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Name: Rachel L.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002


Question:
We are studying about the adhesion of water molecules and how the molecules have a tendency to "stick" to the sides of a cylinder because of charges. My conjecture was that if a beaker was partially filled with water and then a current was run through the beaker, the water would be more likely to be attracted to the sides of the beaker. Am I right in thinking this?


Replies:
Rachel, I do not expect the current through the beaker to attract the water. Running current through a beaker does not add charge to the beaker. Current only causes charge within an object to move. For every electron a current adds to a beaker, another electron comes out the other end.

What just might cause something to happen is touching the beaker just above the water surface with a charged object, something with static electricity on it. Some of the static electricity transfers to the glass, but cannot go anywhere else. Sometimes a piece of glass rubbed with silk or a piece of hard plastic rubbed with fur will work. Touching a piece of plastic to a TV screen and then transferring the charge to the glass might work to.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Rachel,

Even though a water molecule is polar because the its charges are not symmetrically distributed, water itself is electrically neutral -- it has no net charge either (+) or (-). Check out the structure of a water molecule in an introductory chemistry book. You will find that it is rather like a four cornered pyramid with the oxygen atom in the center and with the two hydrogen atoms sticking out toward two of the corners of the pyramid. The oxygen atom carries two unshared pairs of electrons that stick out toward the other two corners of the pyramid. The result is a kind of electrical lopsidedness wherein the hydrogen atoms form a partially (+) side of the molecule and the unshared electron pairs make up the (-) side. So, even though water has a (+) and (-) side, the charge on one side is equal to the charge on the other -- thus, a net charge of zero.

Using your illustration, if it were possible to make one side of the beaker (+) and the other side (-), the water molecules in between would rotate so as to point the (+) end of their dipole toward the (-) side of the beaker and the (-) end of their dipole toward the (+) side of the beaker. Even so, there would be no net movement of all the water molecules toward either side because, for that to happen, the water molecule itself would have to have a charge -- either (+) or (-). In other words, IF the molecule had a net (+) charge, it would move toward the (-) side of the beaker and vice versa. Won't happen because, as described, the molecule has no net charge.

Reorientation of a neutral polar molecule an applied electrical field, is not the same thing as net movement toward either side of the field.

Regards,
ProfHoff 473



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