Teaching about the Earth's Magnetism in High School
Covering the Earth's magnetism in a high school course on Earth sciences addresses two important problems of the science curriculum. |
On one hand, the customary sequence allocates to physics just one year (and that as an elective!), not nearly enough to sample all areas of physics. In particular, the coverage of electromagnetism, deferred to the end of that course, often ends up short.
On the other hand, while physics in high school suffers from lack of time, Earth science could use more substance. It involves rather little math and only limited experimentation, and so all too often ends up mainly as rote memorization. A better strategy may be to qualitatively describe the physical and historical foundations of Earth sciences, making the student appreciate the way understanding comes from observations and the way science evolves. The challenge is for the teacher to assemble such a course, with meaningful and memorable threads tying together the many topics covered.
The Earth's magnetism and the historical evolution of its study are one such thread. Appropriate topics include:
The Web SiteThe material for such a course, in clear plain language, is available for free on the world-wide web, at a site named "The Great Magnet, the Earth." Its home page is http://www.phy6.org/earthmag/demagint.htm and it contains about 20 files. Some of it goes beyond the items listed above (e.g. the link between magnetometers and research on smoking), though such extensions may be appreciated by students who want to explore further. One file gives instructions for performing an experiment of Gilberts', and two are addressed to teachers. Three separate files contain the full text of an hour-long talk "Teaching about the Earth's Magnetism in Earth Sciences Class" given 11.18.2000 as an AAPT lecture at the Baltimore meeting of the National Association of Science Teachers. The site has a Spanish translation (a French one has been started), a large glossary and a list of questions by users (with answers), and it can be downloaded to one's own computer in a choice of compressed formats.
History of ScienceLike two earlier educational web sites by the same author1,2 , this one, too, stresses the history of science. History is a framework logically relating different parts of the subject to each other, and it also adds human interest and stories of discovery, so important in keeping the attention of the class. Some examples;
The second week would be devoted to electromagnetism. The teacher should start with a very brief overview of electricity and electric currents, topics about which students may know little at this stage. Tell about electrons, static electricity and the flow of electrons through wires, e.g. in a flashlight. The flow of electricity may be compared to the more familiar flow of water through pipes.
Then introduce the story of Oersted and Ampere, and of how Faraday rose from apprentice bookbinder to leading scientist. Oersted showed how electric currents created magnetism, while Faraday looked for the opposite effect, for magnetism to create electric current. That turned out to be harder, requiring magnetic fields that varied, or in cases of interest here, motion of electric conductors through fields. Faraday's Waterloo bridge experiment can be viewed as the forerunner of the tether experiment on the space shuttle.
The third week, on the origin of magnetic fields on Earth and Sun, may be the hardest. How come the observed field varies, decade by decade? Could it come from magnets inside Earth which are slowly moving? Halley, of comet fame, thought so. But sunspot magnetism left little doubt: at least some magnetic fields in nature must be produced by electric currents, created by the flow of electrically conducting fluids through the very same magnetic fields. On the Sun, this process is probably related to the 11-year sunspot cycle, and to the fact the Sun's equator rotates faster than the rest. This part of the course brings the student in touch with present-day science, with problems still not completely solved.
The Sun's polar field reverses every 11 years or so, and this forms a bridge to the last section, on geomagnetic reversals and plate tectonics. It is a large subject, and the teacher might well shorten coverage of the 3rd part by 1-2 days to allow more time here. This story starts with magnetic reversals of the Earth's poles, and the way it was deduced from ancient lava flows. Reversals are a topic that seems to intrigue students. Do they expose life on Earth to deadly radiation? No, the atmosphere protects us. Will the trend of the last 150 years continue, until the field reverses somewhere around the year 3500? Possible--but by the past record, not likely.
Next come plate tectonics and the story of Alfred Wegener. To start with, students should be made aware that the elevations on our globe are not distributed smoothly, but cluster around two levels--continents (including continental shelves) and ocean floors. This suggests continents are distinct entities. Did they fit together like jig-saw puzzle pieces? Wegener believed they did, and ended as outcast among scientists. Only 30 years after his death, thanks to a new awareness of magnetic reversals and to new electronic magnetometers, which allowed seafloor magnetization to be mapped, did a clear picture emerge. This too is still an area of active research, as satellite systems allow the motions of different parts of continents to be tracked as well.
It all adds up to an exciting voyage of discovery, for students and teachers alike. Who ever said Earth Sciences had to be dull, or was unrelated to high school physics?
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org
Last updated 25 November 2001