3B Scientific Air Cushion Plate User Manual
PHYSICAL EXPERIMENTS
ON THE
AIR-CUSHION TABLE
U15420
Physical Experiments on the Air-Cushion Table
Table of Contents
Introduction ........................................................................................................................ 5
1. Setup and Possible Uses of the Air-Cushion Table ........................................................... 6
1.1 Components of the experimenting apparatus ......................................................................... 6
1.2 Principle Uses of the Air-Cushion Table ............................................................................. 11
1.3 Setup of the Air-Cushion Table ............................................................................................ 11
1.4 Instructions for Usage .......................................................................................................... 12
1.5 Maintenance and Care .......................................................................................................... 13
2 Description of the Experiments ........................................................................................ 14
2.1 Structure and Properties of Gases ...................................................................................... 14
2.1.1 Motion of a Molecule in High Vacuum ............................................................................... 14
2.1.2 Motion of the Molecules in a Gas ........................................................................................ 14
2.1.3 Dependence of the Number of Impacts with the Vessel Wall on the
Velocity of the Molecules .................................................................................................... 15
2.1.4 Dependence of the Number of Impacts with the Vessel Wall on the Volume...................... 15
2.1.5 Mean Velocity of the Molecules – Temperature of a Gas .................................................... 16
2 1.6 Mean Velocity of the Molecules – Influence on Foreign Molecules ................................... 17
2.1.7 Velocity of Molecules in a Gas Compound ......................................................................... 17
2.1.8 Mixing Temperature of Gases .............................................................................................. 18
2.1.9 Increase of Temperature in Gases when Supplying Energy ................................................. 19
2.1.10 Form and Volume Properties of Gases ................................................................................. 20
2.1.11 Adiabatic Compression and Expansion of Gases ................................................................ 21
2.1.12 Dependence of the Pressure on the Temperature ................................................................. 22
2.1.13 Dependence of the Pressure on the Number of Molecules .................................................. 22
2.1.14 Diffusion of Gases................................................................................................................ 23
2.1.15 Diffusion of a Gas through a Porous Partition ..................................................................... 24
2.1.16 Brownian Motion in a Gas ................................................................................................... 25
2.1.17 Density Distribution in a Gas in the Gravitational Field ..................................................... 26
2.1.18 Local Distribution of the Molecules in a Gas ...................................................................... 27
2.2 Structure and Properties of the Liquids ............................................................................. 29
2.2.1 Configuration and Motion of Molecules in a Liquid ........................................................... 29
2.2.2 Increase of Temperature in Liquids when Supplying Energy .............................................. 29
2.2.3 Diffusion of Liquids ............................................................................................................. 30
2.2.4 Brownian Motion in a Liquid............................................................................................... 31
2.2.5 Evaporation of a Liquid ....................................................................................................... 31
2.2.6 Liquefaction of a Gas through Pressure ............................................................................... 32
2.2.7 Solidification of a Liquid ..................................................................................................... 32
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Physical Experiments on the Air-Cushion Table
2.3 Structure and Properties of Solids ...................................................................................... 34
2.3.1 Configuration and Motions of the Lattice Elements in a Solid ............................................. 34
2.3.2 Melting a Solid ....................................................................................................................... 34
2.3.3 Change of the Aggregation State of a Gas through Compression and Cooling ..................... 35
2.3.4 Heat Conduction in Solids ..................................................................................................... 36
2.4 Processes of Electric Conduction ....................................................................................... 37
2.4.1 Motion of an Electron in a Vacuum Under the Influence of an Electric Field
(Demonstrated By Means of Mechanical Forces)................................................................ 37
