next up previous contents
Next: Waves vs. particles Up: Electricity and magnetism Previous: Electricity

Magnetism

The earliest observations on magnets can also be traced back to the early Greeks (eg. Thales of Miletus; see Sect. 2.3.2). The Chinese literature also has extensive references to naturally occurring magnets (then called loadstones). The fact that magnets align in a unique way, together with the fact that the Earth itself is a magnet, lead to the discovery of the compass. This was of paramount importance to the development of civilization. The earliest known compass appeared in China by the first century A.D.; it arrived in Europe by the twelfth century A.D.




\begin{figure} \framebox [6 in][r]{\parbox[r]{5.5 in}{\scriptsize \bigskip{\em ...  ...ein{\epsfysize=2.5 truein\epsfbox[0 0 612 792]{5.eandm/bio_f14.ps}}}\end{figure}



According to thirteenth-century philosophy, the compass needle points towards the North star which, unlike all other stars, in the night sky, appears to be fixed. Thus, philosophers reasoned that the lodestone obtained its ``virtue'' from this star. Better observations, however, showed that the needle does not point exactly to the North Star and eventually it was shown that it is the Earth that affects the compass. Apart from the roundness of the Earth, magnetism was the first property to be attributed to the body of the Earth as a whole:

Magnus magnes ipse est globus terrestris [the whole Earth is a magnet]. William Gilbert

By the early 17th century the properties of magnets were well known and many folk tales (such as the anti-magnetic properties of garlic) had been debunked. Magnetism was believed to be an effect different from electricity, their intimate relationship had not been discovered.

Careful experimentation with magnets came to a head in the late 19th century. By then reliable batteries had been developed and the electric current was recognized as a stream of charged particles. In 1870 Ørsted noted that a compass needle placed near a wire was deflected when a current was turned on, that such a deflection also occurs when the wire is moved, and he concluded that moving charges generate magnetic effects. These results were furthered by Ampère and who rendered them into a precise mathematical formulation.



\begin{figure} \framebox [6 in][r]{\parbox[r]{5.5 in}{\scriptsize \bigskip{\em ...  ...in{\epsfysize=5 truein\epsfbox[0 -230 612 562]{5.eandm/bio_f15.ps}}}\end{figure}



\begin{figure} \framebox [6 in][r]{\parbox[r]{5.5 in}{\scriptsize \bigskip{\em ...  ...in{\epsfysize=4 truein\epsfbox[0 -180 612 612]{5.eandm/bio_f16.ps}}}\end{figure}


During the same period Faraday made various experiments with moving magnets (as opposed to moving wires). He found that a magnet moving in a coil of wire generates a current: moving magnets generate currents. This result provides the principle behind electric generators, be it small household ones, or the giant ones found in Hoover Dam. The fact that charges in motion create magnets and that moving magnets generate currents demontrates the intimate connection between electric and magnetic phenomena.



\begin{figure} \framebox [6 in][r]{\parbox[r]{5.5 in}{\scriptsize \bigskip{\em ...  ...in{\epsfysize=5 truein\epsfbox[0 -230 612 562]{5.eandm/bio_f17.ps}}}\end{figure}



\begin{figure} % latex2html id marker 2445 \framebox [6 in][r]{\parbox[r]{5.5 i...  ...in{\epsfysize=4 truein\epsfbox[0 -180 612 612]{5.eandm/bio_f18.ps}}}\end{figure}


Faraday also showed that charge is conserved. That is, the amount of positive charge minus that of negative charge is always the same.

The results of all these investigations can be summarized in a series of four equations. These were studied extensively by Maxwell who noted that they are inconsistent with charge conservation, but Maxwell himself realized that a slight modification in one equation would get rid of this problem. The modification proposed by Maxwell is simple, but the results are so momentous that the modified set of four equations are known as Maxwell's equations. Why are Maxwell's equations so important? There are four reasons:

The last point leads to the inescapable conclusion is that light is precisely the object that was described by the wave-like solution of Maxwell's equations (without his modification there are no wave-like solutions); in Maxwell's own words

We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.

It is in this way that the next unification in physics occurred: light, electricity and magnetism are different aspects of the same set of phenomena and are described by a single theory. Because of this we now speak of electromagnetism and not of electric and magnetic phenomena separately.


next up previous contents
Next: Waves vs. particles Up: Electricity and magnetism Previous: Electricity

Jose Wudka
9/24/1998