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Plasma Globes and “Body Capacitance”
Part of a Series of Activities in Plasma/Fusion Physics
to Accompany the chart
Fusion: Physics of a Fundamental Energy Source
Teacher's Notes
Robert Reiland, Shady Side Academy, Pittsburgh, PA
Chair, Plasma Activities Development Committee of the
Contemporary Physics Education Project (CPEP)
Editorial assistance: G. Samuel Lightner, Westminster College, New Wilmington, PA and
Vice-President of Plasma/Fusion Division of CPEP
Advice and assistance: Cheryl Harper, Greensburg-Salem High School, Greensburg, PA and
Chair of the Board of CPEP; T. P. Zaleskiewicz, University of Pittsburgh at Greensburg
(emeritus), Greensburg, PA and President of CPEP
Prepared with support from the Department of Energy, Office of Fusion Energy Sciences,
Contract #DE-AC02-76CH03073.
©2007 Contemporary Physics Education Project (CPEP)
Preface
This activity is intended for use in high school and introductory college courses to supplement
the topics on the Teaching Chart, Fusion: Physics of a Fundamental Energy Source, produced
by the Contemporary Physics Education Project (CPEP). CPEP is a non-profit organization of
teachers, educators, and physicists which develops materials related to the current understanding
of the nature of matter and energy, incorporating the major findings of the past three decades.
CPEP also sponsors many workshops for teachers. See the homepage www.CPEPweb.org for
more information on CPEP, its projects and the teaching materials available.
The activity packet consists of the student activity and these notes for the teacher. The Teacher’s
Notes include background information, equipment information, expected results, and answers to
the questions that are asked in the student activity. The student activity is self-contained so that
it can be copied and distributed to students. Teachers may reproduce parts of the activity for their
classroom use as long as they include the title and copyright statement. Page and figure numbers
in the Teacher’s Notes are labeled with a T prefix, while there are no prefixes in the student
activity.
Developed in conjunction with the Princeton Plasma Physics Laboratory and funded through the
Office of Fusion Energy Sciences, U.S. Department of Energy, this activity has been field tested
at workshops with high school and college teachers.
We would like feedback on this activity. Please send any comments to:
Robert Reiland
Shady Side Academy
423 Fox Chapel Road
Pittsburgh, PA 15238
e-mail: [email protected]
voice: 412-968-3049
Plasma Globes and “Body Capacitance”
Teacher’s Notes
Part of a Series of Activities in Plasma/Fusion Physics
to Accompany the chart
Fusion: Physics of a Fundamental Energy Source
EQUIPMENT LIST:
"Nebula Ball" Plasma Globe (or equivalent)
Available in Radio Shack or many “novelty” stores; from Arbor Scientific; through the
internet, for example at http://coolstuffcheap.com
A Tesla coil.
A copper wire at least two meters long or a set of wires that can be connected to form a
conducting path at least two meters long, some additional copper wire
To build the student capacitor: Two metal tacks, two thin cardboard sheets at least six inches on
a side, a small shoebox and scissors.
Various objects made of wood, plastic, glass and metal
Aluminum foil
GENERAL BACKGROUND:
CAUTION:
In these procedures, students will be using a plasma globe, a Tesla coil and a “grounded” wire.
There are safety issues concerning these. A Tesla coil is a lot like the source of alternating
electrical charge that is inside a plasma globe. It produces electrical charges that alternate sign
typically over 10,000 times per second, and it does so at high voltages. These are typically in the
range of 10,000 to 50,000 volts (A one centimeter spark in air is typically produced by a
potential difference of 30,000 volts). This sounds extremely dangerous, but ordinarily it isn’t.
If the same voltages were produced at low frequencies or with direct current, they would be
dangerous, but high frequency electricity does not penetrate far enough through human skin to
where it could affect a person’s heart. It can produce a painful spark, but this will not result in
injury unless it causes a person to move quickly away and to hit something hard.
An exception to this good news is that the electric fields radiating from a Tesla coil or plasma
globe can interfere with the operation of an implanted defibrillator. If you or a student have such
an implant or have any heart problem at all, it will be safe to observe the activity from 2 or more
meters away, but you or your student should not operate the Tesla coil or touch the plasma globe.
(This information was provided by private correspondence with a representative of Guidant
Corporation, manufacturer of these implanted devices.)
Also an operating Tesla coil should never be brought near a person’s face. It should also not be
brought near a plasma globe. Experience has taught us that a plasma globe can be destroyed by
the sparks from a Tesla coil.
Plasma Globes and “Body Capacitance” – Page T2
Ideally your students should have made a study of static electricity to develop the basic idea of
how electrical polarization is produced in objects. This is the basis of understanding much of
what happens in a plasma globe, and the important ideas are presented below. If your students
haven’t already learned these basics, consider using the text by Chabay, Ruth W. & Bruce A.
