Teaching about the Earth's Magnetism in High School

    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 magnetic compass and its evolution. Robert Norman (1581) and William Gilbert (1600) demonstrated that the force on the compass needle came from the globe of the Earth (evident to us, but not to early mariners!), and their experiments are neat examples of the scientific method. Through them students can also sense how our era of science began.

•     The story of Oersted and his unexpected discovery of a link between magnetism and electricity prepares the student for later study of electromagnetism in physics class. It may also introduce the subject of lodestones and of magnetization by lightning.

•     The slow changes of the Earth's magnetic field, which mystified scientists for a long time. Today they are seen as a signature of the electric currents which produce this field, deep inside the Earth. The core of the Earth is molten iron, slowly circulating, and motion of a conductor through a magnetic field can generate an electric current, which in turn can maintain that field. A "fluid dynamo" for producing currents this way (though not its own magnetic field) was proposed by Faraday, who in 1832 tried to demonstrate it, using the flow of water in a river and the Earth's magnetism. However, only after 1908 was the process taken seriously as a possible source of magnetism in nature, after sunspots were found to be intensely magnetic.

•     The magnetization of ancient lava flows, which suggested that in the distant past, the north-south polarity of the Earth was sometimes reversed. Between 1962 and 1965 seafloor magnetization finally solved the puzzle, providing a detailed chronology of past reversals as well as evidence that continents slowly moved about the Earth's surface. Riding on top of "tectonic plates," their motion resembled the "continental drift" unsuccessfully proposed by Alfred Wegener early in the century. Dynamo theory later explained how such reversals might occur.

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•     Students may be intrigued to hear that Gilbert was the queen's physician and that some copies of his book were defaced, for voicing the heresy that the Earth was not the fixed center of the universe, but rotated around its axis.

•     The unpredictable path of discovery is demonstrated by Oersted, who had the key to a major scientific breakthrough, but was unable to interpret it. They may also enjoy Gilbert's story of an iron bar atop a steeple which strangely became magnetized--by a stroke of lightning, we might guess, though Gilbert missed that clue.

•     They may be interested in the parallels between Faraday's experiment at Waterloo bridge and the space tether experiment on the space shuttle. And...

•     They may sense the drama of Wegener's quest for acceptance of his theory of "continental drift," of Larry Morley's unsuccessful attempts to publish his theory of seafloor magnetization, and of the way plate tectonics provided the final synthesis.

    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?

1. David P. Stern and Mauricio Peredo, Space Physics for Poets, The Physics Teacher, 35, 38-39, Jan. 1997
2. David P. Stern, Using Space to Teach Physics, The Physics Teacher, 37, 102-3, Feb. 1999