re: attracting water

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From: bliss (swise@lwhs.org)
Date: Thu Nov 18 1999 - 21:55:26 PST

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Message-Id: <199911190443.UAA31857@noontide.lwhs.org>
Date: Thu, 18 Nov 1999 21:55:26 -0800
From: swise@lwhs.org (bliss)
Subject: re: attracting water

I'm not sure if this explanation has been thrown into the soup already, but
perhaps my version will clarify the induction explanation for why water is
attracted to a charged object.

In a basic chem text you will find that water naturally undergoes
autoionization, which is a fancy way to say that it breaks into hydrogen
(H+) and hydroxide (OH-) ions. Furthermore, tap water is not pure; it
contains many other dissolved ionic substances. That's why we have to
distill water in order to use it for scientific work.

It would seem to me that with ions present naturally in water, there is no
reason why a negative or positive rod placed near a stream would not cause
an induced charge in the water due to the movement of the ions.

For example, if a negative rod is placed near the stream, the hydroxide
(OH-) and other negatively charged ions will be repelled to the opposite
edge of the stream. The positively charged ions such as hydrogen will be
attracted to the edge closest to the rod. Because electrical force varies
with the square of distance, there is more attractive force between the
positive ions and the negative rod than there is repulsive force between
the negative ions and the negative rod. Consequently, the stream of water,
under such a force, is accelerated toward the rod until it reaches a stable
equilibrium.

The concentration of ions in water is small (1x10-7 for neutral water),
which might lead one to think that the force on the water would also be
small. However, remembering Coulomb's law, the constant K by which small
charges are multiplied is very large (8.99x10+9), which can yield
substantial force at small distances.

And to ask a favor (if you have read this far) -- I've sent a few questions
out to pinhole recently on behalf of my students (about styrofoam, mu
mesons, black light). These are questions we are truly stumped on, and
students are excited to possibly hear from you all. Any help you can have
in answering these questions will directly affect student learning (and
feelings of connection with science). Thank you.

Sarah Wise

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Sarah Wise

Lick-Wilmerding H.S.
755 Ocean Avenue
San Francisco, CA 94112
swise@lick.pvt.k12.ca.us

"Not everything that can be counted counts,
and not everything that counts can be counted."
--Albert Einstein

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