IGCSE
Physics Notes: The Vacuum
A vacuumis is a space with no
matter in it. There is not 'nothing' in a vacuum – there may be
light passing through it, but there is no matter. In fact, the
definition above is a perfect vacuum – no vacuum is ever perfect.
We describe how close a vacuum approaches to a perfect vacuum by it's
'quality'. Other things equal, lower gas pressure means a
higher-quality vacuum. A typical vacuum cleaner produces enough
suction to reduce air pressure by around 20% - not a very good
vacuum. Ultra-high vacuum chambers, common in chemistry, physics, and
engineering, operate below one trillionth (
)
of atmospheric pressure and evacuate all but 100
.
Outer space is an even higher-quality vacuum, with the equivalent of
just a few hydrogen atoms per cubic meter on average.
A good quality vacuum may be easily produced in the laboratory
with mercury, a (long) test tutbe and a glass container. The test
tube is filled with mercury then inserted upside down into the glass
container, the opening below the mercury level as shown. The spce at
the top of the test tube will be a good quality vacuum, able to
support a 760mm column of mercury because of the atmospheric pressure
on the mercury surface in the glass container. This is called a
torricellian vacuum.
Vacuums – or at least low quality, partial vacuums where the
pressure is slightly below atmospheric pressure, are widely used.
Every incandescent light bulb contains a partial vacuum, and vacuums
can be used to form seals so that food stays fresh, for example.
No comments:
Post a Comment