Static Electricity
A FOCUSSED CONCEPT
Copyright ©2000-2004 J. A. Panitz
eLAB
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VISUAL E&M
Introduction ................................................. 2
Review Theory ............................................ 3
Review Theory ............................................ 4
The Experiment ........................................... 5
Getting Started ........................................ 5
Prepare for Data Collection ..................... 6
Observe a Corona Discharge .................. 7
See the Light ........................................... 7
Observe an Electric Wind ........................ 8
Examine the Breakdown of Air ................ 8
Determine the Dome Polarity .................. 9
Perform a Hair Raising Experiment ....... 10
Team Discussion ........................................11
Finish Up ................................................... 12
Supplies and Materials .............................. 13
Supplies and Materials .............................. 14
Introduction
Figure 1 shows a “Van de Graaff” generator.
This device generates static electricity by
rubbing two dissimilar materials together. A
motor inside the plastic cover rubs a Teflon®
pulley against a rubber belt. The static
charge created by this process is transported
by the belt to the large aluminum dome
where it is stored. The charge on the dome
creates a large potential difference between
the dome and the ground terminal at the
base of the generator.
ALUMINUM
DOME
INSULATING
COLUMN
The goal today is to observe static electricity
with the Van de Graaff generator. The
operation of the Van de Graaff generator will
be explored.
PLASTIC
COVER
GROUND
TERMINAL
BASE
LINE CORD
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Figure 1. Van de Graaff Generator
A rubber belt that
passes over the
teflon pulley is the
BELT
second insulator.
As the shaft of the
PULLEY
motor rotates, the
MOTOR
surface of the
SHAFT
pulley acquires a
negative charge
LOWER
BRUSH
and the inner
surface of the belt
a positive charge.
Figure 2. The lower pulley.
The outer surface
of the belt acquires an equal amount of
negative charge by a “corona discharge” at
the pointed ends of the lower brush.
Review Theory
Robert Jemison Van de Graaff conceived the
idea of the generator that bears his name
while working in Oxford in the late 1920’s.
The purpose of the Van de Graaff generator
is to collect a large amount of charge in order
to generate a very high voltage, typically in
excess of 150,000 Volts. The generator uses
a process that is familiar to everyone. If you
rub your shoes across a carpet in dry
weather (typically in the winter) your body
can acquire a static charge. The spark that
occurs when you approach a radiator
indicates that a high voltage was generated
by the static charge on your body
The Van de Graaff generator creates a high
voltage by storing a static charge on a
conducting dome. The charge is created
when two different insulators are rubbed
together. One insulator is typically made of
Teflon® and acts as a pulley. The pulley is
pressed onto the shaft of an electric motor
in the base of the generator. See Figure 2.
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Copyright ©2000-2004 J. A. Panitz
The charge on the inner and outer surface
of the belt is transported to the upper pulley
which is isolated and made of aluminum.
Free electrons in the aluminum neutralize
some of the positive charge on the inside of
the belt. As a result, the upper pulley looses
electrons and acquires a net positive charge.
Review Theory
BRUSH
PULLEY
BELT
Figure 3. The upper pulley.
The positive charge
that remains on the
inside of the belt
moves to the surface
of the lower pulley
where it neutralizes
some of the negative
charge. As the belt
moves, the upper
pulley acquires more
positive charge.
ALUMINUM
DOME
E
The dome rests on a conductive frame. See
Figure 3. As the positive charge on the upper
pulley increases, a corona discharge is
created at the pointed ends of the upper
brush. As the corona discharge occurs a faint
blue-red glow appears at the end of the
wires. Electrons from the sphere feed the
discharge and are attracted to the positively
charged pulley. Before they reach the pulley
they are intercepted by the outer surface of
the moving belt. Since the dome loses
electrons it acquires a net positive charge.
ALUMINUM
PULLEY
MOTOR
DRIVEN
PULLEY
(TEFLON)
E
START
RUN
e
Figure 4. The charging process.
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FRAME
As the belt moves, the electrons on its outer
surface are transported around the lower
pulley and move toward the lower brush. As
the electrons pass the lower pulley a corona
discharge is created at the pointed ends of
the lower brush. Positive ions in the
discharge are attracted to the belt where they
neutralize some of the electrons on its outer
surface. As the belt turns, the process
continues and the dome acquires a greater
positive charge See Figure 4.
The Experiment
6. The pulleys (Figures 2-3).
7. The brushes (Figures 2-3).
Getting Started
1.
