3B Scientific Magdeburg Plates - Manual (Page 5)

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3. Operating principle
A protective mantle of air called the atmosphere sur-
rounds the earth. The air molecules like all other ma-
terials are subject to the force of gravity and thus be-
come concentrated at the earth’s surface. The atmo-
spheric pressure, which is exerted by the air mass, is
highest at sea level and gradually decreases with in-
creasing distance from the earth’s surface. Air pressure
like any liquid exerts equal and constant pressure on
all sides of an object. In an open object equilibrium
always prevails between internal and external pressure.
If the internal pressure drops more than the external
pressure, the state of equilibrium is reestablished by
letting air in. Conversely, if the internal pressure be-
comes greater than the prevailing external pressure,
there is the same tendency to escape. In an enclosed
object the force resulting from the difference between
the internal and external pressures affects the object’s
surface either causing it to compress or in the latter
case causing the object to explode.
The physicist and mayor of Magdeburg Otto von
Guericke was the first to demonstrate the effect of at-
mospheric pressure. His vacuum experiments com-
menced around 1650 and reached their zenith in 1654
in a spectacular experiment attended by Kaiser
Ferdinand III in the town of Regensburg: the famous
Magdeburger hemispheres. To do this von Guericke
evacuated the air out of two copper hemispheres of
42 cm diameter and sealed with an oil and wax satu-
rated leather lining. The air pressed the hemispheres
together with such force that even 16 horses were un-
able to pull them apart.
4. Operation
4.1 Demonstration of the effect of atmospheric
pressure
• Check the sealing ring and the plates for damage
before the experiment.
•
Connect the hose between the hand pump 8 and
the Magdeburg plate 3 as shown in Figure 1.
• Slip the hose connectors inside one another and
push them together by turning lightly with the
fingers.
• Place the desired sealing ring on one plate and
press the plates together.
•
When the smaller diameter ring is chosen, it is
easier to separate the plates by pulling them apart.
An experimental comparison can be made using
the two rings one after the other to show how
force depends on the surface area.
•
A second person may start the process of evacu-
ating the space between the plates.
• Most of the air will have been evacuated after a
few pumps to form a sufficient vacuum for the
experiment.
• Demonstrate the effect of atmospheric pressure
by attempting to tug the plates apart.
•
Loosen the hose connection to the Magdeburg
plate to let air into the apparatus.
Fig.1
4.2 Quantitative measurement
4.2.1 Approximate determination of atmospheric
pressure by experiment
• Assemble the equipment as in section 4.1.
• Pump out the air between the plates.
•
The person holding the Magdeburg plates should
step onto some bathroom scales and observed the
weight indicated as a second attempts to sepa-
rate the plates by pulling the bottom handle ver-
tically downwards.
•
Note the weight indicated by the scales when the
plates separate.
• The following formula is used for the calculation
Pressure = Force/Area
•
To determine the force, subtract the weight of the
Magdeburg plates and the person holding them
from the weight shown at separation. To convert
the force into Newtons, multiply the weight in ki-
lograms by 9.8
• The force pressing the plates together corresponds
to the atmospheric pressure being exerted on the
area encompassed by the sealing ring. Calculate
the area in m² from the inner diameter of the ring.
• 1 N/m² is equivalent to 1 Pa. Since the value will
come out to be rather large, you can convert your
answer into kilopascals.
•
It is recommended that the result be compared
to the actual atmospheric pressure as measured
using a barometer.
• Discuss with the students what might be causes
for any deviations (e.g. inaccuracies in the scales
and the reading of them, not pulling vertically
downwards when separating the plates or residual
air between them).
Also required:
1 Set of bathroom scales
4.2.2 Determining the force acting on the Magdeburg
plates with a data logger
•
Connect the hoses as in Figure 2.
•
Connect the pressure sensor (e.g. U 44220) and a
barometer (e.g. U 44300) to a data logger (e.g.
U 44000).
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