Re: electric charge

["John S. Denker" <jsd@MONMOUTH.COM>]

Larry Cartwright wrote:
>   It has always seemed to me that any day now, any
> minute now, some insightful theoretician is going to come up with a
> bosonic model that corrects our backward thinking and makes "flowing
> charge" seem like the 20th century equivalent of phlogiston and caloric
> and aether.

Relax.  It doesn't take any great insight.  We know the answer.

The recent discussion has gotten slightly tangled up
because there are several seemingly-similar ways of
asking the question that lead to not-actually-similar
answers.

Consider the following notions:
 1) Water is a substance?
 2) Charge is a substance?
 3) Charge is like a substance?
 4) Water obeys a local conservation law?
 5) Charge obeys a local conservation law?
 6) Water flows?
 7) Charge flows?

Analysis:

1) I hope we can agree that water is a substance.  If not, please
stop reading now.  I'm trying to do physics here, not metaphysics.

2,3) Lest I be misquoted (again) let me be very clear:  IMHO
 -- I would not say charge _is_ a substance.
 -- Charge is _like_ a substance in many ways, but not all ways.

There is an _imperfect_ analogy between charge and a substance
such as water.  A water molecule can exist as a water molecule
in isolation.  In contrast, as far as we know, charge
cannot exist separately from some charge-carrying particle.  But
still we can say intelligent things about "charge" itself.  The
charge has certain properties that can be described _without_
saying anything about the charge-carrying particle.

5) In particular, charge obeys a local conservation law.  This
law is quite independent of whether or not the charge-carrying
particles are conserved.  This situation is not at all unusual
in physics.  A sound wave cannot exist independently of the
sound-carrying medium, yet we can say intelligent things about
the sound wave itself, independent of the medium.  For example,
the shape of the soundwave might be preserved across a change
in the medium, as discussed in:
  http://www.monmouth.com/~jsd/physics/reality-reductionism.htm

4) We commonly teach the notion of conservative flow using water
as a familiar example.  But if we look closely, we find that
water is not actually rigorously conserved.  A jug of water,
all by itself, is 0.00001 percent auto-ionized into OH- and H3O+
at STP, so if we look at some ultramicroscopic region, there is
a completely nontrivial chance that an H2O molecule will flow
into the region but some other species will flow out.

6,7) I am taking "flow" (or more precisely, "conservative flow")
to be synonymous to "obeys a local conservation law" so (6,7)
are synonymous with (4,5).
  http://www.monmouth.com/~jsd/physics/conservative-flow.htm

=====================

Suggestion as to terminology:  If you are ever tempted to say
"charge is a substance" it may be better to say "charge is a
conserved quantity".  You don't want to get dragged into a
metaphysical discussion of what a substance is;  people have
fewer preconceptions and misconceptions about what a "quantity"
is.

It is slightly ironic that the abstract quantity, charge, obeys
a stricter conservation law than the familiar quantity, water --
but that the way it is.  We understand charge _better_ than we
understand water!


=====================

As the word implies, metaphysics must come after physics.  It
is generally a waste of time to discuss metaphysics with people
who don't know anything about physics.  It cracks me up when
certain people demand to know whether light is a wave or a
particle;  they don't know what a wave is, and they don't know
what a particle is, so why should they care whether light is
a wave or a particle?

Similarly it cracks me up when people demand to know whether
charge is a substance, without having much idea of what they
mean by "substance".  Why ask the question if the answer
doesn't mean anything?

Charge is an abstraction, but it is no more abstract than
things like mass or energy or momentum or innumerable other
things that are the stock-in-trade of physics.

It is what it is.  It does what it does.  And we know quite
a lot about what charge is and does.  It flows according to
a local conservation law.  It is the source of EM fields,
and is acted upon by EM fields, according to very well-known
laws.