Discuss the theory and the experiment.
Plan a way to share the workload.
Managers: select a discussion question.
5.
Turn on the generator. Notice how the
belt moves over the upper and lower
pulley. Turn the generator off.
2.
Lift the aluminum dome from the top of
the Van de Graaff generator. Place the
dome on the table.
6.
Make a scale drawing of the generator
in your lab notebook. Label each
component and describe its function.
3.
Loosen the three mounting screws at
the base of the generator. Hold the
insulating column. Lift and remove the
plastic cover from the base of the
generator. See Figure 2.
7.
Attach a nail to the dome with tape as
shown in Figure 5. Align the nail with
the groove at the midpoint of the dome.
Do not replace the dome!
DOME
NAIL
Discuss the theory and the construction
of the Van de Graaff generator. Identify:
1.
2.
3.
4.
5.
TAPE COVERS
END OF NAIL
Figure 5. Nail Attached to Dome (Top View).
The aluminum dome (Figure 1).
The insulating column (Figure 1).
The ground terminal (Figure 1).
The motor shaft (Figure 2).
The rubber belt (Figures 2-3).
5
Create a ground electrode. Use a test
lead to connect a metal sphere on an
insulating handle to the ground terminal
on the generator.
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8.
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4.
Prepare for Data Collection
When the plug at the end of the line cord is
inserted into a receptacle the long pin on the
plug will connect to terminal (G). The length
of the pin insures that ground will be the first
connection made at the receptacle. This pin
is attached to the green wire in the line cord.
The Van de Graaff generator can store a
large amount of charge on its aluminum
dome. If you touch the dome the charge will
flow through you to the earth at “ground
“potential. A flow of charge is called a
“current”. Although you would receive a
noticeable shock, the current that would pass
through your heart is not lethal. The shock
hazard is minimized by connecting the lower
brush in the base of the generator, through
the line cord and plug, to the ground terminal
(G) on a wall receptacle. See Figure 6.
The short, flat pin in the plug is “Hot” because
it connects to one side of the 120 V line at
the wall receptacle (H). This pin is connected
to the black wire in the line cord. The
remaining flat pin is “Neutral”. It connects to
the other side of the 120 V line in the wall
receptacle (N). The neutral pin is connected
to the white wire in the line cord. This “color
code” is used for all 120 V (AC) wiring.
H
N
1.
With the generator off, verify that the
green wire in the line cord is connected
to the brush and ground terminal in the
base of the generator. Observe how the
black and white wires are connected.
2.
Turn off the room lights.
G
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Figure 6. A wall receptacle.
Observe a Corona Discharge
See the Light
Corona discharge can be observed in a dark
room after your eyes have dark adapted.
Wait five minutes. The red screen will prevent
you from losing your dark adaptation.
If you move a fluorescent lamp toward a
corona discharge, charge will be induced on
its surface. The induced charge will create
a potential difference across the tube and
the phosphor in the tube will glow.
2.
3.
Turn on the generator. Darken the
screen. Observe the corona discharge
at the end of both brushes. Record your
observations in your lab notebook.
Notice the smell of ozone that
accompanies a corona discharge.
Turn off the generator. Balance the
dome (with the nail attached) on the wire
support. The dome should rotate freely.
Turn on the generator. Darken the
screen. Examine the pointed end of the
nail. Record your observations in your
lab notebook
Move a fluorescent tube toward the end
of the nail. Notice the appearance and
the stability of the glow in the tube as a
function of distance and position.
2.
Record your observations in your lab
notebook.
3.
Turn off the generator.
4.
Touch the ground electrode to the dome.
Hold the insulating rod!
Always perform this step after you
turn off the generator to minimize
the possibility of a shock.
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1.
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1.
Observe an Electric Wind
Examine the Breakdown of Air
The glow associated with a corona discharge
is caused by the ionization of air at sharp
protrusions. The ionization process creates
positive and negative ions. The movement
of the ions creates an “electric wind”.
Electrical “breakdown” is a process that
occurs when a high voltage is applied across
two electrodes. When breakdown occurs a
spark jumps between the electrodes. In air,
the spark creates a conductive path of
positive and negative ions. At atmospheric
pressure, about 3 x 106 Volts is required to
create a 1 m spark in dry air. This value is
called the “breakdown field” (E).
Remove the nail from the dome.
2.
Turn on the generator.
3.
