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Magnetic Fields

#2. Magnetic Fields

People not familiar with magnetism often view it as a somewhat mysterious property of specially treated iron or steel.

A magnetized bar has its power concentrated at two ends, its poles; they are known as its north (N) and south (S) poles, because if the bar is hung by its middle from a string, its N end tends to point northwards and its S end southwards. The N end will repel the N end of another magnet, S will repel S, but N and S attract each other. The region where this is observed is loosely called a magnetic field; a more specific look at the concept of "field" is provided in a later section.

Either pole can also attract iron objects such as pins and paper clips. That is because under the influence of a nearby magnet, each pin or paper clip becomes itself a temporary magnet, with its poles arranged in a way appropriate to magnetic attraction.

But this property of iron is a very special type
of magnetism, almost an accident of nature!

Out in space there is no magnetic iron, yet magnetism is widespread. For instance, sunspots consist of glowing hot gas, yet they are all intensely magnetic. The Earth's own magnetic powers arise deep in its interior, and temperatures there are too high for iron magnets, which lose all their power when heated to a red glow. What goes on in those magnetized regions?

It is all related to electricity.

Matter consists of electrically charged particles: each atom consists of light, negative electrons swarming around a positive nucleus. Objects with extra electrons are negatively (-) charged, while those missing some electrons are positively (+) charged. Such charging with "static electricity" may happen (sometimes unintentionally!) when objects are brushed with cloth or fur on a dry day. Experiments in the 1700s have shown that (+) repels (+), (- ) repels (-), while (+) and (-) attract each other.

Close to 1800 it was found that when the ends of a chemical "battery" were connected by a metal wire, a steady stream of electric charges flowed in that wire and heated it. That flow became known as an electric current. In a simplified view, what happens is that electrons hop from atom to atom in the metal.

In 1821 Hans Christian Oersted in Denmark found, unexpectedly, that such an electric current caused a compass needle to move. An electric current produced a magnetic force!

Andre-Marie Ampere in France soon unraveled the meaning. The fundamental nature of magnetism was not associated with magnetic poles or iron magnets, but with electric currents. The magnetic force was basically a force between electric currents (figure below):

--Two parallel currents in the same direction attract each other.

--Two parallel currents in opposite directions repel each other.

Here is how this can lead to the notion of magnetic poles.
Bend the wires into circles with constant separation (figure below):

--Two circular currents in the same direction attract each other.
--Two circular currents in opposite directions repel each other.

Replace each circle with a coil of 10, 100 or more turns, carrying the same current (figure below): the attraction or repulsion increase by an appropriate factor. In fact, each coil acts very much like a magnet with magnetic poles at each end (an "electromagnet"). Ampere guessed that each atom of iron contained a circulating current, turning it into a small magnet, and that in an iron magnet all these atomic magnets were lined up in the same direction, allowing their magnetic forces to add up.

The magnetic property becomes even stronger if a core of iron is placed inside the coils, creating an "electromagnet"; that requires enlisting the help of iron, but is not essential. In fact, some of the world's strongest magnets contain no iron, because the added benefit of iron inside an electromagnethas a definite limit, whereas the strength of the magnetic force produced directly by an electric current is only limited by engineering considerations.

In space, on the Sun and in the Earth's core, electric currents are the only source of magnetism. We loosely refer to the region of their influence as their magnetic field, a term which will be further discussed later.

#2. Magnetic Fields

Further reading: