Heat, Temperature, Conduction, Convection,Radiation, Gas laws
Heat, Temperature, Conduction, Convection,Radiation, Gas laws
Introduction:
The universe is made up of matter and
energy.The universe is made up of either matter or energy.
Let
us consider about matter.
Matter exist in different states.There
are five states of matter. These are
1.Solid 2. Liquid. 3. Gas 4. Plasma
5. Bose Einstein Condenscent
Out of these, the first 3 states exist
at normal conditions, such as pressure and temperature. The 4th
state exists above 60000 C. The 5th state exists at nearly
00
K.
All these matters can be studied on the
basis of Kinetic theory. According to this--
1. All the matter
is made up of atoms or molecules.
2. These molecules
are in some motion.
3. There is
attraction between the molecules depends
on distance between the molecules.
4. The molecules
are in spherical shape.
Let
us consider the first three states of matters on the basis of kinetic theory.
1. Solid:
In solid, molecules are closely packed because
solid is made by taking 1022 molecules / cm3 .Due to this there is
a strong attraction between the molecules.Hence molecules arrange in a
particular shape.Thus solid have shape and size.The molecules in the solid are
in vibration motion.The vibration
depends on temperature.If the solid is heated then the vibration increases, the
distance between molecules increases, at one particular temperature called melting
point separation between the molecules so increases that the solid is converted into liquid.
2. Liquid:
In the liquid, the distance between molecules is large.The liquid is made by taking 1015 molecules / cm3 .Due to this, the attraction between the molecule is very small. The molecules have vibration and translation motion.The liquid has no shape, only size.As the liquid is heated, at a particular temperature called boiling point, the liquid is converted into gas.
3. Gas:
In case of gas the molecular distance is very large.The gas is made by taking 1010 molecules / cm3 .There is no attraction between the molecules.Each molecules have vibration, translation and rotational motion. The gas has no shape and size.
Heat:
Heat is a type of
thermal energy. Thermal energy is the energy associated with the molecular
motion within an object.
Heat or thermal energy
of a body is the sum of kinetic energies of all its constituent particles, on
account of translation, vibration and rotation motion.
The direction of energy flow is from the substance of higher
temperature to the substance of lower temperature.
Heat is extensive property.
i.e. Heat depends on amount of substance.
e.g. Heat required to boil 2 kg of water and 5
kg of water is different.
The SI unit of heat energy is joule or J. CGS unit of heat is erg.
But usually heat is measured in heat units.
CGS heat unit is calorie or cal. MKS heat unit is kilocalorie or kcal.
1 calorie: The quantity of heat
required to raise the temperature of 1 g
of water by 1°C is called 1 calorie .
1 kilocalorie: The quantity of heat
required to raise the temperature of
1 kg of water by 1°C is called
1 kilocalorie .
1 cal = 4.186 J Usually it is taken as 1 cal = 4.2 J
1 kcal = 4186 J Usually
it is taken as 1 kcal = 4200J
Temperature:
Temperature of a body is the degree of hotness or
coldness of the body. The branch dealing
with measurement of temperature is called Thermometry
and the devices used
to measure temperature are called thermometers.
Highest
possible temperature achieved in laboratory is about 108 K ,while lowest possible temperature
attained is 10-8 K. Branch of Physics dealing with production and
measurement temperature close to 0 K is known as cryogenics.
Temperature is intensive property. i,e. It does not depends on amount of substance. e.g. Temperature required to boil 2 kg of water and 5 kg of
water is same.
Temperature of body is directly proportional to
kinetic energy of molecules of the body.
Absolute zero:
Absolute
zero is the temperature at which a system is in the state of lowest possible
(minimum) energy. As molecules approach this
temperature their movements drop towards zero. It corresponds to −273.15 °C .
Triple point of water :
The single
combination of pressure and temperature at which pure water, pure ice, and pure water vapour can coexist in a
stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a pressure of
611.73 Pa. The triple point of water,
T = 273.16 K, is the standard fixed-point temperature for the calibration
of Kelvin thermometers.
Different Scales of Temperature:
Fig AA. Fahrenheit
scale:
The Fahrenheit scale was developed in 1717 by
the German physicist Gabriel Fahrenheit. Fahrenheit scale is based on two fixed
points,
a . 32
for the freezing point of water
b . 212
for the boiling point of water,
c . the interval between the two being divided
into 180 parts and each
part
is known as 1 0 F . ( Fig A )
B. Celsius
scale:
The Celsius scale was developed in 1742 by the
Swedish astronomer Anders Celsius. Celsius temperature scale also called
centigrade temperature scale. The Celsius scale is based on two fixed points,
a. 0 for the freezing point of water
b. 100
for the boiling point of water
c. the
interval between the two being divided into 100 parts and each
part
is known as 1 0 C . ( Fig A )
C. Kelvin scale:
Lord Kelvin, working in Scotland, developed the
Kelvin scale in 1848. Kelvin temperature scale is the base unit of
thermodynamic temperature measurement in the International System (SI) of
measurement. It is defined as 1/ 273.16 of the triple point of pure water. The kelvin scale is the only
unit of measurement to include the temperature for "absolute
zero.
