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The Spring 2011 issue of The Earth Scientist is focused on modernizing seismology education. Thanks to IRIS, you can download this issue for free as a pdf. Print copies are available in our online store.
This is a drawing of a magnetic field.
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The Magnetic Field

The force of magnetism causes material to point along the direction the magnetic force points. This property implies that the force of magnetism has a direction. As shown in the diagram to the left, the force of magnetism is illustrated by lines, which represent the force. In this diagram, the force points from the positive pole to the negative pole of the magnet. As shown in the diagram, if one side of the magnet is called the positive side, and the other side called the negative side, the force of magnetism flows from the positive side or pole, to the negative pole.

The force of magnetism forces small pieces of iron to line up in the direction the magnetic force points. A compass, in which a sliver of magnetic material can swing freely, is thus forced to point toward the positive pole.

On Earth, the north (positive) pole of the Earth's magnet is in fact at its South geographic pole. A compass needle sure enough indicates North, but if you put a compass needle near a bar magnet, it points AWAY from the north (positve) pole of the bar magnet. This picture shows where the poles are actually found, and also shows that the poles drift over the surface of the Earth over time.

The lines, which represent the force emanating from the magnet, illustrate what is called the magnetic field of the magnet, that is, the force of magnetism coming from the magnet is called the "magnetic field". The magnetic field is strongest where the lines of force come together, and is weakest when the lines of force are far apart. The force of magnetism can exhibit the behavior of more than two poles, but it never exhibits one pole (a mono-pole).

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA