Re: There's work, and then there's work

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 20:10 -0800 1/26/03, David Rutherford wrote:
> How can you say that the negative charges move, but the positive charges
> don't? What makes the negative (or positive) charges special? If the
> positive charges don't move, doesn't this _experimental result_ sort of
> disprove one of the most basic laws of physics, Coulomb's Law?
I would say that it is time that you studied a good introductory
modern physics text, which has answers to many of these questions.

How we can say that one type of charge or the other moves can be
established in a couple of ways, most commonly by the Hall effect. I
will leave you to investigate the Hall effect on your own. Suffice to
say that it is capable of identifying the dominant charge carrier,
which in ordinary conductors is most often the electrons--typically
only one or two per atom, which due to the structure of the
conducting material are allowed to move relatively freely. The rest
of the electrons, and the positive charges are too tightly bound to
be able to move except under extreme circumstances. In
semi-conductors, the charge carriers can be either positive or
negative (sometimes both), and which is which is determined by the
chemical composition of the material. In liquid solutions and in
particle beams and plasmas, the situation is still more complicated,
with charges of both signs often moving relatively freely.

And no, having fixed charges does not disprove Coulomb's law, which
is really all about static charges in the first place. It is a rule
which allows us to calculate forces between charges that are fixed in
position, or are not moving very quickly. Electrons have so little
mass that it is not difficult to get them to speeds where calculating
the forces on them must take into account the time delays between
when a charge arrives at a point and its effects are felt at more or
less distant points. This is the stuff of the theory of relativity.

These things aren't all that hard, but they cannot be decided by just
thinking about them, no matter how well we can think. The proof is in
the laboratory.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

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