The Kelvin scale is based on only one fixed
point, but calibration two fixed points are considered,
a. 0 for Absolute zero
b. 273 K
for the triple point of water and 373 K for boiling point of water
c. the
interval between the triple point and boiling point being divided
into 100 parts and each part is known as 1 0
K . ( Fig A )
The relations between conversions of different scales of
temperature:
Notations : C for 0 C, F for 0 F
and K for 0
K
1. Conversion of 0 C into 0 K :
K =
C + 273 e. g. 27 0 C to
Kelvin scale , K = 27 + 273 = 300 0
K
2 . Conversion of 0 K into 0 C :
C =
K - 273 e. g. 373 0 K to
Celsius scale , C = 373-273 = 100 0
C
3. Conversion
of 0 C into 0 F :
F =
( 9 C / 5 ) +32 e.g. 200 C to
Fahrenheit scale, F = ( 9*20 / 5 ) +32
= 680 F
4. Conversion of 0 F into 0 C :
C = (
5/9) F -32 e.g. 810 F to Celsius scale, C = (5/9) 81-32 = 130 C
Modes of transmission of heat:
Heat transfer is the change of energy between
two bodies due to difference in temperatures. It takes place to balance the
temperature. There are three types of Modes of
transmission of heat,
1. Conduction 2. Convection 3. Radiation
1. Conduction:
Definition : The process of transfer of heat
from the point of higher temperature to the point of lower temperature through
the material in which there is no migration of its particles from one place to
another place is called conduction of heat.
The particles of medium help to transfer heat from hot end to
cold end due vibration of particles at their place but heat energy is handed
over to one particles to another particle.
Conduction do not occur through vacuum conduction is slow
process of transfer of heat.
Good conductor : The materials through which heat conducts easily and speedily
are called good conductors
All metal conducts heat and they are good conductors of heat .Thus utensils , boilers, calorimeters etc are made
up of metals.
Bad conductor : The materials through which heat is not conducted easily and
speedily are called bad conductors.
Plastic, Wool, Thermocol,
Wood are the bad conductor of heat.
Uses :
1. The handle of pressure cooker is of wooden.
2. Ice is covered by wooden sawdust. Sawdust is bad conductor so it prevents the ice to
melt in hot atmosphere.
3. To keep warm in winter we use blanket woollen clothes. It
prevents the conduction of heat from our body to the cooler surroundings.
4. Calorometer it sounded by cotton and it is kept in the wooden
box to avoid the loss of heat to the surroundings while performing the
experiment.
Convection :
The process of transfer of heat from the point of higher temperature to the point of
lower temperature from one place to another through material which is due to actual
migration of particles of material is called convection of heat.
The motion of liquid particles which sets of flow of hot liquid
from bottom to the top and cold liquid from top to bottom are called convection
current.
Uses :
1.Textile mills or factories where coal is burnt for
boilers are equipped with tall chimneys. The hot
currents of air which is smoke filled rises up through the chimneys. Thus convection currents are set up and cool fresh air moves in
through the door and windows.
2.Auditorium, cinema halls, dark rooms are provided with the exhaust fans near the ceilings. The warm
and impure air from the breathing of audience rises up and it is pushed out by the exhaust
fans. The fresh
cool air enters into the hall through the door. For the
same reason rooms have the ventilators instead of exhaust fans.
3.Trade winds : When any region is heated more than its surrounding regions
the air about the hot region becomes hot and moves upward the place of this
region is taken up by the relatively cool air moving in from the surrounding
regions. The motion
of this air is results in the formation of wind. These are
called trade winds.
4.Land and sea breezes : The land near the sea is heated by
the sun to higher temperature during the day and because sea has a greater heat capacity than the
land. The temperature of sea is lower
because of mixture between hot surface layers and the colder layers below
them. The air above land is hot so becomes lighter and rises upwards
and this is replaced by the cooler air moving from the sea towards land and sea breeze is set
up. During night land cools rapidly
while the sea remains hotter because water takes more time to cool. Thus air above the sea is
lighter and so it moves upward and it replaced by the breeze flowing
from land to sea. In this way land breezes caused.