Hold the Electric Whirl by its aluminum
base (Figure 7).
Move it toward
ARM
the dome of the
generator.
Observe the
pointed end of
BASE
each arm.
1.
Hold the ground electrode 0.01 m from
the dome. Record the number of sparks
that occur each minute.
2.
Move the ground electrode slowly away
from the dome. At some distance (d) a
spark will not occur regardless of how
long you wait. Record this distance to
the nearest 0.01 m in your lab notebook.
3.
Find the voltage of the Van de
Graaff generator (V).
Figure 7. The Electric Whirl.
4.
Record your observations in your lab
notebook.
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1.
Determine the Dome Polarity
You can determine the polarity of the dome
with a neon bulb. A neon bulb is a glass tube
containing two metal electrodes and neon
gas below atmospheric pressure. When a
potential difference greater than 90 V is
applied between the electrodes, the neon
gas will ionize and a glow will appear around
one electrode which is called the “cathode”.
The cathode is always at a negative
potential. Examine Figure 8. The neon bulb
indicates the dome is at a positive potential.
Perform the polarity test shown in Figure 9.
1.
Tape one lead of a neon bulb to the
ground electrode. Bend the other lead
away from the electrode.
2.
Move the ground electrode toward the
dome. Record your observations in your
lab notebook.
3.
When the free lead of the bulb is very
close to the dome a glow will appear
around one electrode. Sketch what you
observe in your lab notebook. Show the
polarity of the dome.
4.
Describe the polarity test in your lab
notebook.
5.
Turn off the generator.
6.
Turn on the room lights.
NEON BULB
TAPE
DOME
GROUND
ELECTRODE
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Figure 8. Polarity Test
Perform a Hair Raising Experiment
1.
Hold a small mirror in one hand. Stand
on a plastic stool and place your other
hand firmly on the sphere. Ask a team
member to turn on the generator.
2.
Look at your image in the mirror. If the
air is dry your hair should begin to rise
as shown in Figure 9. Ask a team
member to turn off the generator and
touch the ground electrode to the dome.
Remove your hand from the dome
before the dome is grounded or you
may receive a shock!
5
Remain on the plastic stool for at least
one minute to allow the accumulated
charge to dissipate into the air. Step off
the stool.
Figure 9. A Hair Raising Experiment
Record your observations in your lab
notebook.
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4.
Team Discussion
Summarize the experiment and the
conclusions that were reached. Go to
the blackboard, sketch the essential
features of the apparatus and compare
the results from all the teams.
5.
Discuss and describe the optimum
shape for the dome of a Van de Graaff
generator. Consider the result of using
a highly polished, hollow cube for the
“dome”.
2.
Review the calculation of the generator
voltage. Discuss the uncertainty in your
estimate.
6.
3.
Discuss the corona discharge
phenomenon. From your observations,
describe how a lightning rod functions.
Suppose you are hired by a scientific
supply company to change the polarity
of the Van de Graaff generator used in
this experiment. Discuss and describe
a procedure that could be used to make
the dome acquire a negative charge.
4.
Discuss the hair raising experiment.
Explain the sensations that were
experienced and the appearance of
your hair.
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1.
Finish Up
1.
2.
3.
Enter a brief summary of the experiment
and the team discussion in your lab
notebook.
Quit the software menu.
Manager: Ensure that all components
and hardware used by your team are
returned to their original state and
placed in their original location.
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Supplies and Materials
Van de Graaff apparatus. Figure 10.
1 each Van de Graaff Generator
Science First 10-060.
1 each Ground Electrode.
CENCO CP70734-00.
1 each Test Lead. 24”.
Alligator Clip to Banana Plug
Pomona. 116624-0.
1 each Foot Stool (Plastic).
WalMart 007169109727.
VAN DE GRAAFF
GENERATOR
FOOT STOOL
GROUND
TERMINAL
TEST LEAD
GROUND ELECTRODE
1 each Meter Stick.
Figure 10. Van de Graaff apparatus
1 each Mirror. 3” (Hand Held).
1 each Lamp. Fluorescent.
Sylvania. F15W/TB/WN.
1 each Nail. Finishing. 10d x 3”.
1 roll
Tape. Scotch™ 810.
1 each Neon Lamp. EIKO C2A.
Copyright ©2000-2004 J. A. Panitz
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Supplies and Materials
1 each Screw Driver. Flat head.
1 each. Patch Cord. 36” (Black).
