Page 124 - Science Class 08

Pour a li le amount of water into the container and heat it. The container fills with steam and the
air is pushed out. Now, stop hea ng and close the mouth of the container with a cork ghtly.
You will no ce that the container gets deformed.

It is because, on hea ng, the steam pushes the air of the container outside. When the
container is corted, the steam present in the container condenses into water and the container
gets evacuated. Now, due to external atmospheric pressure the box gets deformed.

Effects of Atmospheric Pressure Measuring Atmospheric Pressure

The atmospheric pressure is maximum at sea Atmospheric pressure is measured by an
level. As we move upward, the air becomes instrument called barometer. It consists of a
thinner and the atmospheric pressure decrea- long glass tube filled with mercury. It is kept
ses. It is, therefore, at very high al tude, we upside down in a bowl of mercury. It does not
have difficulty in breathing. While travelling in ma er if the barometer is lted, as the ver cal
a jet plane at a very high al tude, nose starts height of the mercury column stays the same.
bleeding. It is because, the pressure exerted by The column of mercury is held up by the air
the blood in our body will be much higher than pressure. Since, the atmospheric pressure
the outside pressure. This can cause blood varies day-by-day depending on weather, the
vessels to burst. That is why the pressure inside height of mercury also varies. No pressure is
an aeroplane is maintained at the normal exerted on the top of the liquid in the column
ground level atmospheric pressure. because of the vacuum, so the pressure in the
Fountain pens starts leaking at higher al tude. column is just the liquid pressure which is
In this case also the pressure inside the pen is propor onal to the depth.
much higher than the pressure outside. At normal atmospheric pressure, the points A

Similarly, the mountaineers also suffer nose and B remains the same. So, the pressure due
bleeding at high al tudes because at high to 760 mm of mercury, Hg, is equal to the
al tude, the atmospheric pressure is much less atmospheric pressure. A height of 760 mm is
than our blood pressure. called standard atmospheric pressure, wri en
as, 760 mm Hg. The unit mm Hg is o en called
for, par cularly in vacuum applica ons.

1. Why do nails have pointed p?
2. How pressure is measured?

3. Write any two applica ons of pressure in our daily life.
4. How can you prove that liquids also exert pressure?
5. How can we measure the atmospheric pressure?

Force : Push or pull

Contact force : When the source of force comes in contact with the object
Non-contact force : When the source of force does not come in contact with the object

124 Science-8