Sherwood, that is referenced in this background or Teaching About Electrostatics, an
AAPT/PTRA book by Robert A. Morse, or the CPU Curriculum Unit, Static Electricity and
Magnetism.
Theory of Spark formation:
There are three parts to the process of forming sparks. The first involves the buildup of opposite
electrical charges on two objects, the second is the creation of free electrons and/or positive and
negative ions in the air between the objects due to cosmic rays and common radioactive isotopes
in the air, and the third is the additional ionization of air between the two oppositely charged
objects by energetic collisions between the free electrons and ions with air molecules. The result
of the latter is that electrical current will be carried by the “ionized” air since the breakdown of
air into ions and electrons transforms the air from an insulator to a conductor of electricity.
To fully understand the production of sparks, consider the parts separately at first. Then they can
be combined later.
Build up of opposite charges on two objects
There are many ways in which two close surfaces can become electrically charged. The first one
to consider is charging by rubbing. This only works with insulators, and usually the two surfaces
must be of two different materials. The reason for this is that different insulating materials have
different attractions for charged particles, such as electrons and ions, and rubbing of two surfaces
against one another brings some of the surface molecules of the two objects close enough
together that some electrons can be transferred from one set of molecules to the set that has the
greater attraction for the electrons. Another possibility is that the rubbing can result in some of
the surface molecules fragmenting into ions, some of which can be transferred (See Chabay and
Sherwood, Chapter 14). In either case the surface that either gains negative charges or loses
positive charges will become negatively charged. The other surface will then become positively
charged. (See Figure T-1)
Surfaces about to be
rubbed together
Rubbing results in
some transfer of
electrical charge
Upon separation, the
two surfaces are
oppositely charged
Figure T-1: The process of transfer of electrical charge
Plasma Globes and “Body Capacitance” – Page T3
Examples of this process are the electrical charging of party balloons by rubbing them on a
person’s hair (after which they can stick to a wall in a dry room), combing of dry hair that results
in hair sticking out by electrical repulsion between like charged hairs and the electrical charging
of clothes in a dryer.
If two flat surfaces have been rubbed together to transfer electrical charges from one to the other,
the result is usually that the surfaces will attract one another because of their opposite electrical
charges, but usually no sparks are produced. The attractive force is often too small to notice, but
for light objects like balloons, it is easily noticed.
However, in the case of a balloon sticking to a wall, the balloon had been rubbed on something,
but the wall hadn’t. How does the wall, which wasn’t electrically charged by rubbing, become
charged in such a way that it and the balloon attract one another? That is, how do we apparently
get an opposite charge on the wall? A clue to the process can be seen by rubbing a balloon
(Cheap balloons work best. Don’t use really good balloons made for holding helium.) on dry hair
or fur and bringing it close to small pieces of paper. Notice how the paper is affected.
Like the wall, the paper wasn’t rubbed against another surface. Yet a clearly attractive force has
resulted from bringing the balloon close to the paper. In fact it turns out that an electrically
charged object, such as a rubbed balloon, will attract any originally uncharged object! A
particularly interesting case can be seen by getting a small and smooth stream of water to flow
from a faucet and bringing a rubbed balloon slowly toward it from one side. The stream of water
will be very strongly deflected toward the balloon. (This assumes that the water itself is not
electrically charged. Occasionally, with PVC pipes, the water may be electrically charged and it
will be repelled by any electrically charged object of the same sign of net electrical charge as the
water. To neutralize the water stream, attach a ground wire from the metal of the faucet to the
ground part of an electrical outlet.)
To understand how uncharged objects, including insulators, are attracted by electrical charged
objects, it is necessary to make use of the idea that matter consists of atoms with positively
charged nuclei surrounded by negatively charged electrons, where the charges are equal in
neutral atoms. Because these opposite electrical charges exist in all ordinary objects, it is
sometimes possible to produce net electrical forces by separating positive and negative charges
by tiny amounts without giving the objects net electrical charges. One way in which this can
happen is illustrated in Figure T-2, with the effect produced on a wall by an electrically charged
balloon.
Charged balloon
near the wall
Figure T-2: When an electrically charged balloon is
brought near an electrically neutral wall, the excess
charges in the balloon (illustrated as positive) attract the
opposite charges in the wall (illustrated as negative) and
repel the like charges. This brings the opposite charges
a little closer to the balloon and pushes the like charges
a little further away, producing a net attractive force
between balloon and wall.
Plasma Globes and “Body Capacitance” – Page T4
On the microscopic scale, in which the charge separation occurs, electrons in individual
molecules within the wall are shifted slightly toward the balloon. This makes one side of each
molecule slightly negative and the other side slightly positive, as illustrated in Figure T-3. The
molecules are then said to be polarized.
E
E
Neutral
unpolarized
molecule
Neutral
polarized
molecule
E
A sample of polarized
wall molecules in the
electric field produced by
the charges in the balloon
Figure T-3: Molecules can remain neutral and still produce electrical effects by
polarization, the production of separated positive and negative poles.