Banana Plug to Alligator Clip.
Pomona. 1166-36-0.
1 each Electric Whirl. (Figure 11).
(a) Arm.
(b) Rod.
(c) Base.
Winsco. N-124.
ARM
ROD
BASE
Figure 11. Electric Whirl.
Copyright ©2000-2004 J. A. Panitz
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Tuesday, February 12, 2002
VandeGraaff Hints and Demonstrations
VDG Hints and Demos
(c)1994 William J. Beaty
Shortcuts to articles below
VDG versus your PC
Turn off VDG without being shocked
Humidity Emergency
"Giant" VDG Lightning
Sky Pie
Motor Magic
Dancing Bubbles
Electrostatic Levitation
Magic Blinking Wand
Invisible threads of air
Faux-Doo
Crackle's Hailstorm
Conductive "Insulators"
Plus or Minus?
Paperclip Ray
Zap Gun
Is "Static Electricity" evil?
Educational Purpose
MOTOR
MAGIC
Build the pop-bottle electrostatic motor from plans elsewhere on my web
pages. Use your VDG as a power supply for the motor. If you ground one
bottle of the motor, stand on a plastic insulator, touch the VDG sphere,
and point at the other bottle of the motor, the motor will start to
turn. It is powered by the charged wind coming from your fingertip.
Initially hold your finger a couple of inches from the bottle until you
get the demo working. I managed to slowly increase the distance to the
motor, retuning the motor brushes each time for best operation, until I
could point at a motor which was four feet away! By instead using a
paperclip taped to the VDG sphere and bent so it pointed at the motor, I
managed to run the bottle motor from 10 feet away! This was in dry
weather, when the VDG was working very well.
"Giant"
VDG
Lightning
If you have access to a second large VDG sphere, you can create immensely
long, dim sparks. (Remember, spark length is only limited by the voltage
if the radius of electrode curvature is much larger than the gap between
electrodes, and small electrodes can create VERY long sparks.) Connect
the second sphere to ground, and position it about 6in from the VDG sphere
terminal. Affix to the grounded sphere a 1/4 in. ball bearing, or an
"acorn" type 8-32 nut with a spherical head. Position the spheres so that
the nut is in the gap between terminals. When the VDG is run, the nut will
initiate the spark, and the field between the terminals will provide
energy to allow quick growth. Spark brightness decreases as length
increases, so turn off the lights. In a darkened room, increase the
separation between the spheres until you have long sparks. In this way
I've occasionally managed to produce 24 in. sparks from a Science First
14" sphere. If you instead glue a ball bearing to a thread and lower it
between the spheres, it will trigger the lightning in the same way that
aircraft can trigger lightning when flying near storm clouds.
Turn
off
VDG
without
pain
Turning your VandeGraaff machine *on* is no problem, but how can you touch
the metal switch to turn it off without being zapped? There are several
methods. If you carry a metal object in your hand, such as car keys, you
can touch the switch with the metal object, hold it there, then turn off
the switch with fingers. The painful spark hits the metal, not your skin.
Another way: hold onto a grounded wire when you turn the machine on, and
never let go of ground. All objects near a VDG will become electrified
because of the charged wind emitted by the metal sphere, but if you keep
touching a ground wire, you will stay neutral, and will not receive a
shock when touching the switch. (Note that it does not work to simply
ground the VDG power switch, it is YOU who must be grounded.) And, in
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Tuesday, February 12, 2002
VandeGraaff Hints and Demonstrations
a pinch, it helps to just whack the switch with the palm of your hand.
You'll still get zapped, but it hurts less when it's not your fingertips,
and when you do it fast.
Sky
Pie
Place a small aluminum foil pie pan upside down on the dome of your VDG.
When you turn on the power, it will levitate and fly off to the side.
Alike charged objects repel each other. Big thrills? But wait, what if
you place TWO pie pans nested on your VDG? When you turn on power, the
top one holds the bottom one down, until the top one flies off, which then
allows the bottom one to take off. SOOOO, place an entire stack of thin
foil pie pans upside-down on top of your generator, and get ready for a
pan storm. When run, your generator will loft each pan in sequence and
fling them in various directions. This works best with those little "pie
tart" pans about 10cm dia.
DANCING
BUBBLES
Blow soap bubbles at your VDG terminal. They will initially be attracted,
but then will become charged by ion wind and will then be violently
repelled from the generator sphere. They will also be attracted to any
other object. With practice you can hold your hand above a charged bubble
and keep it aloft by attraction.
