Properties of a Plasma

Properties of a Plasma:

Half-Coated Fluorescent Bulbs

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 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:

voice: 412-968-3049

Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T1

Properties of a Plasma: Half-Coated Fluorescent Bulbs

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:

A half-coated fluorescent bulb can be used to directly study plasmas as electromagnetic systems. This specially manufactured tube is clear for half of its length and is coated with the normal phosphors for the other half. The half-coated fluorescent bulb is special because the plasma inside can be observed, experimented with and studied to a greater degree than is possible with any other easily produced plasma.

Equipment List:

Half-Coated Fluorescent Bulb Kit

(including bulb housing and special cable with current limiting resistor)

Science KIT 46144-00 (or equivalent)

(see

Universal Power Supply (DC 0-375 volts)

Science KIT 69716-01 (or equivalent)

Tesla Coil

Science KIT 61157-02 (or equivalent)

Spectrometer

Science KIT 45492-00 (or equivalent)

Diffraction Grating

holographic gratings by Learning Technologies Inc. are highly recommended

(see

LARGE horseshoe magnet, two strong bar magnets, or an electromagnet that can be used to set up a magnetic field inside the fluorescent bulb tube

Unfortunately horseshoe magnets that are sufficiently large (minimum 10 cm "pole-gap") are quite difficult to locate. At one time they were readily available from "surplus" vendors - - but this doesn’t seem to be the case now.

Background:

A good source of background on the physics of fluorescent bulbs can be found in the article, “Shedding Some Light on Fluorescent Bulbs” by Nicholas R. Guilbert.[*] As described in the article by Guilbert, a fluorescent bulb produces a mercury vapor spectrum. However in normal operation this is not apparent since the inside of the bulb is coated with phosphors that absorb the line spectrum of the mercury and emit light of longer wavelengths in a nearly continuous white light spectrum.

To operate the fluorescent bulb, connect the bulb to an appropriate 350-400 volt d.c. power supply such as described in the instructions that are supplied with the tube. (See Figure T1)

CAUTION: Don’t plug in the power supply until all electrical connections have been made. Be very careful that no electrical terminal or connection is positioned where you or a student could easily contact it. If there are any electrical connections that are exposed, wrap electrical tape around them before plugging in the power supply.

If the d.c. power supply is current limited (10 mA limit), after all connections have been made and the power supply is plugged in, turn it on and turn the voltage up to about 350 volts. (You are now ready for the instructions in the next paragraph.) If the d.c. power supply in not current limited, you will need to use about 20,000 ohms of resistance between the power supply and the bulb to limit the current. Since the resistor used might have to dissipate up to 3 watts of power, some care in the choice of resistors is needed. If you are lucky enough to have a resistor of 20,000 ohms or a little more that is rated at 3 watts or more, just connect this between the power supply and a lead to the bulb. Be sure to wrap the resistor and all exposed wires in electrical tape before plugging in the power supply. If you can’t find a single resistor near 20,000 ohms that is rated at 3 watts or more, the next best thing is to use a set of resistors with higher resistance in parallel. For example, five 100,000 ohm resistors each rated at 1 watt would be equivalent to a 20,000 ohm, 5 watt resistor.

Use a Tesla coil or Van de Graaff electrostatic generator to ignite the bulb, following the instructions that came with the tube. It is very important that, if a Tesla coil is used for this, it not be held near the positive end of the tube for more than about a second. If the tube doesn’t light immediately, pull the Tesla coil away, and raise the voltage on the power supply before trying again. Otherwise the high current from the Tesla coil can damage the tube.

FigureT1: Electrical connections and Tesla Coil to ignite the bulb

You need a strong magnet or electromagnet to examine the effects of magnetic fields on a plasma. The best magnet would be a large and strong horseshoe magnet with poles far enough apart that they can be placed on either side of the tube holding the fluorescent bulb. However, any reasonably strong horseshoe magnet will do. If you don’t have a good horseshoe magnet, either use two strong bar magnets, one on either side of the tube, or an electromagnet.

Expected results and answers to questions in “Spectra of mercury vapor and phosphors in a fluorescent bulb and spectrum of an incandescent bulb” part:

1. Once your teacher has turned on the half-coated fluorescent bulb, darken the room, and observe the color and brightness of each half of the bulb. Since the source of the energy for the coated part of the bulb is the same mercury vapor that is producing violet light in the uncoated half of the bulb, what evidence do you have that some of the mercury spectrum from the uncoated half is not visible?

