HEAT
Heat and Temperature
Heat Temperature
Definition Heat is a form of energy, Definition - Temperature is the
which when absorbed by a body, measure of the degree of hotness or
makes the body hot and when coldness of a body
released by the body, makes it cold
Unit S.I. unit of heat is joule (J) Unit S.I. unit of temperature is
kelvin (K)
Measurement - Heat is measured Measurement Temperature is
using a device called calorimeter measured using a thermometer
Symbol - Q or H Symbol T (for kelvin scale)
t (For other scales)
Different temperature scales
Celsius or centigrade scale (oC)
Kelvin scale (K)
Fahrenheit scale (oF)
Different temperature scales
Celsius or Kelvin scale (K) Fahrenheit Scale
Centigrade Scale (oF)
(oC)
Freezing point of 0 oC 273 K 32 oF
water
Boiling point of 100 oC 373 K 212 oF
water
Number of divisions 100 100 180
between these two
values
Use used for everyday used in scientific earlier used to
temperature calculations only measure body
measurement temperature. Not
used in India now a
days.
Relation between different
temperature scales
Celsius Scale Kelvin Scale Fahrenheit scale
C0_ K 273 F 32
100 - 0 373-273 212 -32
OR
Celsius Scale Kelvin Scale Fahrenheit scale
C _ K 273 F 32
100 100 180
Relation between kelvin and Celsius
scale
C + 273 K
Relation between Fahrenheit and
Celsius scale
C _ F 32
5 9
Questions
1. The body temperature of a normal
healthy person is 37 0C. Find its value in 0F
and K.
Questions
1. Convert 107 0F into 0C and K.
2. Convert 20 K in 0C and 0F.
3. What will be the value of - 40 0C in 0F
scale.
Answers
1. 107 0F = 41.66 0C , 314.66 K
2. 200 K = - 73 0C , -99.4 0F
3. - 40 0C in 0F scale = - 40 0F
Answers
1. 107 0F = 41.66 0C , 314.66 K
2. 20 K = - 73 0C , -99.4 0F
3. - 40 0C in 0F scale = - 40 0F
Clinical Thermometer
Clinical Thermometer
A clinical thermometer reads temperature
from 35C to 42C. The temperature of
human body normally does not go below
35C or above 42C. That is the reason
that this thermometer has the range 35C
to 42C.
Clinical Thermometer
Is the body temperature of every person
37C?
The temperature of every person may not
be 37C. It could be slightly higher or
slightly lower. Actually, what we call normal
temperature is the average body
temperature of a large number of healthy
persons.
Clinical Thermometer
Do not use a clinical thermometer for
measuring the temperature of any object
other than the human body.
Also avoid keeping the thermometer in
the sun or near a flame. It may break.
Clinical Thermometer
.
Clinical Thermometer
Yes, A laboratory thermometer can be
used to measure body temperature.
However it has following two issues;
1. Its scale is not as good as of clinical
thermometer
2. Its size is big.
Clinical Thermometer
Why does the mercury not fall or rise in a
clinical thermometer when taken out of
the mouth?
Clinical Thermometer
A clinical thermometer has a kink near the
bulb. It prevents mercury level from falling
on its own.
Thermometer
Thermometer
Heat transfer
Heat always flows from a hotter
object to a colder object.
Mode of Heat transfer
There are three ways of heat transfer
between the bodies.
1. Conduction
2. Convection
3. Radiation
Conduction
This is the mode of heat transfer in
solids. In this process, the heat is
transferred from one molecule to the
next molecule till it reaches the colder
end. The molecules only transfer heat,
they dont change their position.
Conduction
Convection
This is the mode of heat transfer in
liquids and gases. In this process, the
molecules receive heat and move to
the colder end, thus they change their
position.
Convection
Convection
Radiation
This is the mode of heat transfer
vacuum. In this process, no material
medium like solid, liquid or gas is
required. The heat is transferred in
the form of electromagnetic wave,
which can travel through vacuum.
Radiation
Conductors of Heat
The materials which allow heat to
pass through them easily are
conductors of heat.
Example: Aluminum, Iron and Copper
Insulators of Heat
Air and water are also the poor
conductors of heat
Insulators of Heat
The materials which do not allow heat
to pass through them easily are poor
conductors of heat. Poor conductors
are known as insulators.
Example: Wood, Rubber, Wool, Plastic
Practical Application of transfer of heat
1. A blanket (or saw dust) is wrapped
around ice blocks to prevent them
from melting. It is because blanket
is poor conductor of heat.
Practical Application of transfer of heat
2. A room heater is usually placed on
the floor and ventilators are made
near the roof.
Convection
Practical Application of transfer of heat
3. A room heater has reflector
installed behind the heating rod.
Sea Breeze
A sea breeze is a cool breeze that
blows during daytime from sea
towards land near the coasts.
Sea Breeze
During the day, the land gets heated
faster than the water. The air over the
land becomes hotter and rises up. The
cooler air from the sea rushes in
towards the land to take its place. The
warm air from the land moves towards
the sea to complete the cycle. The air
from the sea is called the sea breeze.
Land Breeze
A Land breeze is a cool breeze that
blows during night time from land
towards sea near the coasts.
Land Breeze
During the night, the land cools faster
than the water. Water, being a poor
conductor, is still warm. The air over
the water becomes warm and rises up.
The cooler air from the land rushes in
towards the sea to take its place. The
warm air from the sea moves towards
the land to complete the cycle. The
cool air from the land is called the land
breeze.
All hot bodies radiate heat. When this
heat falls on some object, a part of it is
reflected, a part is absorbed and a part
may be transmitted. The temperature of
the object increases due to the absorbed
part of the heat.
Examples from everyday life
we prefer light-coloured clothes and in winter
we usually wear dark-coloured clothes. Why
is it so?
Dark surfaces absorb more heat and, therefore,
we feel comfortable with dark coloured clothes
in the winer. Light coloured clothes reflect most
of the heat that falls on them and, therefore,
we feel more comfortable wearing them in the
summer.
How woollen clothes keep us warm in winter?
Wool is a poor conductor of heat. Moreover,
there is air trapped in between the wool
fibres. This air prevents the flow of heat from
our body to the cold surroundings. So, we feel
warm.
Question to think
Suppose you are given the choice in winter of
using either one thick blanket or two thin
blankets joined together. What would you
choose and why?
I would choose two thin blankets joined
together. It is because joined blankets has a
layer of air in between them. This makes them
more insulating than a single thick blanket ,
thus they provide more warmth than the
single thick blanket.
Question to think
We often use electricity and fuels like coal and
wood to keep our houses cool or warm. Is it
possible to construct buildings, that are not
affected much by heat and cold outside?
This can be done by constructing outer walls
of buildings so that they have trapped layers of
air. One way of doing this is to use hollow
bricks, which are available these days.