2.4.2 Deflection of an Electron Radiation in the Electric Field .................................................... 37
2.4.3 Motion of Electrons in a Vacuum Under the Influence of an Electric Field........................ 38
2.4.4 Principle of Electric Conduction .......................................................................................... 39
2.4.5 Influence of Lattice Elements on the Motions of Electrons in an Electric Field ................. 39
2.4.6 Motion of an Electron in a Metal Lattice Under the Influence of the Electric Field
– Ohmic Resistance (Demonstrated By Means of Mechanical Forces)............................... 40
2.4.7 Motion of the Free Electrons in a Metal .............................................................................. 41
2.4.8 Thermal Emission ................................................................................................................ 41
2.4.9 Bound Charge Carriers in an Insulator................................................................................. 42
2.4.10 Behavior of a Free Charge Carrier in an Insulator ............................................................... 43
2.4.11 Electric Conduction in a Semiconductor – Intrinsic Conduction
(Demonstrated By Means of Mechanical Forces)................................................................ 43
2.4.12 Electric Conduction in a Semiconductor – N-Type Conduction
(Demonstrated By Means of Mechanical Forces)................................................................ 44
2.4.13 Electric Conduction in a Semiconductor – P-Type Conduction
(Demonstrated By Means of Mechanical Forces)................................................................ 45
2.5 Nuclear Physics ................................................................................................................... 47
2.5.1 Scattering of Positively Charged Particles Near an Atomic Nucleus .................................... 47
2.5.2 Scattering of Alpha Particles When Passing Through a Metal Foil ...................................... 47
2.5.3 The Rutherford Atomic Model .............................................................................................. 48
2.6 Mechanical Motions............................................................................................................ 49
2.6.1 Vertical, Horizontal and Diagonal Projection ...................................................................... 49
2.6.2 Elastic Collision ................................................................................................................... 49
2.6.3 Change in the Direction of Motion of an Object with a Force ............................................ 50
4
Introduction
Physical Experiments on the Air-Cushion Table
Air cushions are produced and sustained by
means of air continuously emitted from jets in
one of the objects as they move against one another. This prevents any contact between the two
objects. As a “lubricant”, there is a thin gas cushion between them, similar to the oil film frequently used. Due to the much lower viscosity of the
air, friction is reduced to negligible levels.
Using the air cushion makes it possible to conduct many experiments in a much better quality.
A large number of experiments, however, are only
possible by making use of the air cushion.
A disadvantage of the common two-dimensional
air-cushion arrangement is limited visibility. To
observe the motions in two dimensions, it is necessary to step up closely to the setup. Such systems are furthermore very difficult to handle because of complicated stabilization and adjustment
procedures. The use of projection offers new
opportunities. It allows both an expedient reduction in the size of all parts of the experiment setup and a considerable improvement in visibility.
Finally, mechanical collisions proved to be too
inefficient. Since in this case only part of the energy is transmitted, it would have been necessary to take additional measures to compensate
for the loss of kinetic energy. Making use of the
forces between ceramic magnets allows the production of virtually fully elastic collisions. The
fact that an immediate contact between the colliding partners does not occur is no disadvantage
in most cases. This method is highly suited for
model demonstrations, e.g. of the force relations
on a microphysical level.
The gas cushion principle, use of projection and
use of magnetic forces make the air-cushion table a high-quality teaching aid, characterized by
simple operation, high reliability, universal usage and excellent methodological qualities. Some
of the experiments basically cannot be carried out
better with other currently known methods.
The air-cushion table is used mainly in model
demonstrations of microphysical procedures. The
characteristic vividness of models and the excel-
lent visibility make this demonstration a kind of
“window into the microcosm”. However, it is necessary to mind the shortcomings and limits of
modeling. Not only are the procedures highly simplified and represented in a purely mechanical
way, also the motions of the real objects are in
many cases determined by other forces. Furthermore, all procedures occur on one level. Finally,
models contain additional misrepresentations,
which become visible e.g. in the shape and color
of the hover discs.
Due to the relatively high throughput of air and
the small size of the hover discs additional driving mechanisms occur. The effect of these is that
the motion of the small, hover discs will not stop
as long as the airflow continues. This has the great
didactic benefit that many processes can be observed for any required duration, without any
need of intervention.
