In this video, you will find that a gas
is flowing down the staircase as you can
see, or a gas-like substance. Now its motion
is quite similar to these marbles
that are moving down the staircase as
you can see over here. So let us study
about the movement of particles in the
gas in greater detail.
Before that I want you to look at
another video.
In this video you will find that a man
is blowing air into the balloon and the
balloon is bursting.
If you observe closely you will find that
the man while holding the balloon is
blowing air into it and the balloon is
gradually expanding and after a certain
point of time it bursts. So why is it
that the balloon is bursting when simply
what the man is doing is blowing air
into it?
Let us find out.
Over here, a certain gas is kept inside a
particular container and we closely
examine the molecular movements inside the gas. Now
as you can see, these gas molecules are
randomly moving about in every direction
they don't have any particular direction
attached to them and in doing so they are
colliding with one another as well as
with the walls in the container which
they are kept.
So as you can see these gas molecules when
they collide with the walls
in this manner. When these gas molecules
are colliding with the walls, they are 
exerting a force on the inner walls of
the container, that is, in every
direction. If you observe closely, you
will find that this force that is
exerted by the molecules on the inner
walls of the container is perpendicular
to the walls of the container. What have
we studied about the perpendicular
forces? A perpendicular force on a surface
is also known as thrust. So we can say
that these molecules are exerting a
thrust on the inner walls. So since these
molecules are exerting a thrust and since
these walls have a definite area, we can
say that force exerted by the gas
molecules per unit area will be the
pressure exerted by the gas. So if we
consider this for example
to be unit area, then we can say that
these gas molecules that are exerting a
force on this area is equal to the
pressure exerted by the gas molecules. So
we can say that inside any container a
gas that is kept exerts pressure on the
inner walls. So now let us consider what
was happening when the man blew air into
the balloon.
When the man is blowing air into the
balloon he is increasing the amount of
air or the amount of gas inside the
balloon, which means that the number of
gas molecules is increasing. These gas
molecules collide with the inner walls
of the balloon and cause it to expand
in all directions and the point comes
when the balloon exceeds it limits and burst
out due to the excessive gas pressure or air
pressure inside the balloon.
Now consider this animation. Over here a pet water bottle
has been taken, that is, a typical plastic bottle and
a hole is made at the bottom on one
side. Now as you can see, this water bottle
that was initially full is now leaking
from this whole and as it is leaking, the
liquid level in the bottle is coming
down.
Now why do you think that once this
hole is made the bottle starts to leak?
Why is it that liquid flows from the
inside of the bottle to the outside? Now
we have to explain this scientifically
in terms of pressure.
Now you will notice that when water is
leaking from this hole, if you bring
your finger and place it on the hole,
force will be exerted on the finger. If
you make another hole and press your
finger to that as well, you will find
that a certain amount of force is
exerted on that finger by this liquid
that is moving down as well as you can
clearly see from this animation. Now let
us analyse what is going on over here.
If we consider the container in which
water is kept to be the one that you can
see we can say that if we look down into
the molecular arrangement of the
molecules in water, we can see that these
water molecules are also moving about
randomly in every possible direction and
in doing so are colliding or hitting
the walls - the inner walls of the
container. So even in the previous case,
the water molecules were hitting the
inner walls of the bottle.
Now in doing so, when these liquid or
water molecules collide with the walls
of the container, they
exert a certain force on these walls. Now
as you can see, these forces that are
being exerted on the walls
are perpendicular and we have studied that
perpendicular of forces are also known as
thrust. So we can thus say that these water
molecules are exerting an amount of
thrust on the inner walls of the
container and if we consider unit area,
then we can say that force exerted by
liquid molecules or water molecules in
this case per unit area is the pressure
exerted by the liquid inside the
container, in this case the water inside
the bottle.
Now let us recall a familiar analogy,
when two bodies are kept near one
another or in contact with one another
at two different temperatures, that is, let's
say we have
a warm body that has a higher temperature
and it is kept near a cool body that
has a much lower temperature. There will
be a flow or transfer of heat in between
these two bodies and heat will flow from
the body which has a higher temperature
to the body which has a lower
temperature and heat flow will take
place till that time
till the temperature of both the bodies
becomes the same. So in a similar manner,
fluids, that is, liquids or gases flow
from one region to another region only
if there is a pressure difference in
between the two regions.
So just like in the case of heat that flows from
hot body to cold body, fluids will flow from
high pressure to low pressure.
So thus we can 
conclude that fluids, liquids or gases
always flow from a high pressure region to 
 a low pressure region.
So in the case of the water bottle that was
leaking,
fluid always flowed from the high pressure
region to the low pressure region, that
is, from inside the water bottle to outside
it. So what can we say? We can say that
since the fluid, or water in this case
is flowing outside inside the
container, the pressure is high.
And outside the container, the pressure is
low
and since fluids flow from a region of
high pressure to a region of low
pressure, in this case the liquid flowed or
water from the inside of the container
to the outside of the container.
So taking a quick recap, we found that
fluids liquids or gases always flow in
between two points if there is a
pressure difference existing in between
two points,
that is to say that these fluids will
always flow from a region of high
pressure to a region of low pressure. We also
found out with the help of videos and
animations that the molecules inside
fluids always collide in with the walls
of the container in which they are kept.
So if a gas is kept inside a balloon or
water is kept in a bottle, their molecules will be
continuously colliding with the inner
walls in which they have been kept. So
thus due to this, they exert a certain
amount of pressure and it is due to this
pressure difference that a flow of
liquids or gases is enabled.
