Outline of the story told historically. Basic ideas that even young children now know. Keep everything to an introductory level.
First comes Thales of Miletus a.k.a. Θαλής ο Μιλήσιος (635-543 BCE) Greece (Ionia). Miletus is now on the western coast of Turkey in what was then a region of Greece known as Ionia (source of the chemical term ion, but that's another story).
A nice quote from Thales would be nice here.
Some minerals such as magnetite (Fe3O4) are obviously magnetic.
Chinese navigators knew that magnetic rocks align themselves north-south (the south-pointing spoon).
The compass in real sense was created by a Chinese geomantic omen master in late Tang Dynasty, who originally used it for divination.
Find something historical.
The north magnetic pole of a compass points in the general direction of the north geographic pole of the earth. Since opposite magnetic poles attract, this means that the south magnetic pole of the earth is very near to its north geographic pole.
Next comes Peter Peregrinus (as he is known in English) a.k.a. Pierre Pèlerin de Maricourt (presumably his proper french name) a.k.a. Petrus Peregrinus de Maharncuria (his latin title, which means "Peter the Pilgrim of Maricourt"). Peter wrote what is commonly known as the Epistole de Magnete or Letter on the Magnet. It's full title is Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt, militem, de magnete (Letter on the Magnet of Peter Peregrinus of Maricourt to Sygerus of Foucaucourt, Soldier"). It was written on 8 August 1269 during the siege of the city of Lucera — the last remaining stronghold of Islam on the "calf muscle" of the boot-shaped peninsula that is now called Italy.
Peter's work was so complete that no further studies on the properties of magnets were done until the monumental work of William Gilbert (1544-1603) in 1600 — Tractatus sive Physiologia Nova de Magnete, Magneticisque Corporibus, et Magno Magnete Tellure (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth). De Magnete was the text in which Gilbert revealed the results of his research on magnetism and attempted to explain the nature of magnets and the five motions associated with magnetic phenomena. The work met with great acclaim and was republished in 1628 and 1633.
William Gilbert (1544-1603) England
More on ferromagnetism later
The earth is a magnet, geomagnetism, poles: north seeking and south seeking
rule of action: opposite poles attract, like poles repel
types of magnets
A lifting magnet in action — Brooklyn, New York (Source: ToddCam)
informal definition: compare to the other fields
|gravity||force due to||mass||force per||mass||g|
|electricity||force due to||charge||force per||charge||E|
|magnetism||force due to||poles?*||force per||pole?*||B|
|*||Most emphatically, No! The magnetic field has a very odd definition.|
the real definition appears later
dilemma: breaking a magnet, there is no magnetic monopole, even down on the atomic scale there is no magnetic monopole, field lines heal themselves
Still, even though we don't have a formal definition of the magnetic field there's no reason why an informal definition can't suite us for awhile.
properties of magnetic field lines
The symbol for the magnetic field is B (boldface) when describing the full vector quantity and B (italic) when describing the magnitude alone.
The SI unit of the magnetic field is the tesla [T], named in honor of the Serbian-American electrical engineer Nikola Tesla (1856-1943) born in a part of the Austro-Hungarian or Hapsburg Empire that is now the independent nation of Croatia. Tesla was a pioneer in the associated disciplines of alternating electric current and rotating magnetic fields. His basic designs for electric motors, generators and transformers in the early part of the Twentieth Century were little changed by the beginning of the Twenty first Century.
Another unit in common usage is the gauss [G], named in honor of the German mathematician Karl Friedrich Gauss (1777-1855). Gauss is generally regarded as the greatest mathematician of all time. Of particular interest to physicists were Gauss' work on curved surfaces, which were important in the fields of electrostatics and general relativity (all puns intended). The gauss is also a unit in the cgs system that was originally developed by Gauss and is sometimes also known as the Gaussian system.
Each of these units will be defined formally in a later section of this book. Right now I will tell you that the tesla is the bigger unit.