The charges on the balloon produce an electric field. Nearly 200 years ago Michael Faraday
invented the ideas of electrical and magnetic fields to explain how electrical and magnetic forces
could act over distances, seemingly with no causal agent. Faraday made the first objective
attempt at solving the problem for electricity with the idea that a physical electrical field is
associated with each electrically charged object. In his imagination electrical fields radiated
from positive charges and into negative charges, so we can visualize electric fields as directed
lines that go from positive to negative charges. It is these electrical fields, whether or not
visualized as “lines of force,” that are responsible for the forces between electrical charges, with
charges interacting with the field rather than some distant charge. Positive charges experience
electrical forces that act in the direction of the field lines, and negative charges experience
electrical forces that act in the direction opposite to that of the field lines.
It is the electric field produced by the balloon’s net electrical charge, as illustrated by the three
lines in the right part of the above figure that polarizes the molecules. This electrical field is
illustrated as non-uniform. It is stronger near the balloon, and this is represented by the electric
field lines being close together. The field lines then spread apart showing that the strength of the
electric field decreases with distance. The electrical polarization in the wall is along the
directions of the electric field lines.
The overall result is that the electric field is strongest where the net electric charge is negative
(near part of the wall) and weakest where the electric charge in the wall is positive. This
produces a net attractive force between the wall and the balloon, as illustrated in Figure T-4.
Note that the top right arrow above the illustrated part of the wall is longer than the top left
arrow. These indicate the strength and directions of the forces exerted on the net negative and
net positive, respectively, charge regions in the wall. The overall effect on the wall is then
attractive (toward the balloon), and by Newton’s Third Law, the net force on the balloon exerted
Plasma Globes and “Body Capacitance” – Page T5
by the wall must also be attractive. This is why a rubbed balloon will stick to a wall. Note that if
the balloon were negatively charged, the positive part of the molecules would be closest to the
balloon and the negative parts further away still giving an attractive force.
Force on + end
Force on - end
Net Force on wall due to balloon
Figure T-4: Electrical forces on the wall due to forces on separated charges within
the wall. The sum of these forces is a net force toward the balloon. The balloon
experiences an equal and opposite force, as indicated.
While many things around us are composed of molecules, metals have a different internal
structure. The atoms of pieces of metal in solid and liquid states are bound together by sharing
the outer electrons, called valence electrons. These electrons are so weakly bound to particular
metal atoms that when a number of metal atoms are close together, the valence electrons are
freed from the atoms they started with and move freely through the bulk of any piece of metal.
Once the valence electrons are no larger parts of particular atoms, the atomic “kernels” that
remain are positively charged, and it is the mutual attraction of all of the positive atomic kernels
for all of the negative valence electrons that produces the general metallic bond. However, the
important part of the metallic bond for the purposes of understanding the attractive interaction of
an electrically charged object with a metallic object, or any other electrical conductor, is that
there are a large number of charges within any piece of metal that can move freely within the
bulk of the piece. Consequently, if an object, such as a rubbed balloon, with a net electrical
charge is brought near a piece of metal, the metal piece is easily polarized by the shifting of
some electrons, either toward a positively charged object or away from a negatively charged
object. Either way, as seen in Figure T-5, there will be a net attractive force between the charged
object and the polarized metal piece.
Unpolarized piece of
metal, positive and
negative charges
distributed randomly
Negatively charged object
shifts electrons (negative signs)
away from it to the left,
positive atomic kernels move
little
Positively charged object shifts
electrons (negative signs)
toward it to the right, positive
atomic kernels move little
Figure T-5: Electrically charged objects brought near pieces of metal polarize the pieces. A
negatively charged object makes the metal piece more positive on the near side and more negative
on the other side to produce net attractive forces. A positively charged object makes the metal
piece more negative on the near side and more positive on the other side to produce net attractive
forces.
Plasma Globes and “Body Capacitance” – Page T6
The result, then, is that if a net change can be built up by some transfer process, such as rubbing,
then a neutral object can have a charge separation induced by an electric field such as produced
by nearby charges.
So the field between charges, which is necessary for spark formation, can be between two
objects with excess charge or between a charged object and a neutral object.
Ionization of air to form sparks
The second and third parts of the explanation of how sparks form considers how air can be
ionized into separate positive and negative charges in order to form sparks.
From the earlier discussion, it is apparent that when a molecule is acted on by an electric field
due to external electrical charges, as indicated in Figure T-6, the positively charged nuclei will
experience forces in one direction (that of the electrical field) and the negatively charged
electrons will experience forces in the opposite direction.
F
F
Electric Field
Figure T-6: An atom that might be ionized by the forces on a negative electron and
a positive nucleus due to the electric field between two oppositely charged objects.
In those situations in which an electron is loosely bound to an atom or a molecule, it might be
possible for a very strong electrical field to separate the electron from the atom or molecule to
produce a free electron and a positively charged ion, that is ionize them. However, electric fields
produced by static electric charges are seldom strong enough to separate electrons from atoms or
molecules. Rather, ionization occurs more easily when particles of sufficient energy collide with
the atoms and “knock” electrons loose, creating free electrons and positive ions.
