MadSci Network: Physics
Query:

Re: What is a magnetic field and how does it work?

Area: Physics
Posted By: John Dreher, Staff Astronomy, SETI Institute
Date: Mon Mar 24 18:24:55 1997
Message ID: 856226683.Ph


Turns out, you have company.  There are aspects of magnetism that
present some deep problems.  From one point of view, "fields",
whether electric, magnetic, gravitational, or whatever, are
just abstractions.  Over a long period of time, physicists have
observed how various objects interact.  Two particles with charges
on them can exhibit a rather complicated behavior.  The electric
and (for moving particles with charges) magnetic "fields" were invented
purely to simplify the description of how the objects move.  But,
funny thing, as we learned more about these things, particularly after
the invention of relativity theory, it came to seem more and more like
these "fields" described something real. But even to this day, the story
of "fields" is not complete: one of the most active parts of physics
research is Field Theory.  Now that point of view may be a bit much for
your science project, but I wanted you to know that you are smart to
feel a bit puzzled -- they really are mysterious!

When I think of magnetic fields, I think of a mess of invisible
rubber bands running next to each other.  Magnetic fields (at least 
those we can make) always run in circles -- they have no ends.  And they can
be streched like rubber bands too, if you have something, like a conducting
fluid, that can "get a grip" on them.  When you sprinkle iron filings
near a magnet, the little pieces of iron act like little bar magnets
and try to line up with the field lines (rubber bands), so you can sort of
imagine how the lines run even though you can't actually see them. 
If you do this with some paper over a big
bar magnet, you can sort of see how the lines run out one end, loop
around to the other, then continue back through the magnet to make
closed loops.  

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