Historically, the iron oremagnetite, orlodestone, which is found in various parts of the world, especially in Magnesia, Greece, was known over 2000 years ago to swing around if suspended on a string.
By the thirteenth century it was realized that a sphere of lodestone floating in water (perhaps on a piece of wood) acted as a compass. Peter de Maricourt (exact dates unknown), a French scholar and experimenter, also called “Peter the Pilgrim,” wrote to a friend:
“. . .you’d be able to direct your steps to cities and islands,and to any place whatever in the
world. . .”
He also noticed, around 1270, that a steel needle placed anywhere on the surface of a lodestone would align itself in a unique direction, pointing to a “pole” at the top of the lodestone or from another “pole” at the bottom. He called the ends of some of his needles “north-seeking” (N) and “south-seeking” (S).
By making quite a few “lodestone spheres” Peter the Pilgrim noticed that like poles repelandunlike poles attract. Now just as a charged comb can induce charges on scraps of paper and attract them, as we saw in Fig.1.12, so a chunk of lodestone, or merely a kitchen magnet, can magnetize certain materials nearby, as in Figs.3.1and3.2.
We mentioned Dr Gilbert, physician to the Queen of England, at the beginning of the book (p. xi). Around the year 1600, shortly before he died, he suggested that the earth itself was like a large spherical lodestone, and he drew sketches of what he described as “lines of magnetic virtue” around a magnet. If you sprinkle iron filings The online version of this article (doi:10.1007/978-3-319-05305-9_3) contains supplementary material. This video is also available to watch onhttp://www.springerimages.com/videos/978-3- 319-05304-2. Please search for the video by the article title. The supplementary audio material can also be downloaded fromhttp://extras.springer.com.
D. Nightingale and C. Spencer,A Kitchen Course in Electricity and Magnetism,
DOI 10.1007/978-3-319-05305-9_3,#Springer International Publishing Switzerland 2015 77
(just take a nail and file it down) onto a piece of paper which is covering a magnet you will see his lines of virtue. Such lines of magnetic virtue represent the directions of what we now call the magneticfieldor what physicists callB. They show the directions that a free N pole, if there were such a thing, would move along—analogous to theEfield discussed on p. 18.
Note that theBfield lines are continuous, looping round for ever and passing through the interior of the magnet. This is different from the electric dipole. If we had drawn the electric field (E) for a dipole (which we leave as an exercise) we would have found the same shaped external field as the above. However, E fields begin and end on (+) and () charges, whereas B field lines are seen to be continuous loops, running through the magnet itself—they have no beginning or end.
This fundamental difference between magnetism and electricity implies that there is no such thing as an isolated North or South pole. In Fig.3.3itlooks as if the magnet has poles, but if you were to cut one of the poles off you would not find an isolated magnetic pole or a “monopole,” but rather two magnets and thus four poles, as in Fig.3.4.
Fig. 3.1 Three screws (initially unmagnetized) hang from a regular refrigerator magnet
Kitchen magnet
N
N
N S
S S
S N Fig. 3.2 The permanent
kitchen magnet picking up screws. We don’t yet know its
“polarity,”i.e., which side is N or S, but have labeled things arbitrarily for simplicity. We find that the screws behave in a similar way to the scraps of paper in Fig.1.12
This mystified people for a long time. Worse, in the twentieth century a highly respected theoretical physicist, Paul Dirac (1902–1984), a 1933 Nobel Prize winner, suggested, for rather erudite reasons, that IF there were a small number of monopoles in the universe, this would be of theoretical interest. We will not go into this, but we note that monopoles have never been observed.
Meanwhile, the well-established equations of classical electricity and magnetism, especially the four equations given by the famous Scottish physicistJames Clerk Maxwell, given in the Appendix (p. 170), have never been found to be wrong. We will definitely not need his equations, but our book does illustrate their significance in various places, with the appropriate reference pages given in that Appendix.
The subject of electricity in general owes a great deal to Maxwell. His family commented about their 3-year-old boy in a letter this way: “he is very happy, and has great work with doors, locks, keys etc and ‘show me how it doos’ is never out of his mouth.” At school he was shy and made no friends, but at about 14 he suddenly began gaining prizes for mathematics as well as for verse. He graduated from Cambridge at 23, and although he never reached 50 his admirable mind enriched many branches of physics, including optics and thermodynamics.
Our impractical kitchen gedanken idea in Fig.3.4(gedankenis the German word we met in Chap.2to describe a thought experiment) illustrates that a magnetic pole cannot be isolated. However hard we try to cut a pole off, we won’t succeed!
S N
N
S Fig. 3.3 Sprinkling iron
filings on a piece of paper (the magnet would be under the paper) or plotting around with a little compass will show the directions of the magnetic fieldB, which by convention is away from the N and towards the S. (This is analogous to the convention in electrostatics, where the direction ofEis away from the (+) and towards the ().) Also, where lines are bunched the field is strong
N S NS N S
Fig. 3.4 Failing to isolate cut a single “magnetic pole.”
All we get is another magnet
3.1 Lodestones 79
3.1.1 The North
The earth’s magnetic field is not aligned precisely with the earth’s unshakeable axis of spin, and it is the latter that defines earth’s actual poles. The difference is shown (exaggerated) in Fig.3.5. The location of themagneticnorth according to a regular household compass varies slowly over time and is currently near Canada’s Hudson Bay.
Similarly, our ordinary compass will point to the south as being somewhere in Antarctica, a little south of Tasmania.
It is thought by many researchers that the earth’s field has flipped, north for south, possibly hundreds of times—perhaps as recently as 30 000 years ago.
Furthermore, since the interior of the earth is too hot for iron and other magnetic materials to remain magnetic—heat destroys magnetism—it’s clear that we are dealing with something other than a bar magnet inside the earth!
One possible explanation (among many) for the earth’s magnetism is that the molten iron is ionized, and the moving ions cause theBfield.