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Magnetism
Presented by ElectricianEducation.com
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#6. Electromagnetic WavesPerhaps the greatest theoretical achievement of physics in the 19th century was the discovery of electromagnetic waves. The first hint was an unexpected connection between electric phenomena and the velocity of light.Electric forces in nature come in two kinds. First, there is the electric attraction or repulsion between (+) and (-) electric charges. It is possible to use this to define a unit of electric charge, as the charge which repels a similar charge at a distance of, say, 1 meter, with a force of unit strength (actual formulas make this precise).But second, there is also the attraction and repulsion between parallel electric currents. One could then define the unit of current, as the current which, when flowing in a straight wire, attracts a similar current in a parallel wire 1 meter away with a force of unit strength, for every meter of the wires' length.But electric current and charge are related! We could have just as well based the unit of current on the unit of charge--say, as the current in which one unit of charge passes each second through any cross section of the wire. This second definition turns out to be quite different, and if meters and seconds are used in all definitions, the ratio of the two units of current turns out to be the speed of light, 300,000,000 meters per second.In Faraday's time the speed of light was known, although not as accurately as it is today. It was first derived around 1676 by Ole (Olaus) Roemer, a Danish astronomer working in Paris. Roemer tried to predict eclipses of Jupiter's moon Io (mentioned later here in an altogether different connection) and he found a difference between actual and predicted eclipse times, which grew and then decreased again as the Earth circled the Sun. He correctly guessed the reason, namely, as the Earth moved in its orbit, its distance to Jupiter also went up and down, and light needed extra time to cover the extra distance.But what was the meaning of the link between electricity and light?Remember the idea of Faraday which evolved into the "magnetic field" concept--that space in which magnetic forces may be observed is somehow changed? Faraday also showed that a magnetic field which varied in time--like the one produced by an alternating current (AC)--could drive electric currents, if (say) copper wires were placed in it in the appropriate way. That was "magnetic induction," the phenomenon on which electric transformers are based.So, magnetic fields could produce electric currents, and we already know that electric currents produce magnetic fields. Would it perhaps be possible for space to support a wave motion alternating between the two? Sort of: |
magnetic field ---> electric current ---> magnetic field ---> electric current ---> ... |
There was one stumbling block. Such a wave could not exist in empty space, because empty space contained no copper wires and could not carry the currents needed to complete the above cycle. A brilliant young Scotsman, James Clerk Maxwell, solved the riddle in 1861 by proposing that the equations of electricity needed one more term, representing an electric current which could travel through empty space, but only for very fast oscillations.With that term added (the "displacement current"), the equations of electricity and magnetism allowed a wave to exist, propagating at the speed of light. The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line. Actually it fills space, but it would be hard to draw that.
Maxwell proposed that it indeed was light. There had been earlier hints--as noted above, the velocity of light had appeared unexpectedly in the equations of electricity and magnetism--and further studies confirmed it. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam.Then Heinrich Hertz in Germany showed that an electric current bouncing back and forth in a wire (nowadays it would be called an "antenna") could be the source of such waves. (The current also produces a magnetic field in accordance with Ampere's law, but that field decreases rapidly with distance.) Electric sparks create such back-and-forth currents when they jump across a gap--hence the crackling caused by lightning on AM radio--and Hertz in 1886 used such sparks to send a radio signal across his lab. Later the Italian Marconi, with more sensitive detectors, extended the range of radio reception, and in 1903 detected signals from Europe as far as Cape Cod, Massachussets.It was presumed that light from the hot wire of a lightbulb was emitted because the heat caused electrons to bounce back and forth rapidly, turning each into a tiny antenna. When physicists tried to follow that idea, however, they found that the familiar laws of nature had to be modified on the scale of atomic sizes. That was how quantum theory originated.Gradually other electromagnetic waves were found The wave nature of light causes different colors to be reflected differently by a surface ruled in fine parallel scratches--which is why a compact laser disk (for music or computer use) shimmers in all colors of the rainbow. The orderly rows of atoms in a crystal also form parallel lines but spaced much more closely, and they turned out to have the same effect on X-rays, showing that X-rays, like light, also were electromagnetic waves, but of a much shorter wavelength. Later it was found that beams of electrons in a magnetic field, inside a vacuum tube, could become unstable and emit waves longer than light: the magnetron tube where this occured was a top-secret radar device in World War II, and it later made the microwave oven possible.Electromagnetic waves led to radio and television, and to a huge electronic industry. But they are also generated in space--by unstable electron beams in the magnetosphere, as well as at the Sun and in the far-away universe, telling us about energetic particles in distant space, or else teasing us with unresolved mysteries. You can find more about this in the section on high energy particles. |
An electric current generates a magnetic field, and unpaired electrons spinning
on atoms act as small electro-magnets pointing in particular directions ie they
have a "north" and a "south" pole. When they point in the
same direction on all the atoms, the material itself acts like a magnet; it is
called a ¶ferromagnet,
since the simplest example is BCC iron. Of course a lump of iron is not normally
a magnet, but has to be "magnetised" by some other magnet. This is
because the raw material consists of many magnetic crystallites whose magnetic
moments cancel each other until they are aligned.
