
which increase the kinetic energy of the molecules. Due to increase kinetic energy,
molecules start vibration with greater amplitude and make collision with neighboring
molecules. This collision causes the transference of energy from molecule to molecule.
In this way that is transferred from one end to other.
Example:
Solid heat conductor transfer heat by the process of conductor.
Convection:
Definition:
The process, in which heat is transferred from one place to another by the actual
motion of molecules, is called convection.
Since the molecules of liquids and gases are free to more, therefore in liquid and
gasses heat is transferred from one place to another by convection.
Example:
In fluid (liquids and gasses) heat is transferred by the process of convection.
Radiation:
Definition:
The process in which heat is neither transferred by the vibration and collision neither of
molecules, nor by the actual motion of molecules but it is transferred without any
material medium, is called Radiation.
In this process hot body radiates energy in the form of electromagnetic waves.
Heat reaches from sun to earth, by radiation.
Thermos Flask:
It is used to maintain constant temperature.
A thermos flask consists of a double walled glass.
The inner surface of the outer wall and the outer
surface of the inner wall are polished (silvered).
The space between to the two walls is evacuated and then sealed. The vacuum
between the two walls reduces the probability of transfer of heat by
conduction and convection.
Thermal Conductivity:
Definition:
The ability of a substance to conduct heat energy is called thermal conductivity.
Consider a metal block of length L and the cross section area A,
Such that it one end is constantly maintained at lower temperature
T1 and other end at a higher temperature T2. If Q heat is conducted
from hot end to the cold end in t second, then it is found that.
i. Heat is proportional to the cross sectional area
Q  A.
ii. Heat is proportional to the difference of the temperature.
Q  T OR Q  (T2 - T1)
iii. Heat is proportional to the time of the flow.
Q  t
iv. Heat is inversely proportional to the length
Q  1 / L
Combining them, we get: Q  A (T1 – T2 t
L
Q = K A (T1 – T2 t
L
Q = K A  T t T = T2 - T1
L
The constant K is called thermal conductivity of the material of rod. Different materials
have different value for K.
I f L = 1m
A = 1m2 that is if we take 1 meter cube of the substance)

T = 1oC t = 1 second
Substituting values in 1
Q = K x 1 x 1 x 1 Q = K
It shows that, “the thermal conductivity is the quality of heat conducted per second per
meter cube of a substance whose opposite faces are maintained at a temperature
difference of 1oC “.
Thermal Expansion:
Definition:
Increase in the size of the object due to heat is called thermal expansion.
Thermal Expansion in Solid:
When solid is heated its temperature rises and its molecules start vibrating with greater
amplitude. Due to this, the average distance between the molecules increases and the
solid expands.
There are two expansions in solids.
i. Linear thermal expansion. ii. Volumetric thermal expansion.
Linear Thermal Expansion:
Definition:
“ Increase in the length of the rod due to heat is called linear thermal expansion.”
Consider a metal rod having length L, when the temperature is increased by T, let the
increase be T. It is found that,
L  AL1 L  T
Combining them, we get:
L  L1 T L =  L T
Where “ “ is the coefficient of linear expansion. It can be written as:
 = L
L1 T
It shows that, change in length per unit length per degree rise in temperature is called
coefficient of linear expansion.
Consider L =  L1 T
But L = L2 - L1 & T = T2 - T1
L2 L1 =  L1 (T2 – T1)
L2 = L1 +  L1 (T2 – T1) It gives the final length
L2 = L1 1 +  (T2 - T1) after the rise in temperature.
Volume Thermal Expansion:
Definition:
“Increase in volume of the object (space /cube) due to heat is called volumetric
thermal expansion.”
Consider an object having volume V1. When the temperature is increased by T, let the
Increase by V., it is found that,
V  V1
V  T
Combining them, we get V  V1 T
V =  V1 T
Where “” is the coefficient of linear expansion. Above equation can be written as:
 = V
V1T
It shows that, “change in volume per unit volume per degree rise in temperature is
Called coefficient of volumetric expansion.
Consider: V =  V1T
But V = V2 – V1 & T = T2 – T1
V2 - V1 + V1 (T2 – T1)
V2 = V1 +  V1 (T2 - T1)
V2 = V1 1 +  (T2 - T1 )
It gives the final volume after the rise in temperature.
Relation Between Coefficients of Linear And Cubical Expansions:
The coefficient of volumetric (cubical) expansion is three times the coefficient of linear
expansion. i.e.  = 3
Bimetallic strips:
Two metals of slightly different coefficient of thermal expansitives combined together
to form bimetallic strips when this strips is heated, it bend.
Bimetal strips:
In bi metal thermometer the bimetallic strips is in the form of long spiral whose one
end is fixed and other end is connected to a pointer. When temperature raises the
spiral get into lighter position due to different nature of two metals. Due to this pointer
moves over a scale and gives the temperature.
Thermostat:
A device which is used to maintain the constant temperature is called thermostat.
It is used in refrigerator, air conditioner, electric oven etc. to control the temperature.
Fire Alarm:
The bimetallic strip is also used in fire alarm. Due to fire when the temperature rises,
the bimetallic strip bends and touches the contact due to which the current flows and
either the bulb glows or the bell rings giving the warning about life.
Thermal Expansion of Liquid:
Liquid do not have definite shape. Therefore, there is only one expansion in case of
Liquids. This is called volumetric expansion.
Real and apparent expansion:
When a liquid in a vessel is heated, not only the liquid expands but the vessel also
expands. Thus the observed increase in volume of liquid is not the actual increase in
volume of the liquid because we should consider the increase in volume of the vessel.
