Chapter 30
Magnetism
Magnetism, like electricity, has been known since ancient times. The wordmagnetderives from the Greek
̨
& o&, or “Magnesian stone”; Magnesia was a region of ancient Greece where one could find
lodestone, a naturally occurring permanent magnet.^1 Ancient mariners were able to construct primitive mag-
netic compasses by placing these lodestones on cork and floating them in water. You’re undoubtedly familiar
with magnets yourself, from having seen modern compasses and manufactured permanent magnets.
From the perspective of physics, the phenomena of electricity and magnetism are very closely related, and
are described by a single theory ofelectromagnetism. Classical electromagnetism, which we’ll study in this
course, has at its heart four coupled equations calledMaxwell’s equations, named for the Scottish physicist
James Clerk Maxwell. (The more modern theory, calledquantum electrodynamics, requires mathematics that
is beyond the scope of this course.)
We’ll begin by examining both the similarities and differences between electricity and magnetism.
30.1 Magnetic Poles
Just as electricity consists of two kinds of electric charge, magnetism consists of two kinds ofmagnetic pole.
But while the electric charges are calledCand, the magnetic poles are called (for historical reasons)N
andS. The two kinds of magnetic pole behave similarly to electric charges: like poles (twoNpoles or two
Spoles) will repel each other, but unlike poles (anNand anSpole) will attract each other.
The strength of a magnetic pole (analogous to chargeq) is called thepole strengthq. Pole strength in SI
units is measured in units of ampere-meters (A m).
If two magnetic polesq 1 andq 2 are separated by a distancer, then the forceFbetween the two poles is
given by a magnetic counterpart of Coulomb’s law:
FD
0
4
q 1 q 2
r^2; (30.1)
where 0 is called thepermeability of free space,^2 and is equal to exactly4 10 ^7 N/A^2. (Mathematically,
in Eq. (30.1), we write anNpole as a positiveq, and anSpole as negative.)
Although electricity and magnetism are similar in many ways, there is one important difference: while
individual electric charges can occur in isolation,magnetic poles only occur in pairs. In other words, we
neversee an isolatedNpole orSpole by itself: whenever we have anNpole, there will always be anSpole
(^1) Recent research suggests that lodestone is created when the mineralmagnetiteis struck by a bolt of lightning. See P. Wasilewski
and G. Kletetschka, “Lodestone: Nature’s only permanent magnet — What it is and how it gets charged”;Geophys. Res. Lett., 26 , 15,
2275-78 (1999).
(^2) 0 is pronounced “mu-nought.”