ELECTROSTATIC
LEVITATION
Place a large metal sheet or foil-covered cardboard on the ground. It
should be at least 2 to 3 times the diameter of your VDG sphere. Connect
this sheet to earth-ground. Place some small crumpled pieces of foil on
the center of the sheet. Pick up your entire VDG machine, turn it on, and
while holding it by its base, move the sphere down towards the crumpled
foil. With practice you can get the foil to levitate and hang in the air
between the sphere and the ground plate. The VDG attracts the grounded
foil, but then the corona discharge from the edges of the foil chunk will
form a conductive path in the air which allows the metal to acquire a
like-charge from the sphere, which increases the repulsion force. As the
foil drops away, it loses its charge via corona, and is again attracted
upwards. At a particular distance you can get a piece of foil to hang
unmoving in space with balanced attraction/gravity forces and continuous
corona leakage. (Note: there may be ion-wind filaments associated with
this phenomenon. Someone should do the foil-lifting experiment in front
of a Schlieren system and look for grey lines in the air.)
MAGIC
BLINKING
WAND
Connect a small capacitor (.01uF, 250V) in parallel with a small neon
pilot light (NE-2 or NE-2H). Hold one wire between fingers and bring the
other wire towards the sphere of an operating VDG. The wire will
intercept part of the ion current flowing from the sphere, and the bulb
will begin flashing. DON'T touch both leads at once, or you will get a
shock from the capacitor. For safety, you can connect the two leads of
your device in series with 1-meg resistors, then cover everything except
the floating resistor leads with insulating caulk. For dramatic effect
you can mount this assembly in the tip of a metal wand, with one lead
connected to the metal. The closer you bring the wand to the VDG, the
faster the neon bulb will flash. Use a larger capacitor for slower,
brighter flashing, or a smaller one for a fast, dim flicker.
.01 uF 250v
______|
|
|
O---\/\/\----|
1 meg
|_____/|
\|
|______
|
|
|----\/\/\---O
|\_____|
1 meg
|/
NE-2 BULB
FAUX
-
DOO
Sometimes the humidity is too high, and even though your machine does give
sparks, the "hair raise" demo doesn't work. There is just too much
leakage to ground. All is not lost, you can cut up some strips of tissue
2cm x 15cm each and tape them all over your VDG sphere. When operated,
the tissue strips stand out just fine. Note that sometimes this will fail
during *low* humidity because the paper strips become good insulators, so
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Tuesday, February 12, 2002
VandeGraaff Hints and Demonstrations
they attract to the metal sphere and do not become alike-charged. I
discovered that drawing a line on each strip with india ink can help.
India ink is somewhat conductive.
Placing tissue-strips on the sphere is also useful as a visual indicator
of voltage. If the strips do not rise, you know that something is wrong.
If you are experimenting with e-motors, or designing your own VDG, etc.,
tissue strips are just as good as a voltmeter for detecting heavy loads,
total shorts, bad rollers, etc. If you are de-humidifying a dead machine
with a hair dryer, attach tissue strips then operate the machine while
warming the belt. When the tissue strips suddenly rise, you know you're
successful.
Sometimes the humidity is nice and low, yet your audience will have no
long-hair 'victims.' In this case simply whip out your halloween costume
1960s "Cher" wig and stick it on the sphere-terminal. And if the
*demonstrator* lacks hair, this opens up an opportunity for a variety of
humorous banter and setting your audience up by donning an unobtrusive
long-hair hairpiece before the demonstration... ahem!
CRACKLE'S
HAILSTORM
This is the hands-on version of "Volta's Hailstorm". Pour a small pile of
Rice Crispies on top of your VDG and turn on the power. Be prepared for a
mess! If your machine is fairly powerful, you can try standing on an
insulator, touching the VDG terminal, then extending a handful of cereal
(flat palm, fingers spread hard) and they will levitate and fly to the
nearest uncharged surface (your audience!) Note: a totally fresh box of
Rice Crispies will have such a low humidity that the cereal will be
insulating and won't acquire a charge from the terminal. If you open a
new box of cereal before a demo, spray a bit of water into the box and
shake well to distribute the moisture.
PLUS
OR
MINUS?
To determine VDG polarity, turn off and discharge your machine, then
connect a sensitive ANALOG current-meter between the sphere and the base.
