Ask A Scientist Physics Archive

name         David F
status       educator
grade        other
location     MI

Question -   In a demonstration using liquid water, H2O, a polar
molecule in separate a buret to produce a thin stream of flowing
liquid.  A charged rod is brought near to the water stream and the stream
bends toward the charged rod. Since the water has a positive and negative
end like a magnet, why does the water NOT move away from the charged rod?
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Bipolar forces like electric and magnetic fields can be arranged to 
attract or repel.

If the charged rod is positive, the stream of water will be passively 
polarized:
the mobile charges in the water which chose to move towards the charged rod
will be of whichever polarity which is attracted to positive charge, i.e., 
negative charges.
Like charges (positive) will be repelled to "the other end", probably up 
in the tank or tubes supplying the water.

If the rod is charged negative, the water will be polarized in the 
opposite direction:
positive charge near the negative rod.

Either way, the part of the water near the rod is attracted to the rod.
The part farther away is repelled, but then, it is farther away so the 
force is weaker.
Net attraction results in solid objects.
The water is not even a rigid object, so the part nearby simply responds 
to local attraction.

The water's passive polarization is "induced" by the electric field from 
the charged rod.
The water is not like a magnet with its own intrinsic polarization.
It is more like a piece of iron, "passively magnetic", in which magnetic 
polarization
is temporarily induced by the magnetic force from a nearby permanent magnet.
The induced poles are why the iron is attracted to the magnet.
Induced polarization is always formed in the right direction
to be attracted to, rather than repelled from, the magnet.

Likewise the induced polarization of the water
puts mobile charges of opposite sign nearest to the external charge,
and attraction naturally results.

A permanent magnet or a charged rod both contain trapped, stored energy
which keeps the magnetic or electric field pushing out into the world 
around it.
Anything which is passively polarized contains much less or no such energy,
it is merely "bending" in response to the active field,
trying to help relieve the force, spread out the field, reduce the total 
energy.

If you  add up the energy of the field (proportional to the square of the 
magnetic or electric field)
in all the space in and around your experiment,
you would find that adding a nearby passively polarizable object reduces 
the total energy
as does allowing any oppositely polarized object to move closer in 
response to attraction.

Both formation of induced polarization and
movement of the attracted object towards the actively polarized object
tend to reduce the physical energy of the situation.
I think this is about half of a deep logical reason
why induced, passive polarization is always attractive rather than always 
repulsive,
but I don't have handy the articulate way to express it.

Jim Swenson

PS- think about the fact that the attraction goes as the square of the 
charge on the rod, for a given distance.
The force is positive whether the charge is positive or negative, but
there is no discontinuity in slope as the charge goes through zero.

PPS- have you tried making a dripping-water electrostatic 
generator?  (Kelvin electrostatic Generator)
It is not very difficult.   You can pretty much make it from soda cans and 
thin water tubes.
It uses the induced polarization in two drop-wise water-streams:

        ___________________WATER-TANK 2 Liter (grounding optional)
       /             /
       |             |       thin plastic tubes
(-)|  | |       |  | |(+)
    |  o+ |\     /|     |    open metal tubes ~ 3 inch Diameter
    |     | \   / |  o- |
             \ /             water drops 2-5/sec
       o+     /      o-
             / \             criss-cross wires (not touching each other)
(+)|\   /| /   \ |\ o-/|(-)
    | \-/ |/     \| \-/ |    metal tube with
    |  o  |       |     |       leaky metal funnel (screen is good)
                     o
   |_____________________|   catch tray  (grounding optional)

This can charge itself up until the falling drops
swerve sideways to almost miss the bottom cans.

Jim Swenson
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David F,
Water molecules are polarized, one side is positive and one side is
negative.  This is because of the angled shape of the molecule.  Because
water is a liquid, individual molecules are free to rotate.  When you bring
the charged rod near the water, the attractive side of a molecule turns
toward the rod.  The repulsive side turns away.  Now, the closer side is
attractive.  Since electric force is stronger when charges are closer
together, there is a little more attractive force than repulsive force.
This happens with many of the molecules.  The net effect is attraction.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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