Answer: The coated side is brighter than the uncoated side. This is because most of the electromagnetic energy emitted by the excited mercury is ultraviolet, and the phosphors can convert most of this invisible radiation into visible radiation.

1. Examine the light from each half of the bulb separately through either a diffraction grating or a spectroscope or both. Describe in detail the spectra of the two parts. If you have a spectroscope with which you can see the values of the wavelengths, include those along with the colors that you see.

Expected result: The spectrum from the uncoated half will be the visible light line spectrum of mercury. The three most prominent colors (and wavelengths) are violet (436 nm), green (546 nm) and yellow (579 nm). The spectrum from the coated half will be mostly continuous with a few lines brighter than the surrounding spectrum.

1. Now you will examine the light from an incandescent light bulb and compare it to that from the coated half of the fluorescent bulb. If available, use a clear incandescent bulb as well as a normal frosted incandescent bulb.

As seen by the naked eye, is the appearance of the white light from the fluorescent bulb exactly the same as the appearance of the white light from the frosted incandescent bulb? If there is any difference, describe it as best you can.

Expected Result: There may be some slight difference in appearance to the naked eye, particularly as to color “warmth” or “coolness”, “yellowness” or “whiteness.” But there will be a bigger difference with the spectra as seen next.

As seen with a diffraction grating or spectroscope, are the spectra of the white light from the fluorescent bulb and from the frosted incandescent bulb exactly the same for each? For example, are both spectra completely continuous, or are there places in either spectrum where lines seem to stand out? If so, are any of these “lines” at the same wavelengths or of the same colors of lines as the mercury vapor spectrum? A sketch or drawing could be useful. Try to form a hypothesis that could be used to explain some or all of your observations. If you have a clear incandescent bulb, is there any difference in the spectrum from the clear bulb and that of the frosted bulb?

Answer: They are not quite the same. In particular, as stated in the answer to question 2, both have a continuous spectrum, but there are a few lines in the fluorescent bulb spectrum that are brighter than the surrounding continuous spectrum. The lines that “stand out” from the surrounding spectrum on the coated half are the same as some of the lines in the mercury spectrum. Apparently the visible part of the mercury spectrum is superimposed onto the secondary light emitted by the phosphors. The spectrum from the incandescent bulb does not have these brighter areas. The spectra from the clear and frosted incandescent bulbs should look the same, the frosting just scatters the light.

1. If your power supply can be adjusted to safely change the brightness of the fluorescent bulb, ask your teacher to do so, and again observe the spectra from both halves. Does the brightness of the source affect the location of the spectral lines produced?

Answer: No, spectral lines are produced by transitions between energy levels of atoms. The brightness is increased as a result of a higher rate of production of energy levels above the ground state, but the actual energy levels are not affected, and it is the energy level differences that determine the wavelengths.

Expected results and answers to questions in “Reactions to magnets” part:

1.If you have studied the effects of magnetic fields on electrical currents before, think about what you expect to happen as you bring a horseshoe magnet or a pole of any other strong magnet toward the bulb in the middle of the uncoated part of the tube. Visualize the magnetic field as arcing away from the north pole of the magnet (and, for a horseshoe magnet, arching to the south pole). If the magnet is approaching the bulb with its north pole slightly above the center of the bulb (and south pole slightly below for a horseshoe magnet; other magnets vertical), the magnetic field through the bulb will be vertical and down. In a case like this many people expect that the plasma will be deflected downward in the direction of the magnetic field. Move the magnet toward the bulb, and look closely to see if this is what happens. Describe what happens.

Expected result: Magnetic forces are always exerted perpendicularly to both the direction of the current and the direction of the magnetic field as shown in the figure. If the magnetic field is down, then with the student facing the tube with the positive end of the bulb to the right, the deflection of the plasma will be horizontal and toward the student.

2., 3., 4. The results for these parts are described in the procedures.

APPENDIX

Alignment of the Activity

Properties of a Plasma: Half-Coated Fluorescent Bulbs

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.

Alignment of the Activity

Properties of Plasma: Half-Coated Fluorescent Bulbs

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.

B. Scientific Inquiry

A. The Universe

A. Systems

[*] N. R. Guilbert, “Shedding Some Light on Fluorescent Bulbs,” Phys. Teach. 34, 20-22 (Jan.1996).