Magnetic Confinement Demonstration:

Transcription

1 Magnetic Confinement Demonstration: Motion of Charged Particles in a Magnetic Field 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-AC02-76CH Contemporary Physics Education Project (CPEP

2 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 robreiland1@comcast.net voice:

3 Magnetic Confinement Demonstration: Motion of Charged Particles in a Magnetic Field 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: One method of confining a plasma long enough for fusion to take place is to use continuous magnetic fields inside a doughnut-shaped container, similar to a toroid, called a Tokamak. This works because moving charged particles will go around in circles or spirals in an appropriate magnetic field and remain confined within the toroid. One can present a fairly complete explanation of what happens in magnetic confinement at the level of high school or introductory college physics. But it is also possible to demonstrate visually how magnetic fields cause moving charges to spiral around magnetic field lines. After actually seeing the effect, students should be able to follow the explanation of magnetic confinement more easily. Equipment and Materials: Eico Oscilloscope Model 460 (see comments of use of oscilloscope for this activity below) (or similar "older" scope in which the CRT is NOT shielded) Eico DC power supply (battery eliminator) Model 1065S (or equivalent DC supply 6V/20a V/10a) Caution: If the power supply is going to be on for a while without supervision (a common workshop situation) the 6V scale is the safer choice. When doing a demonstration, it is OK to briefly use the 12V scale (up to about 10 volts). air solenoid Science KIT (or equivalent) steel bar magnets (10) Science KIT (or equivalent) Misc. hook-up wires, connectors (Note: See for some of these equipment references) Use of Oscilloscope for this Activity: If you are not familiar with the controls of an oscilloscope, there are a few things that you should try before presenting this activity to students. You will need to be able to generate both a dot and a line on the screen. There will be no need to have any external inputs to the oscilloscope for anything that will be done in this activity. Because of this, and since you only need to produce a dot and a line, use the simplest and cheapest model you have (if you are lucky to have more than one available).

4 Magnetic Confinement Demonstration Page T2 Turn the power on. After the oscilloscope warms up, turn the intensity knob to get the intensity you want. It will take at least a few seconds before the oscilloscope is warmed up enough to produce a dot, line or other pattern on the screen. Once this happens, you can move the pattern to where you want it with the horizontal and vertical position knobs. If nothing appears, use these knobs to bring the pattern to the screen. You probably won t have to use a vertical gain knob, or what may be called the input sensitivity, unless someone has this on a high setting that results in a lot of chaotic motion on the screen. In that case turn the knob to eliminate this problem. There is a knob that controls the rate at which a line sweeps across the screen. It could be marked sweep frequency, frequency, s/cm or something else that indicates a time rate. When you want a line on the screen, adjust this control until the line looks steady. To show students that the apparent line is really a dot that is moving from left to right over and over again, turn the sweep frequency knob to a slower time or sweep rate until the dot can be seen in motion. To get a dot or at least a small pattern for the first part of the activity, turn the horizontal gain knob. If your oscilloscope doesn t have a horizontal gain knob, select external input to produce a dot on the screen. Expected results and answers to questions: Procedure 2: Slowly bring a bar magnet toward the screen holding it horizontally with the north pole toward the dot on the screen, and do your best to keep the magnet oriented parallel to the direction of the electron beam as the beam goes from the back of the oscilloscope to the screen (see Figure 1). Another way to think of this is that if the electron beam were to continue along a straight line out of the screen, it should pass from the north pole of the magnet through the south pole. Observe what happens to the dot as the magnet gets closer. Does the dot move around a little or a lot? Is there any systematic motion of the dot as you move the magnet in closer? Answer: If the magnet is lined up well and moved smoothly, the dot on the screen should show very little movement. The magnetic field produced by the bar magnet is parallel to the direction of motion of the electron beam, and magnetic fields parallel to velocities of charged particles exert zero force on the moving particles. There will probably be a little wobble to the dot because no one can move the magnet perfectly along a line. Procedure 3: Turn the magnet so that a horizontal line from its south pole through its north pole is perpendicular to the electron beam and the north pole is oriented as shown in Figure 2a. Move the magnet slowly toward the screen from the side. If you have an inexpensive oscilloscope, the case probably doesn t shield the electron beam from magnetic fields. You should then be able to get better results by starting the magnet at one side of the oscilloscope case, about half way from back to front. Again move the magnet toward the oscilloscope and observe the effects on the dot (see Figure 2b).