1 T = 10,000 G
The tesla is in fact too big for most practical purposes. As such it is usually divided into microtesla [μT] or nanotesla [nT]. The gauss is also a bit too large, but not as bad as the tesla, so milligauss [mG] and microgauss [μG] are more common.
|1 T||=||10 kG|
|1 mT||=||10 G|
|1 μT||=||10 mG|
|1 nT||=||10 μG|
The following table lists the magnetic field strength for various devices, events, or phenomena. Whenever possible a location was also specified. Like the gravitational and electric fields, the magnetic field grows smaller with increasing distance form the source.
|B (T)||location, event|
|1013||neutron star, theoretical upper limit|
|1010–1011||neutron star, magnetar|
|108–109||neutron star, radio pulsar|
|1000||highest laboratory field, ephemeral|
|100||white dwarf star|
|45||highest laboratory field, sustained|
|16||strong enough to levitate frogs|
|13||strongest superconding magnet|
|2.4||strongest permanent magnet|
|1||strong laboratory magnet|
|0.15||iron bar magnet, at poles|
|400 × 10−6||jupiter, surface mean|
|100 × 10−6||sun, surface mean|
|60 × 10−6||earth, poles|
|45 × 10−6||earth, surface mean|
|30 × 10−6||earth, equator|
|10 × 10−6||am radio broadcast at receiver|
|1 × 10−6||solar radiation on earth's surface|
|180 × 10−9||100 W light bulb at 1 m, peak|
|150 × 10−9||mercury, surface mean|
|150 × 10−9||earth, altitude of geosynchronous orbit|
|50 × 10−9||earth, magnetosphere nose|
|35 × 10−9||moon, surface|
|5 × 10−9||interplanetary space near earth|
|1 × 10−9||earth, magnetosphere tail|
|500 × 10−12||interstellar space|
|100 × 10−12||intergalactic space|
|50 × 10−12||human heart|
|100 × 10−15||human brain|
This section is intended to be a discussion of magnetism on the small scale, not just the magnetism of iron, nickel, and cobalt (although that will be its primary focus). Perhaps it should be titled "micromagnetism".
Everything's due to electron spin. Well, almost everything.
|ferromagnetic||all spins align parallel to one another||iron, cobalt, nickel, gadolinium, dysprosium, heusler alloys|
|ferrimagnetic||most spins parallel to one another, some spins antiparallel||magnetite (Fe3O4), yttrium iron garnet (YIG)|
|antiferromagnetic||periodic parallel-antiparallel spin distribution||chromium, FeMn, NiO|
|paramagnetic||spins tend to align parallel to an external magnetic field||oxygen, sodium, aluminum, calcium, uranium|
|diamagnetic||spins tend to align antiparallel to an external magnetic field||nitrogen, copper, silver, gold, water, organic compounds|
|superdiamagnetic||all spins align antiparallel to an external field||meisner effect in superconductors|
|ferritic stainless||Fe, Cr, C||yes||moderate corrosion resistance,
|austenitic stainless||Fe, Cr, Ni, C||no||high corrosion resistance,
|martensitic stainless||Fe, Cr, C||yes||moderate corrosion resistance,
|nonstainless (high carbon)||Fe, C||yes||low corrosion resistance,
Alloys made expressly for permanent magnets …
Ferromagnetic alloys made entirely of nonferrous metals …
the basic mechanism
|I||"normal"||gamma ferric oxide
|first commercially manufactured in 1937|
|later replaced by layers of ferric oxide (Fe2O3) and cobalt (Co) with similar magnetic characteristics|
|quickly became obsolete|
|IV||"metal"||finely ground metallic iron||later replaced by mixtures of finely ground iron and cobalt|
|magnetic stripes on bank and credit cards, high coercivity, less susceptible to accidental erasure|
The Curie temperature is named for the French physicist Pierre Curie (1859-1906), who discovered the laws that relate some magnetic properties to change in temperature in 1895.
The antiferromagnetic equivalent of the Curie Temperature is called the Néel Temperature in honor of the French physicist Louis Néel (1904-2000), who successfully explained antiferromagnetism in 1936.
|color tv/computer crt display||500|
|location||median (μT)||range (μT)|
|workplace:||clerical worker w/o computer||0.05||0.02-0.20|
|clerical worker w/computer||0.12||0.05-0.45|
|electrical line worker||0.25||0.05-3.48|
|home:||typical US home||0.09||0.03-0.37|
magnetic resonance imaging (nuclear magnetic resonance)