Introduction to Heat and its effect Video Lecture | Science Class 7

heat and its effect
what is heat the experiments conducted
by count ROM Ford and James Prescott in
the beginning of 19th century
established that heat is a form of
energy known as thermal energy many
other forms of energy can be converted
to heat energy and heat energy can be
converted to other forms of energy for
example when you rub your hands together
mechanically energy gets converted to
heat energy and your hands become warm
when fuel is burnt chemical energy
stored in the fuel gets converted to
heat energy in an electric heater
electrical energy is converted to heat
energy when a solid liquid or a gas is
heated its molecules gain energy and
start moving faster the more we heat the
substance the faster its molecules move
heat energy is therefore the energy of
movement of molecules a factor of heat
when we heat a substance the movement of
its molecules increases this increases
the average distance between the
molecules therefore the space occupied
by the molecules that is the volume of
the substance increases we see that heat
causes expansion
expansion in solids solids expand on
heating we can take the following
experiment to show expansion in solids
take the ring and ball approaches this
apparatus consists of a metal ring and a
metal ball at normal temperature the
ball just passes through the ring now
heat the ball
then try to pass it through the ring it
will not pass to the ball has expanded
on heating cool the ball to normal
temperature it will pass through the
ring expansion in liquids the molecules
of a liquid are not so firmly bound to
each other as in a solid therefore on
heating the vibration increase more than
the vibrations of molecules of solids
the result is that liquids expand more
than solid on heating
expansion in liquids experiment to see
the expansion in liquids see the
following activity fill your flask up to
the brim with water take a rubber cork
with a hole in it and insert a narrow
tube into the hole fix the cork firmly
in the mouth of the flask the liquid
will rise a little in the tube note the
level of the liquid heat the liquid
you will notice that the level of the
liquid in the tube goes down initially
as the flask gets heated first and
expands when the heat reaches the liquid
it expands and it's level in the tube
goes up cool the liquid to normal
temperature the level will go down again
the amount of expansion is different in
different liquids for the same increase
in temperature
expansion in gases on heating a gas the
vibrations of its molecules increases
since the molecules are not bound to
each other at all the average distance
between the molecules increases
considerably
hence the expansion will be more in case
of gases than in liquids or solids thus
gases expand more than liquids and
solids on being heated
expansion in gases experiment to see the
expansion in gases see the following
activity fill a flask up to the brim
with water fix the flask to a stand in
an upside down position over a beaker of
water so that the glass tube dips inside
the water heat the flask at the top as
the air in the flask expands it escapes
out of the flask through the tube into
the beaker containing water bubble
software can be seen in the water
you
cool the flasks are there inside the
flask contracts the water rises up the
tube
this shows that gas like air expands on
heating and contracts on cooling
expansion in gases experiment to see the
expansion in gases see the following
activity fill a flask up to the brim
with water fix the flask to a stand in
an upside-down position over a beaker of
water so that the glass tube dips inside
the water heat the flask at the top as
the air in the flask expands it escapes
out of the flask through the tube into
the beaker containing water bubble
software can be seen in the water
you
cool the flasks are there inside the
flask contracts the water rises up the
tube
this shows that gas like air expands on
heating and contracts on cooling
the another example expansion rounders
are really tracks the railway tracks
over which train run our metro of oil
during summer the ion expands to allow
this expansion space has to be left
between two sections of the rail tracks
if this is not done expansion of the
tracks can cause them to bend this can
cause serious accidents
temperature why did the bottle of water
feel cold in the fridge while it was so
hot outside the answer lies in the
concept of temperature temperature
indicates the degree of hotness or
coldness of a body or a place we must
remember that our own sense of what is
hot and what is cold can sometimes be
misleading so you see the terms hot and
cold are only relative terms there
always has to be a reference body with
respect to which we can say whether a
given body is hot or cold therefore it
is important that we use an instrument
rather than our own judgment to measure
temperature when you touch a hot object
you feel hot if you touch a cold object
you feel cold you feel hot in the first
case because heat energy flows from the
hot object to your hand in the second
case as heat energy flows from your hand
to the cold object you feel cooled
how to measure temperature we can use
either the Celsius or the Fahrenheit
scale to measure temperature the Celsius
scale temperature on this scale is
measured in degree Celsius whereas
degree Celsius on this scale we take the
temperature at which pure water freezes
at sea level to be zero degree Celsius
this is called the lower fixed point the
temperature at which pure water boils at
sea level this is called the upper fixed
point it is taken as 100 degree Celsius
we say at sea level because the
temperature at which water freezes or
boils changes as we go to higher
altitude the Fahrenheit scale
temperature is also measured in
Fahrenheit it's lower fixed point is
taken as 32 degree Fahrenheit and the
upper fixed point as 212 degree
Fahrenheit in our country and in most
other countries the Celsius scale is
more commonly used conversion formula
for degree Celsius and degree Fahrenheit
the simple equations given below will
help us to convert one temperature scale
to another number one is Fahrenheit
equals to Celsius into 9 upon 5 plus 32
and second one is
Celsius equals to 5.9 Fahrenheit minus
32 where f is for fahrenheit and c is
for celsius measurement of temperature
the instrument used to measure
temperature is called thermometer there
are 4 different types of thermometer
multi thermometer in 1714 Gabriel
Fahrenheit invented the first mercury
thermometer mercury is used in
thermometers because of the following
reasons mercury remains in the liquid
state for a wide range of temperature
it melts at -38 0.87 degree celsius and
boys only at 356 0.58 degree celsius it
is relatively easy to see because of its
silvery gray color is not stick to glass
it has a fairly uniform rate of
expansion for an inside range of
temperatures alcohol thermometer
thermometers in which alcohol is used
have some advantage over mercury
thermometers alcohol is cheaper and less
harmful than mercury alcohol
thermometers can measure much lower
temperatures than Mulkey however the
main disadvantage of using an alcohol
thermometer is that these thermometers
can measure temperature only up to 78
degree Celsius while mercury
thermometers can measure much higher
temperatures
laboratory thermometer it consists of a
thin glass tube which is sealed at one
end and has a bulb at the other end the
range of temperature which a laboratory
thermometer can measure is from minus 10
degree Celsius to 110 degree Celsius
clinical thermometer the thermometers
used to measure our body temperature is
called a clinical thermometer it is
generally a multi thermometer though
nowadays digital thermometers are
becoming very popular
so in this section we learnt about the
heat and it's a fact one solid liquid
and gas we also learnt about the
temperature and measurement of
temperature