So, the second part of the ionization process is the presence of a few charges that have already
been separated by high energy ionizing radiation in the form of cosmic rays or radiation from
radioactive material in the air (Chabay & Sherwood, Chapter 14). These charges will be
accelerated by the electric fields produced by the charged surfaces, and some of the accelerated
charges will gain enough energy from the electric field to ionize molecules through collisions,
the third part of the process. Being much lighter, the electrons will gain energy more rapidly
than the positive ions and will cause most of the additional ionizations. Each ionizing collision
will release one or more electrons which can in turn gain energy from the electric field and then
ionize other molecules. There will be a cascading increase of free electric charges, which will
grow to the point that they have observable effects in the form of a spark.
Plasma Globes and “Body Capacitance” – Page T7
Now everything is in place to explain how a hand placed on a plasma globe can seemingly attract
streamers (it is really a matter of enhancing the production of streamers).
Reference:
Chabay, Ruth W. & Bruce A. Sherwood, Matter & Interactions II, Electric and Magnetic
Interactions, John Wiley & Sons, Inc. 2002
What is Capacitance?
In Procedure #4 of Part II, students who have put together the foil covered cardboard circles in
slits of the box are told “You have now made a type of capacitor, …” In fact they have made
one of the simplest types of capacitor. It is a “parallel plate capacitor.”
Capacitance is a basic property of ordinary matter. In that sense everything we can see or touch
is a capacitor, but the term capacitor is usually reserved for constructed objects, such as parallel
plate capacitors, that have a relatively high “capacitance.” Capacitance is defined as the ratio of
the amount of electrical charge transferred from one part to another part of a system to the
voltage that produces the charge transfer. An interesting outcome of this definition is that, in the
case of capacitors made of conductors separated by air, the amount of capacitance depends only
on the geometry of the parts. That is, the amount of electrical charge transferred is always
proportional to the voltage used, and neither the voltage nor the electrical charge appear in
formulas for amounts of capacitance of particular systems, such as parallel plate capacitors. The
unit of capacitance is the Farad (F), which is equivalent to a Coulomb of electrical charge per
volt.
For example, the capacitance of a metal sphere of radius one centimeter is about 10-12 F. It
would be relatively easy to construct a parallel plate capacitor out of two circular metal foil discs
of radii one centimeter that would have 1000 times this capacitance. With relatively new
technologies it has even become possible to produce a capacitor the size of the metal sphere with
a capacitance of one Farad!
The geometric factors that give a parallel plate capacitor high capacitance are surface area and
having two parts that are close together. To understand how this works, imagine that a battery is
giving the two parts of a parallel plate capacitor opposite charges, as illustrated in Figure T-7.
+
Battery with wires connecting the positive
terminal to the upper plate of a capacitor and
the negative terminal to the lower plate
Figure T-7
Top and bottom plates illustrating positive charges
from removal of electrons from the top plate and
negative charges from addition of electrons to the
bottom plate
Plasma Globes and “Body Capacitance” – Page T8
Everything in the circuit starts out electrically neutral, and the battery acts like a pump to take
electrons (negatively charged) from the top plate of the capacitor and move an equal number to
the bottom plate. The excess electrons make the bottom plate negative, and the deficit of
electrons makes the top plate positive. The amount of electrical charge shifted from one plate to
the other is determined by the voltage of the battery and the difficulty of continuing to make one
plate more and more negative and the other more and more positive. In particular, with every
additional electron added to the bottom plate, there is a bit more negative charge on the plate that
will repel additional negatively charged electrons. Eventually this repulsion on additional
electrons will equal the force produced in the wires by the battery to push electrons toward the
plate. When that happens, the battery can’t push any more electrons onto the plate.
If the area of the plates is large, then the charges spread out so that more net charge can be held
on a plate with a given voltage before the battery can’t push any more electrons on the plate.
The opposite charges on the two plates are attracting one another. So, the closer the plates are
together, the greater this attraction will be, and the attraction helps the battery to pump more
charge from one plate to another. Note that for a single object, such as the metal sphere, there is
no other oppositely charged object near enough to it to increase the amount of net charge it can
maintain. So the metal sphere has capacitance, but the amount is very small.
Taking the above ideas into account, it is not surprising that the capacitance of a parallel plate
capacitor is directly proportional to the surface area of one of its plates and inversely
proportional to the separation between the two plates.