On the other hand, the limited force effect between the hover discs and between them and the
magnetic barriers determines a specific maximum
speed, which, in the case of very quick hover
discs, has practically been reached already after
one collision.
With the help of these mechanisms, optimal, wellvisible motions usually begin by themselves. The
driving mechanism increases the velocity; the not
fully elastic collisions limit it.
However, both mechanisms can also have an adverse effect by misrepresenting the motions of
interest. Only the knowledge of these processes
and their well directed usage or inclusion by the
experimenter allow full utilization of the great
potential of this valuable teaching aid.
On the following pages you will find a description of the setup and possible uses of the air-cushion table. Then you will find instructions for conducting important experiments.
The illustrations are meant to assist you in your
work. They are taken from the perspective from
which the teacher views the experiment setup on
the air-cushion table.
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Physical Experiments on the Air-Cushion Table
1. Setup and Possible Uses of the Air-Cushion Table
1.1 Components of the experimenting apparatus
Item Quantity Drawing
Air-cushion table 1
Air source 1
Tube 1
Holding device 1
Lattice model 1
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Physical Experiments on the Air-Cushion Table
Item Quantity Drawing
Plexiglas plate 1
Magnetic barrier 2
253 mm (no. 3 and no. 4)
Magnetic barrier 1
233 mm (no. 2)
Magnetic barriere 1
233 mm with slit for airflow
from the side (no. 1)
Magnetic barrier 1
233 mm with opening
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Physical Experiments on the Air-Cushion Table
Item Quantity Drawing
Flat magnetic barrier 1
Magnetic barrier made 1
of 4 magnets
Electrodes 2
Manipulating rod 1
Magnetic hover disc 30
Ø 16 mm, red
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Physical Experiments on the Air-Cushion Table
Item Quantity Drawing
Magnetic hover disc 25
Ø 16 mm, green
Magnetic aluminum 5
hover disc, Ø 21 mm
Magnetic hover disc 25
Ø 28 mm, orange
Magnetic hover disc 2
Ø 48 mm, blue
Magnetic piston 1
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Physical Experiments on the Air-Cushion Table
Item Quantity Drawing
Guide piece for the 1
magnetic piston
Fastening screws for 2
the holding device
Plastic tweezers 1
Aufbewahrungskasten 1
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1.2. Principle Uses of the Air-Cushion Table
The system kit allows for
- nearly frictionless movement of the hover
discs
- through the air cushion
- modeling the interactions between the microobjects and the field
- through magnetic forces
- through electrical forces
- by tilting the experiment surface
- excellent visibility of all experiments
- due to projection with the overhead projector
- little preparation work
- due to clear and simple system setup
- since only few adjustments required
The experimenter can continually adjust the influence factors and directly intervene in the experimental procedure.
All of this ensures a large variety of uses, preferably to demonstrate the behavior of individual
microobjects or microobject systems. Therefore
it becomes possible to create moving, vivid and
highly simplified models of complicated physical objects and phenomena, which one cannot observe directly.
Some of the forces taking effect in model experiments vary considerably from those occurring
between the real objects. In many cases, however, the force-distance relations are very similar,
so that special attention only needs to be paid to
them in quantitative experiments.
Despite this limitation, the air-cushion table is a
versatile, effective and appealing teaching aid
when handled by a qualified and methodologically skilled experimenter. When teachers have
fully understood the operation of the system and
follow the operating instructions for the system
described below, they can demonstrate experiments with physically convincing and effective
results.
1.3. Setup of the Air-Cushion Table
air-cushion table is made up of a frame and
The
a pressure chamber. The cover plate of the pres-
sure chamber has 1089 holes (ø 0.8 mm). This is
the experiment surface. The side of the pressure
Physical Experiments on the Air-Cushion Table
chamber where the impulse valve is located is
connected to the fan using the tube. The experiment surface can be set to the horizontal or inclined position by means of two adjusting screws.
.
.
Five different types of
with the air-cushion table. They are made of colored, transparent plastic or aluminum discs, onto
which cylindrical, ceramic magnets are attached.
