There is often a clear impression of the difference
between Science
and Engineering. However, in many cases the
distinction is murky.
In the area of vacuum science and technology
for example, were
Edison's efforts to improvement vacuum pump
technology driven by
the desire to explore low pressure environments
or solve the
inventor's problems associated with filament
failure? This workshop
will focus on different aspects of the natural
and physical sciences
as they relate to the observations that are
possible because of
the different pressure environments that can
be generated with
a vacuum system. However, it will also direct
your attention to
engineering perspectives of these same phenomena
and the
impact those engineering perspectives have
had on our lives.
The success of this workshop relies on the
predictable events that
occur when the pressure of an environment
is manipulated. Pressure
results from the influence gas molecules have
on their surroundings.
The earth is surrounded by a mixture of gases
referred to as
the "atmosphere". This atmosphere extends
from the earth's
surface to approximately 1000 km, with most
of the atmosphere
below 10 km. Up to that altitude, its composition
is primarily
nitrogen (78%) and oxygen (21%). The remaining
one percent
contains Ar, CO2, He, Kr, Xe, CH4, H2 and
H2O. All gas molecules
in the atmosphere are in constant motion,
undergoing millions of
collisions with other gas molecules or surrounding
surfaces every
second. The results of these collisions is
the exertion of an average
force per unit area in all directions. At
sea level and 25 _C, this
force per unit area is defined as 1 atmosphere
of pressure and
is approximately 100,000 Newtons per square
meter ( 1 Pascal)
or 14.7 pounds per square inch. The internationally
accepted
non- geographic specific definition of one
Atmosphere (1 Atm)
is the pressure exerted by a 760 mm column
of mercury having
a specific gravity of 13.595 g/cm^3 at O_C.
The random motion characteristic of molecules
in the gas state are
key to describing the pressure of an environment.
The kinetic
theory of gases provides an aid to visualizing
the influence pressure
has on a system. One of the main assumptions
of the kinetic theory
of gases is that gas molecules are in constant
motion and thus have
kinetic energy. The velocity at which gas
molecules travels is directly
dependent on the temperature and inversely
dependent to the mass.
Heaver molecules move slower than light ones
and all molecules move
faster if the temperature is increased.
The distance molecules can travel before a
collision reflects the pressure
that is exerted by these molecules. The average
distance a gas molecules
travels before it collides with another gas
molecule is referred to as its
mean free path. At atmospheric pressure the
mean free path is extremely
short, about 6 x 10-5 mm. As the pressure
is lowered the mean free path
distance increases dramatically. More details
about mean free path are
provided in the APPENDIX and an experiment
for determining this
characterizing parameter will be discussed
in the workshop.
One of the first recorded attempt to measure
atmospheric pressure was
Otto Von Guericke's famous Magdeburg hemisphere
experiment reported
in 1672. In this experiment, Von Guericke
tried to separate two evacuated
hemispheres with two teams of horses. The
experiment was conducted at
the time as a bit of successful showmanship.
It will have the same effect
in your classroom today. A smaller scale version
of the experiment using
two vacuum plates made of Plexiglas sealed
with an O-ring will do the
trick. The drawings for making this set of
plates is also provided in
the APPENDIX section of this workbook.
In summary, this workshop will deal with the
interesting science and
engineering aspects of systems that are at
various pressures other
than 1 Atm. The workshop has a "hands on"
formate but there are
several lecture and group exercises interspersed
to help keep the
concepts under consideration in focus. The
overall goal for this experience
is to provide perhaps a different perspective
of the interaction between
science and engineering with ample examples
of how the manipulation
of pressure can bring this perspective to
the students in your classes.
The workshop begins with a general introduction
to today's technologies
that rely on a predictable vacuum environment.
It continues with a
review of popular demonstrations but then
quickly ventures into what
might be considered new ground to find other
possible applications for
vacuum in the high school science curriculum.
The entire workshop has
been designed to provide you with as many
different leaning experiences
as time will permit. However, the intent is
not to send you away saturated
and exhausted but to have you reluctantly
leave stimulated and pleased.
II. LOW PRESSURE TECHNOLOGIES