How the student constructed parallel plate capacitor compares to the plasma globe
The above ideas can help in the understanding of how the parallel plate capacitor constructed by
students is similar to a plasma globe. The air gap between the sheets of foil of the parallel plate
capacitor functions as a nonconductor of electricity so that sparks that could carry electrical
charge across don’t form between the sheets of foil. (Sparks do form in the air between the tacks
and the Tesla coil and ground wire because the field opposite these points is much larger than the
field between the foil sheets.) However, electrical fields can readily cross nonconductors, and
these fields produce the forces that cause the sheet of foil near the ground wire to become
oppositely charged compared to the sheet of foil that is being charged through the spark from the
Tesla coil. In the plasma globe the glass is the nonconductor that prevents electrical charge from
getting across while allowing electrical fields to pass through.
The inside of the glass of the plasma globe can act like one of the sheets of foil of the student
constructed capacitor, even though it is not a very good conductor (note the results of Part II,
Procedure 5 with glass), because it is the first solid surface beyond the source of the streamers
(the ball at the center of the globe). The charges carried as streamers are like the spark from the
Tesla coil and terminate on the glass as the sparks from the Tesla coil terminate on the tack
touching one foil sheet. When foil is placed on the plasma globe, it is the same as the second
sheet of foil in the parallel plate capacitor. Then, when a ground wire (or equivalent, such as a
finger) is brought close, a spark can jump to the ground just as with the student-constructed
capacitor. When the hand alone is placed on the glass of the plasma globe, it acts like the second
sheet of foil. The closer it gets to the glass, the higher the capacitance of the capacitor formed by
the globe and the hand. The greater number or size of streamers doing the charging of the inside
Plasma Globes and “Body Capacitance” – Page T9
of the glass globe (increasing the charge on the glass) is visual evidence for capacitance
increasing as the hand is brought closer to the globe.
Expected Results and Answers to Questions:
Part I: Plasma Globe Streamers
From Procedure 1
Plug in and turn on a plasma globe, and take some time to observe the apparent formation and
disappearance of electrical streamers inside. Do you notice any patterns in the formation,
motions and disappearances of streamers while nothing is in contact with the globe? For
example, is there a particular region inside the globe where the streamers usually form? Is there
a particular region in which they disappear? Do they tend to move up, down or sideways? Are
the streamers usually straight or curved?
Expected Results:
The streamers tend to form with at the central bulb and end at the glass near the bottom of the
globe. Then these ends move up to the top of the globe where the streamer then vanishes. This
happens over and over again.
From Procedure 2
Place one of your hands on the side of the plasma globe. Does this affect the plasma streamers in
any way? While keeping your hand on the globe, move it around. What do you see happening
to the streamers? Place your hand on top of the globe. Are the effects here different in any way
or ways from those that were produced with your hand on the side of the globe?
Expected Results:
The streamers seem to be attracted to your hand. Typically a thicker than normal streamer will
form near the center of your hand. On the sides of the globe, the streamers will still seem to
move up and past your hand while new ones will seem to be attracted by your hand. On the top
the streamers often seem to stay just below your hand. When the streamers strike the glass
where your hand is, they brighten whether your hand is on the top or the sides of the globe.
From Procedure 3
Instead of placing a hand on the plasma globe, try resting a variety of light non-metallic objects
on the top of the globe. You might use sheets of paper, small blocks of wood, sheets or pieces of
plastic. Do any of these produce the same effects you saw with your hand on the globe? Are the
effects different when you are touching the objects with your hand? Does it matter whether this
touching is close to or relatively far from the globe surface? In particular note any effects when
the objects are held against the side of the globe, as opposed to the top. As a control, note
exactly where your hand is in relation to the plasma globe while touching some of these objects
and observing the effects, and remove the object while keeping your hand in the same location.
Did it matter whether the objects were between your hand and the plasma globe?
Plasma Globes and “Body Capacitance” – Page T10
Expected Results:
There should be no effects with any of these objects. There may be exceptions with very thin
objects when you are touching them. In those cases the effects will be the same as when your
hand is very close to the globe with no object between hand and globe.
From Procedure 4
Repeat Procedure # 3 with small sheets of aluminum foil.
Expected Results:
There should be little or no effect until you touch the foil. Then streamers will seem to be
attracted to the foil, as they would to your hand.
From Procedure 5
Bring a metal object, such as a key or the point of a nail close to the foil that you just placed on
top of the plasma globe, but don’t let it touch the foil. Can you get a spark to form between the
foil and the metal object? When this happens, is there a streamer on the other side of the glass?
Is there ever a spark in the glass itself? Note that, if the key you are using has a plastic handle,
you may have trouble producing a spark unless you are careful to hold onto the metal part of the
key rather than the plastic handle.
Expected Results:
Small sparks will form, and they will be much larger when you are touching the metal parts of
the objects. There is never a spark within the glass itself.
From Procedure 6
Cut out two roughly square sheets of aluminum foil. One should be about an inch across, and the
other should be about 6 inches across. Place one on top of the plasma globe, and smooth it down
to match the curvature of the globe. Observe any differences between the streamers with and
without this sheet of foil. Repeat with the other sheet of foil. Next, observe any new effects if
you are making contact using one finger on the foil. Do this in turn with each sheet of foil. Does
the size of the foil make any difference?
