Studying the Electric Field Near a Plasma Globe

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: T. P. Zaleskiewicz, University of Pittsburgh at Greensburg, Greensburg, PA and President of CPEP

Prepared with support from the Department of Energy, Office of Fusion Energy Sciences, Contract #DE-AC02-76CH03073.

©2002 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

Studying the Electric Field Near a Plasma Globe

Teacher’s Notes

Part of a Series of Activities in Plasma/Fusion Physics to Accompany the chart Fusion: Physics of a Fundamental Energy Source

Introduction:

This activity complements the plasma/fusion activity “The Physics of Plasma Globes”* which concentrated on analyzing the spectra in a plasma globe. However, in that activity, students were encouraged to “play” with the plasma globe. As part of the “playing” they would discover that its is possible to get sparks to jump from a metal disk (such as a coin) placed on the globe to a pointed conductor or even your finger, indicating the presence of an electric field. The focus of this activity is that electric field.

Equipment and Materials:

"Nebula Ball" Plasma Globe or equivalent (In stores such as Radio Shack, from supply houses such as Arbor Scientific, and through the internet, for example at http://coolstuffcheap.com.) Electric field probe (See page 3 of Student Activity for construction details) Digital Multimeter (Science KIT 64971-02 or equivalent) Oscilloscope – best is a 20 MHz one similar to Science KIT 45077 but others may work Optional: Tesla Coil (Science KIT 61157-02 or equivalent) Wooden dowel (at least 25 cm long) Aluminum foil (Note: See http://www.ScienceKit.com for some of these equipment references)

Background and Suggestions:

A plasma globe operates with a high-frequency, high voltage source that produces ionization of the gases between the central electrode and the surrounding glass of the plasma globe. The frequency of the source is typically on the order of 10,000 cycles per second (10 kHz), and the voltage is typically several thousand volts. The resulting plasma consists of electrons stripped off some of the gas atoms and molecules inside the globe plus the positive ions that are left after the removal of these electrons. The visible streamers of plasma are made of this ionized gas. The electric field produced by the source oscillates at the frequency of the source and drives the electrons and ions back and forth at this frequency.

* Part of the series of activities in Plasma/Fusion physics to accompany the CPEP chart Fusion: Physics of a Fundamental Energy Source

Studying the Electric Field Near a Plasma Globe – Page T2

In the first part of the activity, the students will make a “probe” of a metallic disk (for example a coin) and a wire which is connected to ground, with a gap between them. Because of the electric fields, sparks can be made to jump from the disk to the wire. In the second part, they will use an “antenna” which is connected to an a.c. voltmeter or an oscilloscope to detect the electromagnetic radiation from the plasma steamers.

Since any accelerating electrically charged particle will radiate electromagnetic energy, the electrons and ions in the plasma streamers will produce radiation, and this radiation can go through the glass of the globe and be detected by antennae that respond to the electrical components of these electromagnetic fields. The kind of antenna that will respond well is one that contains charged particles that are free to move within the antenna, such as metals. Metals contain electrons which are involved in forming the metallic bonds but which are not tied to any particular atom. They are shared throughout the metal and free to move within.

An oscillating electric field, such as that from the electromagnetic waves radiating from a plasma globe streamer, will push and pull on the mobile electrons in metals and cause them to oscillate back and forth. This then constitutes an alternating electrical current (a.c.) that is readily recorded by an a.c. voltmeter or an oscilloscope that is attached to the metallic antenna through a metal wire.

The radiation from the plasma globe streamers has the same frequency as the plasma source. The wavelength is just the speed divided by the frequency. Then, for a frequency of around 10,000 cycles per second (10 kHz) and a speed in air of 3 x 108 m/s, the wavelength is over 10,000 meters (10 km!). Since any realistic antenna plus the wire that connects it to the meter or oscilloscope will be much smaller than this, the antenna plus wire can be considered together as the antenna that responds as a unit to the electromagnetic field.

This creates a measurement problem in that the entire wire to the meter or oscilloscope will respond to the electric field simultaneously for practical purposes. This results in a measurement of electric field strength from the radiation that decreases less with distance than would be recorded by an ideal probe that would respond at one definite point in any particular measurement. There are a number of ways in which this can be minimized, but the simplest approach is to connect the free end of the wire to a wide but thin piece of metal. A simple and effective way to do this is to cut a square of aluminum foil ten to twenty centimeters on a side and clip the end of the wire to the center of the square. It won’t significantly affect the measurement if the foil is crimped and torn some in the process. To use the antenna, the plane of the foil is kept perpendicular to a line from the center of the plasma globe to the probe, or said another way, the foil should then be kept so that its plane is parallel to the tangent plane of the nearest point on the globe.

With this, a much greater fraction of the energy from the plasma radiation will be received at the location of the end of the wire than will be directly received by the wire between the end and the meter or oscilloscope.

If you use an oscilloscope, a typical display might use a time base of 10 μsec/div or a sweep frequency of 105 Hz. The amplitude of the signal may be as high as 1.5 volts.