If the magnetic moments or "spins" on the atoms are in opposite
directions on the atomic scale, they also cancel, and the material is called an ¶anti-ferromagnet.
Manganese flouride (MnF2) is a simple example. The moments on the Mn atoms at
the corners of the cube point in one direction, and at the centre of the cube
they point in the opposite direction. Since there are equal numbers of each
(when many of these unit cells are stacked together), they cancel exactly.
The most famous anti-ferromagnetic, ¶manganese
oxide (MnO) helped earn the Nobel prize for C. Shull, who showed how
such magnetic structures could be obtained by neutron diffraction (but not with
the more common X-ray diffraction). This material also has the simple BCC ¶rock
salt structure, but here the basic unit is doubled in all three
directions; the Mn moments in one plane point in one direction, and in the
opposite direction in the adjacent plane.
¶Magnetite
or "loadstone" has been known as a magnet from antiquity. It is one of
the common oxides of iron (Fe3O4) and is also cubic, with iron in two valence
states. The formula might be simplistically written FeO.Fe2O3 with Fe++ as FeO
and Fe+++ as Fe2O3. The Fe+++ occupy the tetrahedral holes, and half the
octahedral holes, with the Fe++ occupying the other half. (The charge-ordering
of Fe++ and Fe+++ at low temperature (110K) produces the famous Verwey
transition). The magnetic moments on the octahedral sites are antiferro-magnetic
and cancel (not shown), while on the tetrahedral sites they are ferro-magnetically
aligned. Such a mixture of anti- and ferro-magnets is called a ferr-i-magnet.
Many magnetic structures are much more complex. Neutron diffraction, and
especially the Rietveld method for powder diffraction, has been used to solve
these more complex magnetic structures, such as that of ¶MnTa4S8.
Single crystal techniques using polarized neutrons and strong magnetic fields
are needed however, to understand the most complex magnetic structures.
The discovery of new types of magnets has had great industrial importance - try
counting how many small electric motors are used in a modern automobile - most
made from synthetic magnets. Or consider the importance of magnets in
communications and other electronic equipment. The so-called ¶hard
magnets, whose structure was again found using neutron diffraction, are
examples of these important new materials. This material (Nd2Fe14B) consists of
layers of iron (orange) with interleaved neodinium (purple) and boron (blue):
neutrons show that hydrogen (white) can also be accommodated
Some of the most exiting recent results are being obtained with Giant
Magneto-Resistive (GMR) oxides such as ¶(La,Ca)MnO3.
Already such materials are being used by IBM to make computer hard drives of
much higher capacity. This GMR material has a familiar perovskite-type
structure, which is subtly distorted with temperature. The complex magnetic
structure is not shown, but this, together with the structural distortions, are
important for understanding the unique properties of these materials. The
details of the valence and spin ordering in this material is the ILL Grenoble's
most most cited
current work.
More unusual structures can be formed when the magnetic moments are aligned at
different angles to each other; for example ¶Er6Mn23
has a particularly interesting magnetic structure with moments on both types of
atom. Even more complex structures are produced when the moments form a spiral
structure extending over many unit cells. Such difficult structures can only be
obtained with neutron diffraction from single crystals; they help us understand
better the subtle balance of forces in these materials.
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