Thus, Real expansion = apparent expansion + expansion of the vessel.
Anomalous expansion of water:
When water is heated from 0oC to 4oC the water does not expand but it contracts and
its density increase. When water is further heated from 4 to 100oC, its volume increases
and density decreases. Since volume decreases when the temperature is raised from
0oC to 4oC. Therefore when temperature is lowered from 4oC to 0oC the volume
increases. This is called anomalous expansion of water.
Effect of anomalous expansion of water:
i. In winter season:
At 0oC the surface water of ponds, lakes freezes to ice but at the bottom the
temperature of water remains at 4oC, this helps fish and other forms of marine
life to survive in winter season.
In winter season the water supply pipe lines burst when the temperature falls
below at 4oC. Below 4oC, the water expands and causes the pipeline to burst.
ii. In rainy season:
Lot of water seeps through cracks in rocks, in winter this water expands on
freezing and exerts very high pressure raising the rocks to breaks.
Thermal expansion in gasses:
Gasses also expand on heating. For example, in summer when the temperature is very
high the tubes of bicycles get burst. Due to very high temperature the air in the tube
expands and exerts very high pressure on the walls of the tube, causing it to burst.
Gas laws:
There are four fundamental quantities, pressure, volume, temperature and mass to
explain the behavior of gasses. The relation ship between these quantities.
Boyle’s Law:
Statement # 1:
“When mass and temperature of a gas are kept constant, the volume of a given mass is
inversely proportional to the applied pressure.”
Statement #2:
“When mass and temperature of a gas are kept constant, the product of pressure and
volume always remains constant.”
Statement #3:
If ‘P’ is the applied pressure and ‘V’ is the volume then mathematically Boyle’s law is
given as: V  1/P
V = constant x 1/P
PV = constant
If ‘P1’ and ‘V1’ are the initial pressure and initial volume.
P1V1 = K …………..1
If “P2’ and ‘V2’ are the final pressure and final volume.
P2 V2 = K ………… 2
Equating 1 and 2
P1V1 = P2V2
Charle’s law:
When mass and pressure are kept constant, the volume of a given mass of gas is
directly proportional to the absolute temperature.
Mathematical form:
If ‘V” is the volume and ‘T’ is the temperature then mathematically Charle’s law given are:
V  T
V = KT
V/T = K
If ‘V1’ and ‘T1’ are the initial pressure, volume temperature.
V1 = K ……………………1
T1
If ‘V2’ and ‘T2’ are the initial pressure, volume and temperature.
V2 = K ……………………2
T2
Equating 1 and 2 V1 = V2
T1 T2
Pressure law:
Statement:
When mass and volume of a gas are kept constant, the pressure of given mass of a gas
is directly proportional to the absolute temperature.
Mathematical form:
If ‘P’ is the pressure and ‘T’ is the absolute temperature then mathematically it is given
as: P  T
P = constant T
P = constant
T
General gas equation:
Definition:
The combine mathematical form of Boyle’s law and Charle’s law is called general gas
equation: Fom Boyle’s law PV = K …………………i
From Charle’s law V/T = R ……………….ii
Combining 1 and 2 PV = K …………..1
T
If ‘P1’ , ‘V1 ‘ and ‘T1’ are the initial pressure , volume and temperature.
P1V1 = K …………1
T1
If ‘P2’ , ‘V2’ and ‘T2’ are the initial pressure, volume and temperature.
P2V2 = K …………2
T2
Combining 1 and 2 P1V1 = P2V2
T1 T2
If we take the mass of the gas as 1 mole, can be written as
PV = K
T
Where ‘R’ is the universal gas constant and has a value
R = 8.313 J / mol
Fpr ‘n’ no of moles
PV = nR
T
This is called general gas equation. PV = nRT
Specific Heat:
Definition:
Amount of heat required to raise the temperature of 1kg of a substance by 1K or 1oC is
called specific heat. OR
Amount of heat per unit mass per degree rise in temperature is called specific heat.
Mathematical form:
Mathematically is given as: C = Q
mT
Where C = specific heat Q = amount of heat supplied.
m = mass of substance T = rise in temperature
Above equation can also be written as CmT = Q
Q = CmT
Q = Cm (T2 – T1)
Unit:
The unit of specific heat is J/kgC or j/Kg K
Law of heat exchange:
Heat always from hot body to cold body but the total amount of energy remains same. OR
Heat lost by hot body is equal to the heat gained by cold body.
Thus:

Measurement of specific heat of solid:
An instrument which is used for the measurement of heat is called calorimeter. It is a
copper vessel enclosed in a wooden box insulating material in between. The vessel is
covered by a wooden lid with a hole for the thermometer and another hole for stirrer
to stir the contents of the calorimeter.
The specific heat of solid say zinc can be determined as explained below:
Mass of calorimeter = mc
Mass of calorimeter + water = M
Mass of water = mw = M - mc
Mass of zinc = m
Initial temperature of cal = T1
Initial temperature of zinc = T2
Final temperature of mixture = T3
Specific heat of calorimeter = Cc
Specific heat of water = Cw
Specific heat of zinc = C
According to law of heat exchange Heat lost by = heat gained by
Hot zinc calorimeter + water
Cm(T2 – T3 = Cc mc (T3 – T1) + Cw mw (T2 – T1)
C = (Ccmc + C wmw) (T3 – T1)
M (T2 – T3)
Above relation is used to determine the specific heat of zinc.