3.9 WATER PRESSURE
What happens if there is a small hole in a bucket of water? Water
leaks from the bucket of course! A jet of water shoots out from the
hole. The force that pushes the jet of water is called water pressure.
You may have noticed that, as the water level in the bucket goes
down, the jet of water slows down too. In a few minutes it is only a
trickle, and it stops when the water is level with the hole. The water
pressure obviously gets less as the water level goes down.
We can do a simple experiment to check how water pressure depends on the depth of the water.
All we need is an old plastic bottle. With a pointed object such as a nail, make three holes, all the
same size. One hole must be near the bottom of the bottle, one
half way up, and one quite near the top. Cover all the holes with
tape (or with three fingers). Fill the bottle with water, and take it
outside. Uncover all three holes at the same time and observe
what happens.
As the diagram shows, the pressure depends on the depth of the
water. The jet of water is weakest where the water is shallow
near the top of the bottle. It is strongest where the water is
deepest at the bottom of the bottle.
If we make holes all round the bottom of the bottle, we observe
that the jets of water shoot out equally in all directions. This tells
us that, at the same depth, the pressure is equal in all directions.
We can sum up with a generalisation: Water pressure increases
with the depth of the water and acts equally in all directions.
Measuring pressure with a water
manometer. A water manometer is
an open, U-shaped tube, half filled
with water. The water rises to the
same level on both sides of the U.
The tube is made of glass or plastic
so that the water levels can be seen.
A scale of cm and mm allows us to
measure any changes in the water
levels. A flexible plastic or rubber
tube is attached to one side of the U.
water
If we blow gently into the plastic or
rubber tube, the water level goes
down on one side of the U and up on
the other side. The difference
between the levels measures the
pressure of the air in your lungs in cm
of water. A water manometer can
only measure quite small pressures,
so be careful not to blow too hard!
A water manometer
The diagram shows a manometer being used to measure pressures at different depths in a
bucket of water. This is another experiment which confirms that the pressure increases with the
depth. If we point the manometer tube in different directions, we can also confirm that, at the
same depth, the pressure acts equally in all directions. This includes upwards and downwards as
well as sideways.
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1. (i) What is water pressure, and (ii) in what
direction does it push? (iii) How does water pressure
change with depth?
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2. (i) Describe how to use a water manometer. (ii) In
what units does this manometer measure pressure?
(iii) What is the pressure shown by this manometer?