5 Magnetic Confinement Demonstration Page T3 Does the dot move around a little or a lot? Is there any systematic motion of the dot as you move the magnet in closer? Answer: The dot should be deflected in a direction perpendicular to the orientation of the magnet. Since the magnet is horizontal with north pole on the left, the dot deflection will be up. Forces by magnetic fields on moving charges are perpendicular to both the direction of motion of the charges and to the direction of the magnetic field where it acts on the moving charges. (See General Background for more on this. This background information is also for students, but in case it seems better for them to explore what happens before they try to understand the physics behind it, you can copy it separately from the procedures. You can then either hand it out to your students right away or after they ve carried out their observations.) Procedure 5. Slowly bring a bar magnet toward the screen holding it horizontally with the north pole toward the center of the line on the screen (as you did in Procedure 2). Describe what happens to the line as the magnet gets closer. (Expected results for Procedures 5, 6, 7 are together after Procedure 7) Procedure 6. If you have a solenoid that can be powered by a d.c. power supply, connect the solenoid to the power supply, put some iron bars inside the solenoid to increase the magnetic field that will be produced, and firmly tape them in place with duct tape to keep them from falling or magnetically surging out. Turn the power supply to about 10 volts, and move the solenoid toward the screen holding it horizontally with one end toward the screen as was done with the bar magnet in Procedure 5. Again describe what happens. Procedure 7. An alternative to Procedure 6 is to put the solenoid close to the screen in the same orientation as in Procedure 6, and smoothly increase the voltage to the power supply. In either case you are increasing the magnetic field that the electron beams are moving through. Expected results in procedures 5, 6 and 7: Your students should see the line on the screen rotate (see Figure T1). The reason for this is illustrated in the three figures in the General Background. The presence of a magnetic field lined up with the center beam has no effect on the position where this beam hits the screen. However, beams hitting the screen to the left or the right of the center (see Figure 3) have components of velocity perpendicular to the magnetic field. The magnetic field will start these beams onto spiral paths. The rotations of all of the beams, except for the center beam, gives the appearance that the entire line on the screen is physically rotated by the magnets. S N Figure T1: Appearance of the previously horizontal line on the screen as a magnet produces a horizontal magnetic field that acts on the left and right components of the electron beams.

6 Magnetic Confinement Demonstration Page T4 If the magnetic field used is perfectly uniform, all beams will rotate through the same angle, and the line will be straight at the new angle. The lesser turning at the ends of the line with the magnet indicates that the magnetic filed produced by the magnet is weaker farther from the magnet. Solenoids come closer to giving a uniform magnetic field than do bar magnets because they generate stronger fields that are nearly uniform across the inside diameter of the solenoid. Question: 1. When you used a bar magnet in Procedure 5, did the entire line rotate together as you moved the magnet closer and closer to the screen? What does this suggest about the uniformity of the magnetic field where it acts on the electron beams? Answer: No. The magnetic field radiating from a bar magnet spreads out considerably and greatly weakens over distances of a few centimeters. It won t be as strong away from the center, and the ends of the line on the oscilloscope will not rotate as much as the center of the line. Question: 2. If you also used a solenoid in Procedure 5, did the entire line rotate together as you moved the solenoid closer and closer to the screen or as you increased the power to the solenoid without moving it? What does this suggest about the uniformity of the magnetic field where it acts on the electron beams? Answer: Not quite, but it comes closer than what happens with a bar magnet. The magnetic field produced by a solenoid near its end is nearly uniform, and it becomes weaker at a lesser rate with distance than does the field produced by a bar magnet.

7 Magnetic Confinement Demonstration Page T5 APPENDI Alignment of the Activity Magnetic Confinement Demonstration 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 Understandings about scientific inquiry B. Physical Science Content Standards Structures of atoms Motions and forces Conservation of energy Interactions of energy and matter 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

8 Magnetic Confinement Demonstration Page T6 Alignment of the Activity Magnetic Confinement Demonstration 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. 1. THE NATURE OF SCIENCE AAAS Benchmarks (abridged) Grades 9-12 B. Scientific Inquiry 2. THE NATURE OF MATHEMATICS B. Mathematics, Science, and Technology 3. THE NATURE OF TECHNOLOGY C. Issues in Technology 4. THE PHYSICAL SETTING A. The Universe D. The Structure of Matter E. Energy Transformations F. Motion G. Forces of Nature 11. COMMON THEMES A. Systems B. Models C. Constancy and Change D. Scale 12. HABITS OF MIND B. Computation and Estimation