(Note that Digital microamp meters can be destroyed by accidental
discharges, so use a moving-needle meter instead.) When running, the VDG
will produce a few tens of microamperes in the same direction as the high
voltage polarity. If you lack a microamp meter, a tiny NE-2 neon pilot
light will serve. When connected between sphere and base, one electrode
will glow orange when the machine is run. The orange electrode is the
negative one, the dark electrode is positive.
Analog microamp meters are useful for other things: when repairing a VDG
or building a new one, measure your machine's output current rather than
the voltage. Connect the microamp meter leads to the upper comb and the
lower comb (or simply tape the meter leads to the sphere and the grounded
motor assembly.) Run your machine and tweak the comb spacing for maximum
meter reading. The higher the current, the faster the recharge rate, and
the better it will work during humid days. Ever wondered if other types
of roller or belt materials would work better? Well, try the alternate
materials and see if they give higher microamp readings! FYI, typical
readings are 5 uA (microamps) and up. A healthy VDG might give 10 to 20
uA. A fast machine with a very wide belt might give as much as 300 uA.
PAPERCLIP
RAY
Tape a short piece of wire or an unbent paperclip to the side of the
sphere of your VDG machine. Bend the wire so it points outwards. When
the VDG is running, a stream of charged wind spews forth. This stream is
a genuine Ion Beam. It will electrify distant surfaces, charge whole
people if they are standing upon an insulator, and will run e-motors and
fluorescent tubes at a distance. When this air gets on your clothes and
the humidity is low, it makes them cling to your body as if wet. Also, it
feels surprisingly cold. The air is attracted to conductive surfaces, and
this disrupts the usual "boundary layer" of air which insulates the
surface (ionized air has larger "wind chill" than normal.) Warning: never
direct the ion beam towards a computer, it can induce electrostatic
discharges INSIDE the computer case and keyboard.
CONDUCTIVE
"INSULATORS"
A surprising number of "insulators" behave as conductors when used with a
VDG machine. Wood, cardboard, paper, twine, floors, and shoe soles can
all behave as conductors as far as a VDG machine is concerned. Why?
Well, consider a 12-volt, 1-amp, 12-watt flash lantern. In this device
the light bulb has a resistance of 12 ohms and the wires contribute nearly
zero ohms. As far as the flash lantern is concerned, material which is
far more conductive than the 12-ohm lightbulb is behaving as a conductor,
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VandeGraaff Hints and Demonstrations
while anything far less conductive is behaving as an insulator. Now look
at a VandeGraaff machine. A typical output is 30 microamps at 300,000
volts, giving a load resistance of V/I, or 10,000,000,000 ohms. As far as
your VDG is concerned, "insulators" must have far more resistance than
this. And "conductors" have far less. If a piece of wood has a billion
ohms of resistance from end to end, your VDG machine will "think" that it
is a conductor!
THREADLIKE
FLOWS
OF
ELECTRIC
WIND
When in an e-field, hairs and other sharp objects create tiny coronas
which emit "electric wind". These invisible flows of air are extremely
narrow and fast, and their effects can be made visible by using dry-ice
fog.
Materials:
- VDG machine
- wire and tape (or clip-leads)
- Tray of warm water sitting on insulator
- chips of dry ice
- dark paper (submerged in the water for contrast)
Procedure:
Drop several CO2 chips in the water so that a thin layer of fog forms.
Use tape and a wire to connect the tray to the sphere of your VDG.
Charge the tray with respect to ground.
Move your hand slowly over the fog, keeping your hand a few inches above
it. You'll see small mysterious furrows being carved in the fog by
the invisible, narrow threads of "electric wind."
If your hands are extremely clean (no sharp microscopic defects), then
instead try waving a torn bit of paper over the mist. The sharp paper
fibers seem to generate these "threads" of charged air fairly well.
If humidity is very low, then perhaps the paper should be made moist.
Wave your hand fast, and the spots in the mist will follow your hand's
motions. Pull your hand back, and the spots still appear.
Form a "thread", then wave a charged object near it. The spot in the
mist moves, indicating that the "thread" is being deflected.
Use a soda straw to blow hard across a "thread".
in the mist will move only a small amount!
The corresponding spot
Drop some short (1cm) pieces of hair onto the charged water surface.
They will stand on end, emit "threads" upwards, and narrow flows of
entrained mist will be seen to project upwards from the fog layer.
Also see Air Threads article.
