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Magnetism

magnet and iron filings
Iron filings around a magnet showing it's north and south poles. The magnetic field lines are visible in the pattern of the filings.

Ancient Greeks wrote about a type of peculiar rock that attracted iron. People also noticed that when a sliver of lodestone was suspended or floated, it always turned to one position - a north-south direction. In addition, one end of this sliver of lodestone always points toward geographic North and the other to the South. This became an excellent method for determining direction - a compass. The end of the compass that points to geographic North was called the north magnetic pole of the compass. The magnetic compass was brought to Europe in the Middle Ages from the Chinese who had been using the compass for over 1500 years.

Image of Chinese compass
The image above shows a Chinese compass from approximately 300-200 B.C.E. The spoon shape was made from lodestone and could spin on the smooth plate beneath it, pointing south. Our long and rich history with magnetism is the result of a combination of conditions. One is that iron is a common element in Earth's structure from core to crust. Iron has an arrangement of electrons that makes it possible to develop and maintain magnetism. Iron found in the crust as magnetite can be magnetic. It is also a peculiarity of the Earth that it is magnetic. The ability to use a magnet as a compass made exploration easier and magnetism more important.

Magnetism is one of a few fundamental phenomena in the universe. Most of us have experienced the strange effects of magnets. As we move a magnet slowly toward a metal surface the attraction between the magnet and the metal can suddenly become very great. With a very strong magnet, the magnet and the metal may leap toward each other and be very difficult to separate. But the strangest effect is felt when two magnets are moved close together. Sometimes we feel the attraction, and sometimes there is an even stranger repulsion. When two ends of the magnets repel each other, it almost feels as if there is something between the magnets, pushing them apart. If we slowly move the repelling ends together, that mysterious repulsion gets stronger and can even push the magnets sideways rather than allow the magnets to touch. There is a force on the magnets, even though their surfaces never touch. This was called "action at a distance" in the 1600s and was a very unsettling to many physicists including Newton.

If you place a bar magnet under a sheet of glass or clear plastic and sprinkle iron filings on top of the glass, a very beautiful pattern appears. The picture below shows just such an arrangement.

Image of Christian Aristotelian Cosmos Click on image to enlarge.

The iron filings seem to follow certain lines in arcs from one end of the magnet to the other. Scientists began to recognize that there is a region of influence (a field) in the space around a magnet. This invisible field is responsible for the "action at a distance". The field is stronger near the magnet and weaker farther away. The field can be 'felt' or 'seen' when we push magnets together or sprinkle iron filings around the magnet.

Scientists began to draw this invisible field as lines around a magnet as in the drawing below. Scientists have agreed to an arbitrary convention that the field lines point from the north magnetic pole to the south magnetic pole. The field is strongest where the field lines are drawn closely together. The needle of a compass placed near this bar magnet would rotate until its north end pointed in the direction of the local magnetic field line and tangent to the line.

bar magnet field
When two magnets are arranged end-to-end with opposite poles adjacent as shown below, the field lines between the magnets go from the north end of the bottom magnet to the south end of the top magnet. Close to the top magnet, field lines emerge from the north end and connect with the south end. The same is true close to the bottom magnet. However, at a distance, the field from the north of the top magnet connects to the south end of the bottom magnet. It is as if there was only one magnet.

bar magnet field
This type of connection does not occur when two magnets are arranged end-to-end with opposing poles adjacent (in the case shown below two north poles are adjacent). Between the ends the magnetic fields oppose each other and are forced sharply to the side. To the side of each magnet the field lines curve from north to south, however, the field lines are pushed away from the opposing field becoming flatter near the opposing magnet and bulging away on the far side.

bar magnet field
The above models assume a permanent magnet such as a bar magnet. Bar magnets are examples of ferromagnets. The name comes from iron (ferric), the most common element to display this behavior, yet nickel, cobalt, chromium and a few other elements are also ferromagnetic. While any atom with an unpaired electron can have a magnetic field, the atoms of these special elements act together in groups called domains, locking together their magnetic poles. Each domain, ranging in size from 0.1 mm to 1.0 mm, is a tiny magnet. When an external magnetic field is applied under the right conditions, all of these domains are induced to line up creating a large magnet. In addition, ferromagnets tend to stay magnetized long after the external field is removed.

bar magnet field

Image courtesy of C R Nave HyperPhysics

For more information visit the HyperPhysics Magnetic Domain page.

The next section, Electricity, contains important background information to concepts intimately involved in magnetism.

Next Step: A Magnet in Space »

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