Carried by the air cushion, these hover discs simulate the moving objects.
The experiment surface is delimited by a flat plastic frame. It is also possible to attach magnetic
barriers allowing for almost fully elastic collisions of the hover discs. Therefore it becomes
possible to demonstrate interactions with the vessel walls.
To create an electric field, two rod-type electrodes
can be placed on the experiment surface. A model effect of an electric field can also be attained
by inclining the air-cushion table to the desired
degree.
The impulse valve can be used to create an airflow parallel to the experiment surface influencing the motion of the hover discs. This can be
used to increase the speed of the hover discs.
The fan ensures a sufficient air cushion over the
experiment surface. Its performance is continuously adjustable and can be adapted to the conditions of the experiment. The fan is equipped with
a delivery connection and a suction connection.
While experimenting with the air-cushion table,
the delivery connection is used, the suction connection can be used for other physical experiments (e.g. with the transparency panel apparatus).
The lattice model is made up of 25 ceramic magnets, which are suspended by thin steel wires. This
system oscillates with little absorption. It is used
to demonstrate e.g. the interaction of a metal lattice with the moving charge carriers as a model.
It is inserted into the holding device in the same
way as the Plexiglas plate. The height in which
these components are located above the experiment surface can be adjusted as individually suited for each experiment by means of the setscrew.
The settings can be easily reproduced using the
scale marks.
hover discs are supplied
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Physical Experiments on the Air-Cushion Table
1.4. Instructions for Usage
The air-cushion table is placed onto the overhead
projector so that the arrow on the pressure chamber points to the projection screen. The magnetic
barriers (fig. 1) are placed onto the air-cushion
table so that their numbers (no. 1 to no. 4) match
the markings at the edges of the experiment surface. The magnetic barrier with the slit at the
bottom is arranged at the side of the table where
the tube of the fan connects. The air flows through
the slit and over the experiment surface when
activating the impulse valve.
The pressure chamber and the fan are connected
by the tube (fig. 2). The tube should run as straight
as possible. Only one position is possible when
connecting it to each of the devices. This is why
both ends of the tube and the connectors of the
devices are marked with a line. The tube is connected to the device so that both lines meet. Then
it is turned slightly to the right or to the left.
Next, the experiment surface is aligned horizontally by means of the adjusting screws at sides 2
and 4 by means of the spirit levels.
When needed, the impulse valve is pressed several times for approx. 1 second. The fan has to be
set to a sufficient performance level, since otherwise the pressure of the air cushion is too low
and the hover discs will sink onto the experiment
surface.
To install the lattice model, the holding device is
screwed onto the frame of the air-cushion table.
The lattice model is then inserted into the groove
of the holding device (fig. 3) The influence of
the lattice model on the motion of the hover discs
strongly depends on the height of the lattice over
the experiment surface. The holding device,
which is marked with a scale (fig. 4) can be infinitely adjusted to the appropriate height using a
setscrew.
This allows for demonstrations of the behavior
of conductors, semi-conductors and insulators.
The electrodes are used to create an electric field.
They can be applied in two positions. Placing
them onto their base will create a gap between
the experiment surface and the electrodes. This
gap is large enough for the aluminum hover discs
to fit through. These are then charged in accordance with the polarity of the respective electrode.
The electrodes can also be turned around so that
their bases point upwards. Then the aluminum
parts touch the experiment surface and the hover
discs contact the electrodes.
The voltage applied should be over 20 000 V.
When the voltage is lower, the electrodes have to
be arranged closer to each other.
An especially well-suited voltage source is the
electrostatic generator.
The influence of the electric field on the motion
of the hover discs can also be demonstrated by
slightly tilting the experiment surface. The degree of inclination then corresponds to the
strength of the electric field.