VDG
VERSUS
PC
A VDG is a constant current source. During normal operation there is
a large e-field around the device, but there is also a flow of charge
between the sphere and ground. This flow is composed of charged
air, and while some of it manages to get to ground, much of it
is attracted to surfaces, and lots of it travels far beyond the
region immediately around the machine. If the humidity is low, it will
build up on insulating surfaces where it creates large e-fields and
high voltages. In other words, an operating VandeGraaff SPEWS CHARGED
WIND which wanders around charging up EVERYTHING in the room, including
the walls, ceilings, people, etc. The fans on your PC will suck the
charged air into the case, where it will electrify all non-metal surfaces,
cause huge electrostatic fields and sudden sparks, and generally trash
circuitry left and right. It also tends to collect on ungrounded
plastic parts such as keyboards. It is wise to avoid doing VDG demos
in the same room with an operating computer. If this cannot be avoided,
then keep your demo short, keep the VDG as far as possible from your
computer, VCR, etc., keep the VDG turned off or shorted with a ground
clip except during actual use, and avoid days with extremely low
humidity. Better yet, do a Tesla coil demo instead!
ZAP
GUN
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Tuesday, February 12, 2002
VandeGraaff Hints and Demonstrations
If you can locate a "Zerostat (tm)" record-cleaning gun (Discwasher Inc.), you
can perform the following. Tape strips of tissue all over your VDG
sphere. Turn it on so the strips stand out, then turn it off. The strips
remain standing. Now "shoot" the sphere with the Zerostat gun. The
strips will collapse!. This "gun" contains a Barium Titanate piezo
crystal connected to a sharp needle in the gun's tip. Squeezing the
trigger send a few microamps of charged wind out through the gun's tip.
Ionized air is a conductor, so the presence of ionized air near the
generator allows the charge on the sphere to leak away.
THE
TERM
"STATIC
ELECTRICITY"
When explaining VandeGraaff machines it's probably a good idea to avoid
the words "Static Electricity." A VDG machine is simply an electric power
supply which has a characteristic of high voltage and low current output.
This is in contrast to a dry cell battery. Dry cells are electric power
supplies which give high current and low voltage output.
While it's true that electric charge, charge imbalance, voltage, current,
power, and energy exist, it is NOT true that there is a "stuff" called
Static Electricity. Just because voltage and current may vary, that's no
reason to invoke a new kind of "electricity" called "static."
VandeGraaff machines and batteries do not differ as much as we might
think. After all, if enough VDG machines are connected in parallel, their
currents add up and they can light a normal incandescent bulb. And if
enough dry cells are connected in series, their voltages add up and they
can attract lint, raise your hair, charge your body, cause corona
discharge, and make giant sparks.
Electrostatics, or "Static Electricity," is a class of effects in the same
way that "biology" or "weather" are classes of effects. Your hand is
"biology", yet your hand is not made out of biology. Clouds are
"weather", yet clouds are not composed of weather. And, while scuffing
your shoes on the rug involves "static electricity," scuffing your shoes
does not create any substance or energy called static electricity. If we
always call it "electrostatics" instead of "static electricity", we won't
be so confused about it's nature. If we say "surface charge", we won't be
so surprised when it moves or flows ("Static" must be unmoving, right?
Wrong, surface charges can and do flow.) Assume that the words "static
electricity" breed confusion and ignorance, then avoid speaking them.
EDUCATIONAL
USE
The VandeGraaf machine is a fun demo tool in museums, but it's also useful
in science teaching. It can be used to demonstrate two important things:
electric fields and electric forces. Everyone encounters magnets and
magnetic fields, but few are aware that *electric* fields exist. These
fields are usually hidden under the label "static electricity" and are
ignored. This is unfortunate, since knowledge of electric fields leads to
the understanding of sparks and lightning, voltage and circuits, and even
the physical basis of chemistry and biology! In the functioning of the
everyday world, e-fields are MUCH more important than magnetic fields, yet
all the emphasis is placed on the latter. Students have difficulty
understanding voltage because voltage *IS* electric fields, and if we
don't understand electric fields, we will be befuddled by "voltage." The
VandeGraaff machine is extremely useful because it produces electric
fields which are strong enough to be measured, manipulated, felt directly,
played with, and finally grasped at an intuitive level.
IDEAS FOR MORE?
Write me at billb@eskimo.com
Created and maintained by Bill Beaty. Mail me at: billb@eskimo.com.
If you are using Lynx, type "c" to email.
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