The study of magnetism is a science in itself. Electrical and magnetic phenomena interact; a detailed study of magnetism and electromagnetism could easily fill a book. Magnetism exists whenever electric charges move relative to other objects or relative to a frame of reference.
The Earth has a core made up largely of iron heated to the extent that some of it is liquid. As the Earth rotates, the iron flows in complex ways. This flow gives rise to a huge magnetic field, called the geomagnetic field , that surrounds the Earth.
Earth’s Magnetic Poles And Axis
The geomagnetic field has poles, as a bar magnet does. These poles are near, but not at, the geographic poles. The north geomagnetic pole is located in the frozen island region of northern Canada. The south geomagnetic pole is in the ocean near the coast of Antarctica. The geomagnetic axis is thus somewhat tilted relative to the axis on which the Earth rotates. Not only this, but the geomagnetic axis does not exactly run through the center of the Earth. It’s like an apple core that’s off center.
Charged particles from the Sun, constantly streaming outward through the solar system, distort the geomagnetic field. This solar wind in effect “blows” the field out of shape. On the side of the Earth facing the Sun, the field is compressed; on the side of the Earth opposite the Sun, the field is stretched out. This effect occurs with the magnetic fields around the other planets, too, notably Jupiter.
As the Earth rotates, the geomagnetic field does a complex twist-and-turn dance into space in the direction facing away from the Sun. At and near the Earth’s surface, the field is nearly symmetrical with respect to the geomagnetic poles. As the distance from the Earth increases, the extent of geomagnetic-field distortion increases.
The Magnetic Compass
The presence of the Earth’s magnetic field was noticed in ancient times. Certain rocks, called lodestones , when hung by strings, always orient themselves in a generally north-south direction. Long ago this was correctly attributed to the presence of a “force” in the air. It was some time before the reasons for this phenomenon were known, but the effect was put to use by seafarers and land explorers. Today, a magnetic compass can still be a valuable navigation aid, used by mariners, backpackers, and others who travel far from familiar landmarks. It can work when more sophisticated navigational devices fail.
The geomagnetic field and the magnetic field around a compass needle interact so that a force is exerted on the little magnet inside the compass. This force works not only in a horizontal plane (parallel to the Earth’s surface) but vertically, too, in most locations. The vertical component is zero at the geomagnetic equator , a line running around the globe equidistant from both geomagnetic poles. As the geomagnetic latitude increases toward either the north or the south geomagnetic pole, the magnetic force pulls up and down on the compass needle more and more. The extent of this vertical component at any particular location is called the inclination of the geomagnetic field at that location. You have noticed this when you hold a compass. One end of the needle seems to insist on touching the compass face, whereas the other end tilts up toward the glass.
Practice problems of these concepts are found at: Magnetism Practice Test
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