Magnetic Properties of Matter
What causes magnetism?
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Magnetism is a fundamental property of nature. Because there is no "Unified Theory of Physics", our knowledge of magnetism, like everything else, is incomplete. Angular momenta of electrons (and to a lesser extent, nuclei) impart some magnetic characteristics to all materials. Magnetism extends as a field far beyond the atomic level, however, and is intimately connected with electrical phenomena. Any current, moving charge, or changing electrical potential also generates a magnetic field.
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Magnetic flux lines radiating from a bar magnet.
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A piece of lodestone attracting bits of iron.
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To the ancients, magnetism was a recognized but mysterious force. Greeks living in Magnesia occasionally found rare and wonderful brownish-black rocks that had the power to attract objects made of iron. Known as lodestones, these were chunks of iron oxide (magnetite) that had probably been struck by lightning. Similar stones were discovered in Asia and by the 12th Century the Chinese used them to make compasses for navigation. Today even small children are familiar with the attraction and repulsion of hand-held permanent magnets, the modern successors to lodestones.
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Today we understand that the static magnetic fields associated with lodestones and permanent magnets derive principally from the total angular momentum of electrons within those materials. Lone electrons possess spin, a quantized fundamental property of nature denoted by the letter S. In addition to S, electrons orbiting a nucleus also possess orbital angular momentum (L). Together S + L = J, or total angular momentum, is the property primarily responsible for bulk magnetism. Nuclei and other subatomic particles also possess spin angular momentum, but this effect is too weak to affect gross magnetic properties of a material.
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The relationship between electricity and magnetism was not appreciated until 1819 when Danish physicist Hans Christian Oersted accidentally noticed the deflection of a compass on a nearby table while performing experiments with electrical currents in his laboratory. In 1826 André-Marie Ampère formally demonstrated the relationship between current and strength of the resultant magnetic field (B), whose direction is found by the right-hand rule.
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Ampère's Law and Fleming's Right Hand Rule. With the thumb pointing in the direction of current flow, one's fingers curl in the direction of B.
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The Faraday-Lenz principle of magnetic induction. A voltage (V) is generated in the coil proportional to the rate of change of the magnetic field (dB/dt). The induced current opposes the applied field.
Maxwell Equations. All results in classical electromagnetism can be derived from these four equations together with the Lorentz force law!
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Not only do electrical currents produce magnetic fields, but changing magnetic fields induce electrical currents. In 1831 Michael Faraday demonstrated the principle of magnetic induction by measuring the voltage (V) produced in a coil by a moving magnet. Three years later Heinrich Lenz showed that the current induced was so directed as to oppose the change in magnetic flux. The resultant Faraday-Lenz Law can be written
V ∝ −(dB/dt)
where dB/dt represents the rate change of the magnetic field. The negative sign reflects Lenz's principle that the induced current creates a "counter field" in a direction opposite to B.
In the last half of the 19th Century a number of European physicists worked out further details of electromagnetic phenomena. Their names are legendary -- Carl Friedrich Gauss, Hendrik Antoon Lorentz, Joseph Henry, Heinrich Hertz, among others. James Clerk Maxwell made perhaps the most important contribution unifying magnetism, electricity, and light under a common wave-based electromagnetic theory.
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In the 20th Century Albert Einstein successfully recast the Maxwell equations into a relativistic framework. Paul Dirac, Enrico Fermi, Richard Feynman and others subsequently integrated magnetism into quantum mechanics creating the new field of Quantum Electrodynamics (QED). In the 21st Century the boundaries of theory are being expanded to explain magnetic behavior on both very large (galaxy-size) and very small (sub-sub-atomic) levels. Still, no single model exists that explains all electromagnetic phenomena.
References
Mourino MR. From Thales to Lauterbur, or from the lodestone to MR imaging: magnetism and medicine. Radiology 1991;180:593-612. (A fascinating historical account taking the reader from ancient times to the early 1970's).
"Magnetism." Wikipedia, The Free Encyclopedia. (It's Wikipedia!)
"Magnetism." Encyclopaedia Britannica Online. (Highly recommended free chapter with many illustrations, advanced high school level review, better than Wikipedia)
Summary equations and definitions for classical electromagnetism (pdf)
Mourino MR. From Thales to Lauterbur, or from the lodestone to MR imaging: magnetism and medicine. Radiology 1991;180:593-612. (A fascinating historical account taking the reader from ancient times to the early 1970's).
"Magnetism." Wikipedia, The Free Encyclopedia. (It's Wikipedia!)
"Magnetism." Encyclopaedia Britannica Online. (Highly recommended free chapter with many illustrations, advanced high school level review, better than Wikipedia)
Summary equations and definitions for classical electromagnetism (pdf)
Related Questions
We have a 3.0 Tesla MR scanner at our hospital. I know this is a very strong magnet, but what exactly is a tesla?
We have a 3.0 Tesla MR scanner at our hospital. I know this is a very strong magnet, but what exactly is a tesla?