Expected Results:
When you are not touching the foil, there will be little to no effect in either case. With the larger
sheet of foil, you might at times see more streamers near it than there would be in the same area
without it, but the effect will be small. When you touch the sheets, streamers will again seem to
be drawn toward them, and one large streamer will tend to form toward the centers of the sheets.
The only difference is that there will be more streamers converging toward the larger sheet.
From Procedure 7
Repeat procedure 6 with the sheets of foil held to the side of the globe with a moderately long
insulator, such as a Popsicle stick.
Expected Results:
The difference between the effects is easier to see with the foil sheets on the side. More
streamers concentrate on the glass inside of the larger sheet, and there is noticeably more
brightening at the glass. Foil area does matter.
Plasma Globes and “Body Capacitance” – Page T11
From Procedure 8
Replace the small sheet of foil with a coin of similar area. Typically, this would be a quarter. Is
there any significant difference between the effects with the small sheet of foil and the coin when
you touch neither? Is there any significant difference between the effects with the small sheet of
foil and the coin when you touch both? Compare the effects in the same way between the large
sheet of foil and the coin.
Expected Results:
The differences will be small in all cases, as long as the testing is done with the object at the top
of the globe. A noticeable difference can be seen between using the small sheet of foil and the
coin on a side of the globe held in place by a nonconductor. In this case streamers may be seen
to accelerate slightly toward the foil, as they approach it, and linger there for a fraction of a
second before moving on. This may be hard to see. The same effect will be definitely more
pronounced when the quarter is held against the side of the globe with a nonconductor. It is the
greater mass of the object that matters. When mass is enhanced by using a hand against either
the foil or the coin, the effect is much greater.
From Procedure 9
Examine the glass just inside of any metal you have placed on the globe. Note the additional
illumination of the glass when you touch the metal with a finger or your hand. Now place the
large sheet of foil on the globe and observe the brightening of the glass on the inside. Does this
brightening occur at all points across from the foil, as it did with metal of smaller area, or is it
more limited? Next place all fingers and the palm of one hand a side of the globe and answer the
same questions. Remember these observations as you observe the object made of cardboard and
foil in the next part.
Expected Results:
With large metal objects or your entire hand against the glass, the regions of brightening will no
longer be at all contact areas. There are apparently limits to how much of the glass can be
affected in one region at a time.
Question: 1. From this activity and previous experiences, do you think that the electrical
streamers in a plasma globe and sparks in general are attracted to everything placed on or near
the globe? Are there materials that attract streamers and sparks better than other materials?
What is the evidence for your answer?
Answer:
Streamers and sparks are attracted to all metal, to the human body, to a lesser extent to glass and
very little to most other materials like plastic and wood (there may be a small effect with some
forms of wood).
Question: 2. You may have noticed that the streamers tend to disappear near the top of the
globe, but streamers stay near the top when your hand is on top of the globe or when a conductor
is there while your hand or finger is in contact with the conductor. Streamers are formed when
there is a difference in concentrations of electrical charge between two points. They form more
readily in gases that are more easily ionized, such as gases at lower densities. Does placing your
hand on the globe have more of an effect on the electrical charge difference between the ball at
Plasma Globes and “Body Capacitance” – Page T12
the center of the globe and the glass globe or on the ease of ionizing the gas in between central
ball and outer glass globe? In answering this, think about where the hand is and what it could
most easily affect.
Answer:
It is unlikely that your hand can have much of an effect on the gas inside the glass. It is more
likely that it affects the electrical charge on the nearby glass.
Question: 3. Is the area of a conductor (metal or hand) in contact with the outer surface of the
plasma globe or the mass of this conductor more important in keeping streamer close to the part
of the glass just inside the conductor?
Answer:
The area produces an effect, but the mass is much more important. This can be seen by the large
difference the mass of your body makes when you touch the globe or a metal object on the globe.
Question: 4. The ball inside a plasma globe is made to alternate sign of electrical charge
typically over 10,000 times per second. If an electrical streamer forms between this ball and the
inside of the enclosing glass, is it likely that the inside surface of the glass stays electrically
neutral while the streamer is present? For example, if the ball has just turned positive, is it likely
that the inside of the glass is becoming positive or negative or that it is remaining neutral? In
answering this, think about what you know about polarization of electrical charges in materials.
Draw a picture of what you think happens in the glass.
Answer:
When the ball inside the globe changes sign of electrical charge, the glass will very quickly
develop the opposite sign of electrical charge on its inner surface.
Question: 5. Do you ever see sparks forming within the glass of the plasma globe? Can
electrical charge make its way through the glass? If it can’t, and if sparks form on the outside of
the globe, between a sheet of foil and a metal object, there must be something that does get
through the glass that can affect electrical charge. This something is called an electric field.
What characteristics should an electric field have to explain its ability to generate sparks?
Answer:
Sparks don’t form within the glass, and they don’t get through it. The electric field that can get
through can act on electrical charges beyond the glass to produce the observed sparks.
