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2.
Pressure in liquids
Liquids
exert pressure due to distribution of their own weight.
To see how liquids exert pressure, try the following experiment. Take
three tins of different sizes or diameters. On tin number I, make three
holes at the same height. On tin numbers II and III, make three holes at
different heights. Place three long tapes to close the three holes on each
on the tins. Now fill the tin with water. Remove the tapes quickly and
observe the streams coming out of each of the holes.
You
will observe the following :
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Water
stream will start pouring out through the holes. This means that water
is exerting pressure in all direction.
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In
tin I, the water stream comes out evenly irrespective of the direction
of the hole. This means that the pressure is equal at the same height or
depth.
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In
Tin II and III, the water stream coming out of the lowest hole reaches
the farthest. This shows that the pressure exerted by liquid increases
with depth. Also the pressure is acting perpendicular to the liquid
surface.
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Since
there is no difference between the streams coming out of
tins II and III, the pressure exerted by liquid is independent of
the size of the container, but depends only on the height or the depth
of the liquid. (This is markedly different from what happens when a
solid is applying pressure or weight, as seen in the earlier section).
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in a liquid can be measured by constructing a simple measuring device called
a manometer. Take a U-tube made up of glass. Mount it on a wooden stand.
Fill the U-tube partially with coloured water. The water level will
rise at the same level in both the limbs (the reason for this is explained
later). This level is due to atmospheric pressure on the liquid level. Mark
this level on the wooden stand. Divide the height of the wooden stand
equally. This is an arbitrary calibration of the pressure. Take a rubber
tube and connect it to one arm of the U-tube. On the other end of the rubber
tube attach a funnel. Stretch a thin rubber membrane on the mouth of the
funnel. This will be the manometer you can now use to measure liquid
pressure.
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To
know more about the behaviour of liquid pressure try and do the following :
1.
Take a beaker filled with water. Insert the manometer at different
levels. You will notice, as seen earlier, that greater the depth, higher is
the pressure.
2.
Take two beakers, one filled with water and one filled with a salt
solution. If you keep the manometer funnel at same depths in the two
beakers, you will notice that pressure is dependent on the density of the
liquid. Higher the density, higher the pressure.
3.
Take
a beaker filled with water. Insert the manometer at a certain depth. Rotate
the manometer mouth in all direction. You will notice that the pressure at a
certain depth inside a liquid is equal in all directions.
4.
In
a beaker filled with water, insert the manometer funnel at a fixed depth,
move
the funnel back and forth at the same depth. You will notice that the
manometer pressure does not change. This means that the pressure inside a
liquid is same at all places at the same depth.
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