Plasma Globes and Body Capacitance

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 SourceTeacher's NotesRobert 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 CPEPAdvice 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 CPEPPrepared with support from the Department of Energy, Office of Fusion Energy Sciences, Contract #DE-AC02-76CH03073.©2007 Contemporary Physics Education Project (CPEP) PrefaceThis 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 NotesPart of a Series of Activities in Plasma/Fusion Physics to Accompany the chart Fusion: Physics of a Fundamental Energy SourceEQUIPMENT 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 foilGENERAL 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.) Plasma Globes and “Body Capacitance” – Page T2Also 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. 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 objectsThere 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 Rubbing results in Upon separation, the rubbed together some transfer of two surfaces are electrical charge oppositely charged Plasma Globes and “Body Capacitance” – Page T3Examples of this processFigure are T-1:the electrical The process charging of transfer of party of balloonselectrical by charge 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. 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, Charged balloon producing a net attractive force between balloon and near the wall wall. Plasma Globes and “Body Capacitance” – Page T4On 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. EE Neutral Neutral unpolarized polarized E molecule molecule A sample of polarized wall molecules in the electric field produced by the charges in the balloonFigure 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 Plasma Globes and “Body Capacitance” – Page T5 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 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 - endNet Force on wall due to balloonFigure 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 Negatively charged object Positively charged object shifts metal, positive and shifts electrons (negative signs) electrons (negative signs) negative charges away from it to the left, toward it to the right, positive distributed randomly positive atomic kernels move atomic kernels move little 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 T6The 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 sparksThe 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 FElectric FieldFigure 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 Plasma Globes and “Body Capacitance” – Page T7 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.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. 2002What 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 Top and bottom plates illustrating positive charges terminal to the upper plate of a capacitor and from removal of electrons from the top plate and the negative terminal to the lower plate negative charges from addition of electrons to the bottom plate Plasma Globes and “Body Capacitance” – Page T8Figure T-7 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 globeThe 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 Plasma Globes and “Body Capacitance” – Page T9 the globe and the hand. The greater number or size of streamers doing the charging of the inside 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 StreamersFrom Procedure 1Plug 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 T10Expected 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 T11From 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 T13Answer: 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 CapacitanceFrom 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 T15When 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 T16APPENDIXAlignment of the Activity Plasma Globes and “Body Capacitance” with National Science StandardsAn 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 T17Alignment of the Activity Plasma Globes and “Body Capacitance” with AAAS BenchmarksAn 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

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