Question: 6. Considering your answers to the previous four questions, when there is a relatively
large conductor on the outside surface of the glass of an operating plasma globe, how might this
affect the charges on the inside of the globe? At this point you are hypothesizing. It is not
important that you get this right or that you fully understand what the causes are of the observed
and hypothesized effects. You will be starting the final activity in this set with the hypothesis
that you have just written down and possibly with others from other students. You may even
develop more hypotheses as you proceed. Eventually you will likely eliminate all but one of
these hypotheses as you develop a better understanding of what happens inside a plasma globe.
Plasma Globes and “Body Capacitance” – Page T13
Answer:
All reasonable hypotheses are ok at this point. The best would deal with the fact that the external
conductors can increase the polarization of the glass in response to the alternating charges on the
ball inside the globe.
Part II: Plasma Globe Streamers and Capacitance
From Procedure 5
Plug in the Tesla coil and adjust it until you can feel it vibrating lightly. At this time, if the point
of the Tesla coil isn’t near anything, there should be no noticeable sparks. Yet the pointed end of
the Tesla coil is reversing electrical charges over 10,000 times per second! You might then
wonder why there are no sparks. Move the point of the Tesla coil near a variety of different
objects. You should see that sparks are formed between the point of the Tesla coil and many of
the things it gets close to, but also look for cases in which the sparks are longer or thicker than in
other cases and note if there are objects or materials near which the Tesla coil can’t form sparks
at all. Be sure to try glass and plastic objects as well as metal objects.
Expected Results:
No sparks will normally form between the Tesla coil and wood or plastic. Small sparks will
form with many glass objects (Glasses are made with a wide range of additives and different
concentrations of these. Many of the additives include metals such as lead and sodium.) Larger
sparks will form with any metal, especially if it is being touched by someone’s hand. Large
sparks will form with a person’s hand.
From Procedure 6
Remove one of the cardboard circles from its slot. Bring the point of the Tesla coil near the tack
point of the remaining cardboard circle which is still in a shoebox slot. How does the size of the
resulting spark compare to those you found when you brought the point near to nonconducting
objects? In particular what is the largest distance or gap between the point of the Tesla coil and
the tack point that can be bridged by a spark?
Expected Result:
The resulting spark will be much larger than any that can be formed between the point of the
Tesla coil and any nonconducting object.
From Procedure 7
Bring the point of the Tesla coil near the free end of the grounded wire, and note the size of the
largest spark that forms. How do they compare to sparks observed in part 6.
Expected Result:
These sparks will be as large as any that can be produced with the Tesla coil.
From Procedure 8
Replace the second foil-covered circles in its shoebox slots and make sure that the tack points are
outward and the foil surfaces are parallel and not touching. Bring the free end of the grounded
wire near to one tack point and bring the point of the Tesla coil near to the other tack point. You
should see sparks from the tack points on both sides and no sparks between the parallel sheets of
Plasma Globes and “Body Capacitance” – Page T14
foil. If any sparks form between the parallel sheets of foil, adjust the strength of the Tesla coil
down and try again until you get no sparks between the foil sheets but still get sparks from the
tack points. Note the sizes of the sparks between a tack point and the Tesla coil and between the
other tack point and the end of the grounded wire. How do these sparks compare to those that
you saw in the previous three procedures? If you had to reduce the electrical output from the
Tesla coil in this procedure, quickly redo the tests from the previous three procedures for better
spark size comparisons. You should see relatively large sparks from the tack points on both
sides.
Expected Results:
Large sparks can form between the Tesla coil and the nearest tack point and also between the
other tack point and the end of the ground wire while no sparks form between the two sheets of
aluminum foil. Apparently something other than electrical charge goes across the gap.
Questions: 1. The Tesla coil doesn’t produce sparks unless there is something close to its point.
Does the size of the spark depend on whether the object is a conductor (usually something metal,
but the human body is a conductor at the voltages involved) or a nonconductor? Does the size of
the spark depend on whether the part nearest the Tesla coil is sharp or flat? Does the size of the
spark depend on whether or not the object the Tesla coil is brought near is grounded?
Answer: Sparks form best near conductors, and most easily near pointed parts of conductors.
Weak sparks sometimes form near glass, but no sparks form near plastic or wood. From this it
may be hypothesized that some types of glass are weak conductors of electricity. A grounded
conductor will form significantly larger sparks with a Tesla coil than any other object will.
Equivalent to a grounded conductor is any large conductor, including a human body.
Questions: 2. Consider the air gap between the sheets of foil in your capacitor as like the glass
of a plasma globe in that both are nonconductors. You may have been uncertain about whether
or not sparks were going through the glass, but now you’ve seen that sparks can form on both
sides of your capacitor without sparks going through the air between the sheets of foil. Use the
following illustrations of a capacitor and of a plasma globe to model what happens to an
operating plasma globe. Specifically, identify and label the parts of the capacitor that correspond
to the following parts of a plasma globe: The central ball that all of the streamers seem to come
from, the streamers, the glass and your hand. Note that one thing seems to be missing in the
plasma globe system. That is a second set of streamers or sparks. This could be produced by
replacing the hand with foil on the outside of the glass and moving your hand so that it doesn’t
quite touch this foil. If you want to try this modification, be prepared for a small shock. What
do you think is happening to your body when your hand is brought near an operating plasma
globe?