Studying the Electric Field Near a Plasma Globe – Page T3

There is a legitimate question about whether the meter and/or oscilloscope are measuring oscillating electric fields from the plasma streamers or simply from the electronics that is driving the streamers. Fortunately there is a simple test for this. It involves the use of a Tesla coil, which is based on the same electronics that is used to power the streamers in a plasma globe.

If you have a Tesla coil, you may want to use it to demonstrate to your students that the formation of a spark or streamer adds significantly to the radiated electric field. Or you might want to have students make measurements as a function of distance from the Tesla coil with and without sparking to complement the plasma globe measurements.

If you don’t have a Tesla coil, it still can be worth pointing out that while measurements made with a probe connected to an a.c. voltmeter or an oscilloscope will measure a radiated electric field when the coil is operating with no conducting object close enough for sparking to occur, simply sliding a conductor that is already close to the tip of the Tesla coil a little closer to form a spark results in an increase in readings of the radiated field of about 10% to 20%. The spark (or streamer in the case of a plasma globe) does make a difference and radiates it’s own electromagnetic fields. In the case of a plasma globe the part of the field coming directly from the streamers may actually be a lot more than 20% of the total since the existence of plasma around the source in all directions should significantly shield external probes from the electric field generated inside.

To study the field produced by a Tesla coil with and without sparking, simply place the Tesla coil horizontally on any wooden or plastic platform or table and away from all conductors but the one your students will be manipulating. Place a chunk of metal, such as a key close to the tip of the Tesla coil, turn on the Tesla coil and adjust the gap between the tip and the chunk of metal so that sparking is not quite happening. Use a probe with an a.c. voltmeter or oscilloscope, as you did in the main experiment, to measure electric field as a function of distance. Then repeat the measurements after the chunk of metal has been moved toward the tip of the Tesla coil and a continuous spark is forming.

One additional thing that can be done with the Tesla coil that can’t be done with a plasma globe is that the sets of reading can be taken with a variety of voltage levels by turning the power control to different settings.

Expected results and answers to questions:

From Procedure 2.

Question: In a darkened room, have the disk touching the globe and then record the approximate lengths of sparks you can get to form between the center of the disk and the wire as you slowly move the wire toward the disk. Try this on the top of the globe and at several different heights on the sides of the globe. Do you notice any differences in spark lengths achieved as a function of location on the globe?

Answer: There may be small variations between the top and bottom areas or none at all. The outcome is fairly symmetrical at this resolution.

Studying the Electric Field Near a Plasma Globe – Page T4

Question: You can touch the globe with your hand and get all of the streamers on one side of the globe. If you do this and have someone else tests for sparks on the disk, is there any variation in spark length depending on whether the disk is on the side with the streamers or on the other side?

Answer: The sparks will be a lot smaller if someone’s hand gets most of the streamers on any part of the globe other than where the probe is. This shows that the detected electric field is produced by the plasma streamers.

Question: If there is a large streamer ending on the top disk as a result of someone having a hand there, does this affect the length of sparks that can be produced either at the top or at the sides? What do you conclude about electric field strengths near the ends of streamers vs. elsewhere on the globe?

Answer: As above, the sparks will be largest at the ends of streamers. Anything that keeps plasma streamers away from the probe or results in smaller streamers by the probe will reduce the spark size.

From Procedure 3:

Question: Test for spark length as a function of probe distance by holding the disk a few millimeters from the globe while bringing the end of the wire toward its center. Repeat this at greater distances from the globe until you can no longer see a spark form. Qualitatively describe the size of the sparks as a function of disk distance from the globe.

Answer: The sizes of sparks rapidly drop with distance from the globe.

From Procedure 4:

Question: Place the antenna on the globe, and move it around on the surface. Do you detect any variations with position?

Answer: As in the case of the sparks there will typically be little variation with position.

Question: Keep it in one place for a while. Are there variations in time?

Answer: There will be variations, but they will tend to be small compared to the size of the measurement.

Question: In particular do the readings depend on whether or not here is a streamer near the antenna? Test this by having someone put his/her hand on parts of the globe away from the antenna.

Answer: The readings will be a lot smaller when streamers are stronger away from the probe.

Studying the Electric Field Near a Plasma Globe – Page T5

From Procedure 5:

Since your body is a conductor and can affect the electric field strength, it is best for quantitative results if you don’t directly hold onto the antenna. An alternative is to tape the end of the antenna (the end of the wire not plugged into the meter) to one end of a wooden dowel or stick that is at least 25 centimeters long. Then, by holding the other end of the dowel or stick, you can change the position of the antenna without getting any part of your body near it.

Repeat the measurements made in Procedure 4. Note any significant difference between the new readings and those recorded in Procedure 4.

Expected Result: With the hand away from the probe the readings should be a lot smaller. The student’s body acts as an extension of the antenna.

From Procedure 6:

Starting on one side of the globe, move the antenna horizontally and record meter readings vs. distance from the globe. Before you do this, decide whether it would be best to measure distances to your antenna from the center of the globe or from the nearest surface of the globe. If you decide later that the possibility you didn’t chose would have been better, you can easily change all of the measured distances by adding or subtracting the radius of the globe.

