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HEAT (CHAPTER 4)
In winter We feel cold inside the
house. We come out in the sun, you
feel warm.
In summer, We feel hot even
inside the house.
HOT AND COLD
Tea is hot
and ice is cold. ?
Our sense of touch
to decide whether an object
is hot or cold.
A reliable measure of the
hotness of an object is its
temperature. Temperature is
measured by a device called
thermometer.
MEASURING TEMPERATURE
The
thermometer that measures our body
temperature is called a clinical
thermometer.
A clinical thermometer consists of a
long, narrow, uniform glass tube. It has
a bulb at one end. This bulb contains
mercury. Outside the bulb, a small
shining thread of mercury can be seen.
The scale we use is the Celsius scale, indicated by °C. A clinical thermometer reads
temperature from 35°C to 42°C.
Reading a thermometer
First, note the temperature difference
indicated between the two bigger marks.
Also note down the number of divisions (shown by smaller marks) between these
marks. Suppose the bigger marks read
one degree and there are five divisions
between them.
Then, one small division
can read - 1/5 = 0.2 °C.
The normal temperature of human
body is 37°C. Note that the temperature
is stated with its unit.
The temperature of every person may
not be 37°C. It could be slightly higher
or slightly lower.
We call
normal temperature is the average body
temperature of a large number of healthy
persons.
The clinical thermometer is designed
to measure the temperature of human
body only.
The temperature of human
body normally does not go below 35°C or above 42°C. That is the reason
that this thermometer has the range
35°C to 42°C.
LABORATORY THERMOMETER
Different types of thermometers
are used for different purposes.
The maximum and minimum
temperatures of the previous
day, reported in weather
reports, are measured by a
thermometer called the
maximum -minimum
thermometer.
There
are other thermometers. One such
thermometer is known as the laboratory
thermometer.
The highest(110°C ) and
the lowest (-10°C ) temperature it can
measure. The range of a
laboratory thermometer is
generally from –10°C to 110°C.
TRANSFER OF HEAT
A frying
pan becomes hot when kept on a flame because the heat passes from the
flame to the utensil. When the pan is
removed from the fire, it slowly cools
down.
The heat
is transferred from a hotter object to a colder object.
In all cases heat flows from a hotter
object to a colder object.
The process by which heat is
transferred from the hotter end to the
colder end of an object is known as
conduction.
In solids, The heat is transferred by the process of
conduction.
The materials which allow heat to
pass through them easily are
conductors of heat. For examples,
aluminum, iron and copper.
The
materials which do not allow heat to
pass through them easily are poor
conductors of heat such as plastic and
wood.
Poor conductors are known as
insulators. The water and air are poor
conductors of heat.
When water is heated,
the water near the flame gets
hot. Hot water rises up.
The cold water from the
sides moves down towards
the source of heat.
This
water also gets hot and rises and water from the sides moves down.
This process continues till the whole
water gets heated. This mode of heat
transfer is known as convection.
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.
To
receive the cooler sea breeze, the windows
of the houses in coastal areas are made
to face the sea. At night it is exactly the
reverse. The water cools down more
slowly than the land. So, the cool air
from the land moves towards the sea.
This is called the land breeze.
When we come out in the sun, we
feel warm. The heat from the
sun cannot reach us by
conduction or convection as there is no
medium such as air in most part of the
space between the earth and the sun.
From the sun the heat comes to us by
another process known as radiation.
The transfer of heat by radiation does
not require any medium. It can take
place whether a medium is present or
not.
When we sit in front of a room
heater, we get heat by radiation.
A
hot utensil kept away from the flame
cools down as it transfers heat to the
surroundings by radiation.
Our body, gives heat to the surroundings and
receives heat from it by radiation.
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.
KINDS OF CLOTHES WE WEAR IN
SUMMER AND WINTER
Dark surfaces absorb
more heat and, therefore, we feel
comfortable with dark coloured clothes
in the winter.
Light coloured clothes
reflect most of the heat that falls on them
and, therefore, we feel more comfortable
wearing them in the summer.
Woollen clothes keep us warm
In winter
In the winter, we use woollen clothes.
Wool is a poor conductor of heat. 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.
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