Answer: The ball in the center of the plasma globe is like the Tesla coil. It is a high voltage,
high frequency source of alternating electrical charge. The streamers inside the plasma globe are
like the spark that forms between the Tesla coil and the tack. The glass of the plasma globe is
like the gap between the sheets of foil. The gap between the glass and your hand as you bring it
near to the glass is like the gap between the second tack and the end of the ground wire. The
ground wire is like your hand.
Plasma Globes and “Body Capacitance” – Page T15
When your hand is brought near an operating plasma globe, it alternates in electrical charge in
response to the alternating charge in the globe, but your body stays neutral. So, while your hand
is alternating in electrical charge, the rest of your body is also alternating in electrical charge in
the opposite sense keeping the net charge of hand and the rest of the body continuously zero.
Questions: 3. How is your hand like a grounded conductor? Has your body become part of the
“capacitor”?
Answer: A conductor is grounded by being connected to the ground itself or to any other large
conductor. Compared to a plasma globe the body that your hand is attached to is a relatively
large conductor.
Questions: 4. In Part I you probably noticed that the illumination on the inside of the glass of
the plasma globe didn’t occur everywhere that your fingers and palm touched when you had your
entire hand on the glass. Also the illumination probably shifted around to areas near different
parts of your hand. Relate this to the fact that sparks did not form between the sheets of
aluminum in the capacitor you constructed. Think in terms of how much charge is polarized per
area when the area is large.
Answer: The difference between sparks forming between a Tesla coil and the tack in your
capacitor and not forming between the sheets of foil is one of surface area. Roughly the same
amount of opposite electrical charge is caused to alternate onto the point of the tack as onto the
sheet of foil, but the surface areas of the two are vastly different. The charge is spread over a
much greater area on the sheets of foil and is never bunched together enough to accelerate
charges in the air gap sufficiently to produce a spark.
Evidence of this is the illumination of the glass inside where your hand touched the globe. The
illumination forms only where the glass is charged enough to be impacted by the spark-like
streamers. The amount of electrical charge that can be on the glass is limited, and so produces
illumination in a fraction of the area. The larger the part of your hand against the glass, the less
the average charge per unit area on the inside surface at any time. Therefore, there is a limit to
how much larger objects can be to cause the number of streams the form near them to increase.
Questions: 5. You have investigated how the effects with your capacitor depend on whether or
not grounding is involved. Use these results to try to explain how electrical streamers inside a
plasma globe are strengthened when you have a hand on the globe.
Answer: If you try to form sparks with your capacitor without grounding the other side, the
sparks are smaller than with grounding. Grounding is then needed for production of the largest
sparks. Grounding the plasma globe on one part with your hand also produces larger sparks or
streamers. The non-grounded areas cannot compete very well for streamers with the grounded
area.
Plasma Globes and “Body Capacitance” – Page T16
APPENDIX
Alignment of the Activity
Plasma Globes and “Body Capacitance”
with
National Science Standards
An abridged set of the national standards is shown below. An “x” represents some
level of alignment between the activity and the specific standard.
National Science Standards (abridged)
Grades 9-12
A. Science as Inquiry
Abilities necessary to do scientific inquiry
X
Understandings about scientific inquiry
X
B. Physical Science Content Standards
Structures of atoms
X
Motions and forces
X
Conservation of energy
X
Interactions of energy and matter
X
D. Earth and Space
Origin and Evolution of the Universe
E. Science and Technology
Understandings about science and technology
X
G. History and Nature of Science
Nature of scientific knowledge
X
Plasma Globes and “Body Capacitance” – Page T17
Alignment of the Activity
Plasma Globes and “Body Capacitance”
with
AAAS Benchmarks
An abridged set of the benchmark is shown below. An “x” represents some level
of alignment between the activity and the specific benchmark.
AAAS Benchmarks (abridged)
Grades 9-12
1. THE NATURE OF SCIENCE
B. Scientific Inquiry
X
2. THE NATURE OF MATHEMATICS
B. Mathematics, Science, and Technology
X
3. THE NATURE OF TECHNOLOGY
C. Issues in Technology
4. THE PHYSICAL SETTING
A. The Universe
D. The Structure of Matter
X
E. Energy Transformations
X
F. Motion
X
G. Forces of Nature
X
11. COMMON THEMES
A. Systems
X
B. Models
X
C. Constancy and Change
X
D. Scale
X
12. HABITS OF MIND
B. Computation and Estimation
X
Plasma Globes and “Body Capacitance” – Page T18