5. Mansoons are convection currents on large scale.
Radiation :
Radiation of heat is defined as mode of transparent heat in the
form of electromagnetic waves for which material medium is not necessary.
The electromagnetic will travel with a velocity equal to a velocity of light.
( C= 3 x 108
m/s ) Due to this radiation is fastest process of transfer of heat.
The heat from sun reaches to earth by radiation process.
Comparison between Conduction, Convection. Radiation :
|
Basis |
Conduction |
Convection |
Radiation |
|
Definition |
The process of transfer of heat through the
material in which there is no motion of its particles from one place to
another place |
The process of transfer of heat from one place to
another through material which is due to motion of particles of material |
The mode of transparent heat in the form of
electromagnetic waves for which material medium is not necessary. |
|
Cause |
Due to
temperature difference. |
Due to
density difference. |
Independent of temperature difference, occurs at
all temperatures |
|
Transfer of
heat |
By heated
solid substance. |
By
intermediate substance. |
By
electromagnetic waves. |
|
Occurrence |
Occurs in
solids, through molecular collisions. |
Occurs in fluids,
by actual flow of material particles. |
Occurs at a
distance and the intervening medium is not heated. |
|
Speed |
Slow |
Slow |
Fast |
|
Medium |
Only solids |
Solid,
Liquid, Gas |
No medium |
|
Uses |
The handle
of pressure cooker is of wooden. |
Auditorium cinema halls, dark rooms are provided
with the exhaust fans |
Heat from sun is received by radiation |
Thermal Conductivity :
Fig BConsider the metal rod. One
end of the rod is
kept in steam bath
and other end is kept in ice bath. The heat from hot
end flows towards
cold end. Initially the
temperature off each part is rising
but after some time the temperature of
each part remains constant. This state is
called steady state. At steady state temperature
of each
part remains the same but decreases from hot
end to cold end.
The
fall of temperature between two points such as M and N
separated by a distance d. The temperature at M
is θ1 and at N it is θ2
. At steady state the amount of heat
entering from hot end is entirely transferred
to cold end. Fig B
Steady state :
The temperature at which rate of
heat absorbed by material is equal to the rate of heat evolved is called
temperature gradient.
Temperature gradient :
The fall of temperature with the distance is called temperature gradient.
Temperature gradient = (θ1
- θ2 ) / d
Where θ1
is temperature of at hot end and θ2 is temperature at cold end , these are separated
by distance d.
SI unit of temperature gradient is 0K / m
Law of Thermal Conductivity :
The amount of heat Q flowing at steady state through the cross
section of conductor is directly proportional to the
1. Area of cross section A
2. Temperature gradient (θ1 - θ2
) / d
3. Time t for which heat
flows.
Thus Q α A ; Q α (θ1 - θ2
) / d ; Q α t
Q
α A x t x (θ1 - θ2
) / d
Q = k x A x t x (θ1 - θ2
) / d
where k is
constant of proportionality and is
called
Coefficient of Thermal conductivity.
When A = 1 m2 ; t = 1 s ; (θ1 - θ2 ) / d = 1 0K / m, then Q = k.
Coefficient of Thermal conductivity :
It is defined
as the amount of heat flowing at
steady state through the unit cross section of conductor in unit time for unit
temperature gradient.
SI unit of Coefficient of Thermal conductivity is J/ m.s. 0K,
or W / m. 0K
CGS unit cal/ cm. s . 0C,
MKS unit kcal/ m. s . 0C.
Gas Laws :
The characteristics of gases are described in terms of
following four variables
1. Mass 2. Volume 3. Pressure 4.Temperature
Gas laws are study of for fixed mass any two of
quantities like pressure, volume and temperature, when the third is kept
constant.
Due to this three gas laws are formed These are
1. Boyle's law 2. Charles’s Law 3. Gay-Lussac’s Law
1. Boyle's law :
In 1662,
Robert Boyle discovered that there is a relation between the pressure and the
volume of a fixed mass of gas at a constant temperature.
Boyle’s law states that
"At
constant temperature, the pressure of a fixed mass of gas varies
inversely with its volume".
So P α 1 / V; or PV = constant at constant temperature.
This indicates that at constant
temperature, product of pressure and volume of a fixed mass of gas is
constant.
If a fixed mass
of gas at constant temperature T occupying volume V1 at pressure
P1 undergoes expansion, so that volume changes to V2 and
pressure to P2, then according to Boyle’s law :
P1 . V1 = P2
. V1 = Constant.
2. Charles’s Law :
In 1787, Jacques Charles discovered that if the
pressure is kept constant, the volume of a gas increases directly with the temperature for a
fixed mass of gas.
Charles’s law states that
"At
constant pressure, the volume of a given mass of a gas is directly proportional
to its absolute temperature".
