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Re[2]: Earth's Magnetic Field



=On Tue, 11 May 1999 10:05:42 -0600 Philip Zell <Zell@ACT.ORG> writes:
=> I think it is a mistake to mark magnets' poles based on their
=use >in compasses. . Mark "north" where the lines of magnetic force
=exit the bar magnet, because this is the convention used in physics.

`BUT.... how can we tell if the lines of force are entering or exiting
`from
`N and S poles of a magnet ??? This reminds me of the old question,
`How can you tell whether a kitten is male or female without turning
`it over?"
`The classic answer is "Give it a warm saucer of milk and watch the
`kitten drink it. If HE drinks it, it's a MALE. If SHE drinks it, it's
`a FEMALE.

`Herb Gottlieb from New York City
`(Where the lines of magnetic force seem to connect the poles of
`a a bar magnet but are never seen entering or exiting the poles)

Your point is well taken, Herb. I've never done this, but how
about deflecting a beam of electrons in a cathode ray tube? The
direction in which the beam is deflected will distinguish N from
S. So which is which? Start with F = q v X B, v and B vectors
and X representing the cross product; in our thought experiment,
let's assume the directions of v and B are well defined; q is
negative. So, pretend a pencil-shaped beam of electrons is
moving along a line from your eyes to your screen; this line is
perpendicular to the screen. In the plane of the screen,
perpendicular to the beam's line of flight, is a horizontal line;
a bar magnet lies along this horizontal line, to your right of
the beam. The bar magnet's N pole is next to the tube, so that
the B lines of force point from right to left across the beam.
Using the right hand rule, if q is negative, the beam is
deflected downward (toward the bottom of the screen). If the S
pole is next the tube, the beam is deflected upward.

Philip Zell
zell@act.org