L8 : Heat transfer - Heat, Science, Class 7 Video Lecture

hello friends this video on heat part 8
is brought to you by exam fear calm no
more fear from exam we have discussed
all about the hotness or coldness of a
body how do we measure the hotness using
temperature and how do we measure
temperature using a thermometer so we
discussed all that so now we want to
understand how exactly heat gets
transferred from one object to another
like when you place the thermometer
under your tongue so we say that the
bulb is getting heated up why because
the bulb is getting heat from the from
your heated body so here it is actually
getting transferred from your body to
the bulb so here we are going to discuss
about how he gets transferred under one
circumstances it will get transferred
from one body to another now have you
tried any of these have you ever seen
something being cooked in your house in
the kitchen so even if you have not
cooked it yourself have you ever seen
your mom mom cooking something in the
kitchen so what does she do she normally
takes a vessel maybe a pan or something
and she puts it on the stove right so
the vessel contains the vegetables or
whatever she wants to prepare and what
happens how the food gets prepared the
food gets prepared with heat now as you
heat the vessel so the vessel gets
heated up and inside whatever you have
maybe the vegetables and all they get
fried up or they get cooked up they get
boiled so how this heat transfer takes
place from the stove to that vessel how
the vessel is getting heated up because
heat is being transferred from the fire
to the vessel so from the source of heat
it is actually getting transferred to
another object and again from the
message it is being transferred to the
contents which are present inside the
vessel like the vegetable salt now I
leave so whatever so what's happening
heat transfer is happening similarly if
you have a candle which is well lit and
if you try to place you
and over the time maybe keeping some
distance how do you feel you feel warm
so why do you feel warm because the heat
is getting transferred from the candle
to your hand
so heat transfer is taking place again
when you wash clothes how do you dry
them so the clothes contain a lot of
water immediately after you wash them so
when these wet clothes are kept in the
Sun what happens the Sun gives a lot of
heat so the temperature is increasing so
the heat is being transferred from the
Sun to the clothes and over some time
over the period of time these clothes
get dried up so in all these scenarios
we see that the heat is being
transferred from the source to some
other object now here in this section we
are going to discuss how is each being
transferred from one object to another
what are the various ways by which heat
can be transferred so how do we define
heat transfer it is the transfer of heat
energy from one system to another or
from one part of the system to another
by virtue of temperature difference now
when I talk about heat transfer it is
not necessary that it always has to be
from the stove to the vessel it also
happens that the same object gets
experience heat transfer from one part
to another so let us see if you have a
pan so that pan handle is also made up
of the same metal so what happens now
when you heat the pan so initially the
band becomes very hot but gradually the
handle also starts becoming hotter so
gradually the heat gets transferred from
one part of the same pan to the other
and how this heat transfer happens due
to temperature difference so he will get
transferred only when when the heat will
get transferred from one object to
another or from one part to another only
when those two objects or those two
parts of the object are at different
temperatures if both of them are at the
same
temperature then there will be no heat
transfer taking place so temperature
difference plays a major role in fact
the most important role in the process
of heat transfer for example what
happens when you touch a glass of cold
drink which is completely chilled where
as soon as you touch it you feel cold
right now you touch it for a few seconds
and then you remove your hand what
happens to your fingers so your fingers
become cold while then why is it cool
even after you have left the glass
that's because while here were touching
the glass there was heat transfer while
there was heat transfer because there
was temperature difference between the
two objects what are the two objects in
this case the first object is the glass
the second object is your hand so in
this case the temperature of your hand
was more so let us say this was the
temperature of your hand and this was
the temperature of the glass so in this
case temperature of your hand was more
than the temperature of the glass
because the glass was very children so
as soon as these two comes in contact as
soon as you touch the glass heat starts
getting transferred from one object to
another and heat always flows from a
body which is at a higher temperature to
a body which is at a lower temperature
so heat starts flowing from your hand to
the glass so therefore your hand is
actually losing heat so any luzie
what happens you tend to become colder
and that is why when you leave the glass
even after that your hand has become
colder because it has lost a lot of heat
to the glass and if you touch the glass
now it will be comparatively a little
more warm than before similarly when you
touch a hot glass of coffee what happens
in this case the hand becomes hot
because in this is the heat transfer
takes place from the cup to your hand
because here the cup is at a higher
temperature than your hand so this is
how heat transfer takes place so heat
transfer will take place only if there
is a temperature difference now in case
you are holding a glass which is a
room temperature it is neither cold nor
is it hot so in that case the
temperature of your hand also remains
the same that's because there was no
heat transfer which took place because
both the objects were at the same
temperature so there was low temperature
difference so this is how we define heat
transfer now the question is thank you
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