So V α T ; or
V / T = constant at constant pressure.
This indicates that at constant pressure, the
ratio of volume of a fixed mass of gas to absolute temperature of gas is
constant.
If a fixed mass
of gas at constant pressure P occupying volume V1 at absolute
temperature T1 undergoes
expansion, so that volume changes to V2 at absolute
temperature T2, then according to Charles's law :
V1
/ T1 = V2 / T2
= constant.
For each
degree change in temperature, the volume of sample of a gas changes by the
fraction of 1/273.5 of its volume at 0 oC.
So Vt = V0 ( 1 + t/ 273 ).
Gay-Lussac’s Law :
Gay-Lussac's
law states that
“At constant volume,
the pressure of a given mass of a gas is directly proportional to its absolute
temperature”.
So P α T; or
P / T = Constant at constant
volume.
This indicates that at constant volume, the ratio
of pressure of a fixed mass of gas to absolute temperature of gas is constant.
If a fixed mass
of gas at constant volume V , pressure P1 at absolute
temperature T1 undergoes
changes pressure to P2 at absolute temperature T2, then
according to Guy-Lussac's law :
P1
/ T1 = P2 / T2
= constant.
For each
degree change in temperature, the pressure of sample of a gas changes by the
fraction of 1/273.5 of its pressure at 0 oC.
So Pt = P0 ( 1 + t/ 273 ).
Ideal or perfect gas equation :
It is seen from gas laws for 1 mole of gas :
According to Boyle's
law P α 1 /V,
According to Gay-Lussac's's
law P α T,
Hence , combining these, we
get, P α T / V,
Or P V / T = Constant = R or P V = R T
Where R is Universal gas
constant = 8314.9 J / kg. mol. 0K .
For n
moles of gas P V
= n R T .
The gas which
obeys equation P V = n R T is
called Ideal or Perfect gas.
Heat Capacity :
Let us consider, the same amount of water and oil. To
rise the temperatures of these through 1 0 C the
amount of heat required is different.
Thus
it is seen that when heat energy is absorbed by a substance, its temperature
increases. If the same quantity of heat is given to equal masses of different
substances, it is seen that the rise in temperature for each substance is
different. This is due to different substances have different heat capacities.
Heat capacity of a
substance is the quantity of the heat required to raise the temperature of the
whole substance by 1 0 C.
If the mass of the substance is unity then the heat capacity is called
Specific heat capacity or specific heat.
Specific
heat is the amount of heat required to raise the temperature of unit of mass substance through1 0 C
.
C
= Q / m ∆t
Where Q
is quantity of heat absorbed by a body, m is mass of the body, ∆t is rise in
temperature and C is Specific heat
capacity of a substance.
Specific heat depends on the nature of the material of the
substance. The specific heat of a material is an
extensive property since its value is proportional to the size of the system
being examined.
S.I
unit of specific heat J /kg . 0K
MKS unit of specific
heat is kcal/kg . 0C,
CGS unit of specific
heat is cal / g . 0C.
Specific heat of gases:
When a gas is heated, it expands and its
volume increases. On the other hand if gas is
not allowed to expand then the pressure will increase. By allowing the
gas to expand the pressure is maintained a constant and gas does some amount of
mechanical work against the atmospheric pressure at the cost of its heat
energy. In the case of gases a change in temperature causes a considerable
change in the pressure and volume . Hence there are two specific heats of gas.
1. Specific heat at constant pressure (C p)
2. Specific heat at constant volume ( C V)
1. Specific heat at
constant pressure (Cp)
Specific
heat at constant pressure is the amount of heat required to raise the temperature
of 1 kg gas through1 0 C,
keeping pressure constant.
Cp
= Q / m ∆t
S.I unit of specific heat at constant
pressure J /kg . 0K
MKS unit of specific
heat at constant
pressure is kcal/kg . 0C,
CGS unit of specific
heat at constant pressure is cal / g . 0C.
2. Specific heat at
constant volume (C v)
Specific
heat at constant volume is the amount of heat required to raise the temperature
of 1 kg gas through1 0 C,
keeping volume constant.
Cv
= Q / m ∆t
S.I unit of specific heat at constant volume
J /kg . 0K
MKS unit of specific
heat at constant
volume is kcal/kg . 0C,
CGS unit of specific
heat at constant volume is cal / g . 0C.
Relation between two
specific heats :
Specific heat at constant pressure (Cp) and Specific
heat at constant volume (Cv) are related Mayor's relation as
Cp - Cv = R / J
where R is universal
gas constant and J is Joule's constant.
Ratio of Specific
heats:
Cp / Cv = γ = 1.4
Comments
Post a Comment