The strength of the fan is adjusted until the hover
discs just begin to move freely. This ensures a
relatively low level of noise. When the airflow is
stronger, disturbance caused by the noise of the
fan cannot be avoided. For this reason, it should
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Physical Experiments on the Air-Cushion Table
be positioned behind the experimenting table or
inside it. This will reduce the noise level reaching the classroom. Further
be reached by wrapping sponge rubber around
the fan or lining it with Piatherm or the like. Special care should be taken, though, to ensure that
the air can enter the suction nozzle unimpeded.
1.5. Maintenance and Care
The air-cushion table is a high-quality apparatus, which requires special care. Its stability has
certain limits because of the consistency of the
necessarily transparent material.
noise reduction can
- Avoid damage caused by dropping, hitting,
bumping, dragging or sliding.
- Keep all parts clean and free from dust.
- Remove dust with an anti-static cloth. Strong
rubbing of the table surface causes electrostatic charging which may considerably affect
the experiments.
- To keep the pressure chamber clean, do not
place the airflow generator near dust accumulations.
- Keep the bottom sides of the hover discs clean
at all times. They can be easily cleaned using
ethyl alcohol.
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Physical Experiments on the Air-Cushion Table
2 Description of the Experiments
2.1 Structure and Properties of Gases
2.1.1 Motion of a Molecule in High Vacuum
Components:
Air-cushion table with fan
Overhead projector
Magnetic barrier, long 2 Pieces
Magnetic barrier, short 2 Pieces
Hover discs l Piece
Model simulation
Real Object Model
Vessel containing Experiment surface of
the gas the air-cushion table
Walls of the vessel Magnetic barriers
Gas melecules Hover discs
How to proceed:
Align the air-cushion table horizontally and attach
the magnetic barriers.
Turn the fan to a medium setting. Place the hover
disc onto the experiment surface and give it an
impact so that it hits a magnetic barrier in the
middle at an angle of 45°.
Result:
The motion of the hover disc is straight and uniform. When it hits a barrier, the direction of its
motion changes. The speed is unchanged. The
hover disc rebounds at the same angle at which it
hits the barrier. The law of reflection applies.
2.1.2 Motion of the Molecules in a Gas
Components:
Air-cushion table with fan
Overhead projector
Magnetic barrier, long 2 Pieces
Magnetic barrier, short 2 Pieces
Hover disc, red 16 Pieces
Model simulation
Real Object Model
Vessel containing Experiment surface of
the gas the air-cushion table
Walls of the vessel Magnetic barriers
Gas molecules Hover discs
How to proceed:
Align the air-cushion table horizontally and attach
the magnetic barriers.
Place the 16 red hover discs anywhere on the
experiment surface so that the spaces between them
are approximately 1 cm. Then turn the fan to a
setting in which all hover discs are sure to lift off.
Result:
Each hover disc moves in a straight and uniform
way as long as it does not hit any other hover disc
or a magnetic barrier. When two hover discs
collide, their speed and direction of velocity usually
changes. These collisions cause a transmission of
kinetic energy. When hitting the magnetic barrier,
only the direction of velocity changes.
Interpretation:
The gas molecule moves in accordance with the
laws of classical mechanics.
Interpretation:
Elastic collisions occur between the molecules
of a gas and when molecules hit the vessel wall.
Along the distance covered between two collisions, the “free length of path”, the motion of the
molecules is straight and uniform.
14
Note:
This experiment can be developed from the one
described above in 2.1.1. by placing three additional
orange hover discs onto the experiment surface one
after the other while keeping the fan turned on. The
collisions between the discs and the transfer of
kinetic energy caused by them can be especially well
observed when using a low number of discs.
2.1.3 Dependence of the Number of Impacts
with the Vessel Wall on the Velocity of
the Molecules
Components:
Air-cushion table with fan
Overhead projector
Magnetic barrier, long 2 Pieces
Magnetic barrier, short 2 Pieces
Hover discs 2 Pieces
Stop watch or master clock l Piece
Physical Experiments on the Air-Cushion Table
Note:
The hover disc can also be set into motion so that
it hits the barriers at a perpendicular angle.