Repeat the above starting at the top of the globe and moving the antenna vertically to greater and greater heights. Does the pattern of readings vs. distance depend on whether the measurements are made horizontally or vertically? Graph meter readings vs. distance for both sets of data. If you have a graphing calculator or graphing software with a computer, try to find functions that are consistent with the data. Form a hypothesis that might explain your results. If your hypothesis is testable in a simple way, find out how well it holds up.

Expected Results: The readings should smoothly drop with distance in both cases. The simplest result should occur with measurements from the center of the globe.

The simplest expectation that readings will drop off as the square of distance (inverse square law) will not happen because the probe is not truly a point probe and because metal in the supporting table may reflect some of the radiation so that it isn't radiated with perfect spherical symmetry. Such reflections and/or radiation from other sources may results in the pattern of reading not being the same vertically and horizontally.

From Procedure 7:

Question: With the antenna at some distance from the globe such that you get stable readings that aren’t close to zero find out what happens to the readings when someone touches the globe.

Answer: Unless the person touching the globe is close to the probe, the readings will decrease significantly.

Studying the Electric Field Near a Plasma Globe – Page T6

Question: Are there any additional variations that depend on where this person is standing at the time or what part of the globe her hand is touching? Summarize the results.

Answer: There should be little variation due to the part of the globe touched. Making a smaller number of larger plasma streamers by touching the globe seems to reduce the rate at which electromagnetic energy is radiated.

If the person touching the globe moves close to the probe, the readings should start to increase. The person is conducting electrical currents driven by the electric field radiated, and this can affect the probe.

From Procedure 8:

To study the electric fields in more detail you can use an oscilloscope to determine the frequency of the generator in the plasma globe. Use the same antenna that was attached to the multimeter in the previous investigation, and attach it to the oscilloscope leads. Adjust vertical gain until you can easily see a vertical blur at least a few centimeters high with the probe close to the globe. Then increase the sweep frequency (or time base) and make fine adjustments until waves are clear and stable. Use the sweep frequency (or time) and the number of complete waves showing on the scope to determine the frequency (or period and then frequency) of the source.

Question: Does this frequency change with location relative to the probe? Do the amplitudes of the waves seem to vary with location in roughly the same pattern as was found using the multimeter?

Answer: The frequency should stay the same, but the amplitude will decrease with increased distance from the globe. It will be hard to quantitatively compare the two methods.

From the Questions Section:

Question 1: What evidence do you now have that the charged particles in the plasmas of plasma globes and fluorescent lights are being driven back and forth as types of “alternating” currents rather than as the type of one-way currents (direct currents) produced by batteries?

Answer: The clearest evidence comes if an oscilloscope was used to examine the electric field. In this case the alternating electric field shows up as sinusoidal waves on the oscilloscope screen. Electric fields radiating from direct currents are very weak and not oscillating. Consequently, the oscillating pattern on the oscilloscope screen is strong evidence for the plasma source being an alternating current. Also an alternating current voltmeter would pick up very little from a direct current source, and the presence of strong readings on the meter shows that the source is alternating.

More indirectly, a direct current source to the glass of a plasma globe would generate a net charge on the glass, as can be sensed on the glass of older TV screens or computer monitors. Even after these are turned off, it is possible to sense the electric field from the charged screen with the hairs on the back of a hand. There is no such effect of residual charge on the glass of a plasma globe once the globe is turned off.

Studying the Electric Field Near a Plasma Globe – Page T7

Question 2: What evidence do you now have that energy is concentrated in plasmas, especially the plasma streamers inside plasma globes?

Answer: The first indication is that when a hand or finger is placed against the glass to keep the end of a streamer in place, the glass soon gets hot in that area but not at other places on the globe.

Then it can be seen that when sparks are formed outside of the globe, as in Procedures 2 and 3, they are always formed near the ends of streamers.

Finally, it has been found that the streamers are radiating electromagnetic energy. Since energy is conserved, they must have energy in some form in order to radiate it.

Question 3: What are the features of the emitting and receiving antennas that you observed in this activity? Do you know of any reasons that the size of a receiving antenna might be important in determining how easily it can pick up an electromagnetic signal?

Answer: Emitting antennas in the form of the plasma streamers can radiate electromagnetic energy when powered by an alternating current source.

Receiving antennas in the form of wires can convert electromagnetic energy into alternating currents, which can be measured with alternating current meters and oscilloscopes.

The larger the receiving antenna surface, the greater the signal recorded. The reason for this is that with more surface area of conductor there are more conducting electrons that can be driven by the radiated electromagnetic field and this constitutes a greater alternating current signal at the meter or oscilloscope. Studying the Electric Field Near a Plasma Globe – Page T8

APPENDIX

Alignment of the Activity Studying the Electric Field Near a Plasma Globe 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 G. History and Nature of Science Nature of scientific knowledge X

Studying the Electric Field Near a Plasma Globe – Page T9

Alignment of the Activity Studying the Electric Field Near a Plasma Globe 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 X D. The Structure of Matter 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