Download Properties of a Plasma: Half

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 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
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 www.ScienceKit.com)
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 http://www.starlab.com/)
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
N. R. Guilbert, “Shedding Some Light on Fluorescent Bulbs,” Phys. Teach. 34, 20-22
(Jan.1996).
*
Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T2
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.
Power Supply
-
+
Current
limiting
resistor(s)
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,
Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T3
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.
2. 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.
3. 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?
Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T4
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.
4. 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.
Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T5
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.
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
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
Properties of a Plasma: Half-Coated Fluorescent Bulbs – Page T6
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.
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
E. Energy Transformations
X
F. Motion
X
G. Forces of Nature
X
11. COMMON THEMES
A. Systems
X
B. Models
C. Constancy and Change
X
D. Scale
X
12. HABITS OF MIND
B. Computation and Estimation
X