Magnetism

Discussion

history

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 [animate].

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

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fundamentals

magnetic elements

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)

magnetic field

informal definition: compare to the other fields

A Comparison of Force Fields
phenomena origin field symbol
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.

Selected Magnetic Field Values
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
1–4 MRI
1 strong laboratory magnet
0.45 large sunspot
0.15 iron bar magnet, at poles
0.10 refrigerator magnet
0.001 sun, 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

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Magnetic Field Exposure When Using Various Devices
device B (μT)
color tv/computer crt display 500
electric stove 1000
hair dryer 1000
maglev train 100

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Average Daily Exposure to Magnetic Fields
location median (μT) range (μT)
earth's surface 45 40-60
workplace: clerical worker w/o computer 0.05 0.02-0.20
  clerical worker w/computer 0.12 0.05-0.45
  machinist 0.19 0.06-2.76
  electrical line worker 0.25 0.05-3.48
  electrician 0.54 0.08-3.40
  welder 0.82 0.17-9.60
home: typical US home 0.09 0.03-0.37

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ferromagnetism

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.

alloys

Steel …

Alloys made expressly for permanent magnets …

Ferromagnetic alloys made entirely of nonferrous metals …

magnetic recording

the basic mechanism


[magnify]

media formats

media shapes

substrates

ferromagnetic material

transition temperatures

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.

animal magnetism (magnetotaxis?)

MRI

magnetic resonance imaging (nuclear magnetic resonance)

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