Model simulation
Real Object Model
Vessel containing Experiment surface of
the gas the air-cushion table
Walls of the vessel Magnetic barriers
Gas molecules Hover discs
How to proceed:
After aligning the air-cushion table horizontally,
attach the magnetic barriers.
Turn the fan to a setting in which two hover discs
placed above each other are sure to lift off. Give
this doubled hover disc an impact so that it hits
one of the barriers in the middle at an angle of
45°. Count the impacts with the wall occurring
within a given period of time (10 seconds).
Then repeat the experiment with only one of the
hover discs at a higher velocity.
Result:
The higher the velocity of the hover disc is, the
more often it will hit the magnetic barrier within
a specific period of time.
Interpretation:
The higher the velocity of the molecules is, the
more impacts of the gas molecules will occur with
the vessel wall. Since these impacts cause the
pressure, higher molecule velocities cause higher pressure.
2.1.4 Dependence of the Number of Impacts
with the Vessel Wall on the Volume
Components:
Air-cushion table with fan
Overhead projector
Magnetic barrier, long 2 Pieces
Magnetic barrier, short 2 Pieces
Hover disc, red l Piece
Hover disc, grün l Piece
Stop watch or master clock
Model simulation
Real Object Model
Vessel containing Experiment surface of
the gas the air-cushion table
surrounded by magnetic
barriers
Walls of the vessel Magnetic barriers
Gas molecules Hover discs
How to proceed:
Align the air-cushion table horizontally and attach
the magnetic barriers.
Turn the fan to a setting in which the hover discs
are sure to lift off. Then place both hover discs
into one corner of the experiment surface, first
holding them with two fingers and then quickly
releasing them. Count the number of impacts one
of the discs performs with the vessel walls within
a specific period of time (5 seconds).
15
Physical Experiments on the Air-Cushion Table
Then reduce the area available for the hover discs
to half its size. To do this, lift up magnetic barrier
no. 2 and reattach it so that it separates the
experiment surface into two halves, with its ends
snapping into the recesses provided in barriers
no. 3 and no. 4. Now set both hover discs into
motion in the same way. Count the number of
impacts one of the hover discs performs with the
magnetic barriers within the same period of time
as in the previous experiment.
Result:
In the first experiment, the number of impacts with
the barriers is lower than in the second. By reducing
the area to half its size, the number of impacts
increases to approximately the double amount.
Interpretation:
Reducing the volume of a vessel containing a gas
causes an increase in the number of impacts of
the gas molecules with the vessel walls within a
specific period of time. Since the number of impacts occurring in a specific period of time with
a specific wall is an indicator of pressure, the
conclusion is that reducing the volume increases
the pressure.
Note:
Evaluation is easier when counting only the
impacts with magnetic barrier no. 2 in each of
the experiments. This, however, requires longer
times of measurement.
It is also possible to ascertain the total amount of
impacts of both discs. In this case it is recommended that one student counts the impacts of
the red disc while another student counts those
of the green. The results are then added up.
2.1.5 Mean Velocity of the Molecules
– Temperature of a Gas
Components:
Air-cushion table with fan
Overhead projector
Magnetic barrier, long 2 Pieces
Magnetic barrier, short 2 Pieces
Hover discs, red 16 Pieces
Model simulation
Real Object Model
Vessel containing Experiment surface of
the gas the air-cushion table
Walls of the vessel Magnetic barriers
Gas molecules Hover discs
How to proceed:
Align the air-cushion table horizontally and attach
the magnetic barriers.
Position all hover discs in one corner of the
experiment surface so that the spaces between
them are approximately 1 cm.
The fan is turned to a medium setting.
Sequentially observe the motions of each of the
hover discs. Draw attention to the velocity of each
disc in relation to the velocity of all other discs.
Gradually turn down the fan so that all hover discs
come to a stop and then turn it up again so that
they are sure to lift off. The same observations
are repeated at a lower velocity.
Result:
The velocity of each hover disc changes with each
impact. While an impact with the vessel wall
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