Charge and Humidity
Name: Jennifer G.
My question involves explaining why static charge does not build up on
humid days. I have always taught that the positive side of a polar water
molecule will be attracted to the electrons on the Van deGraaff, pick up
an electron, then be repelled since it is negative. My chemistry
background is weak but my coworker's is not and she drew an electron dot
diagram to try to figure out where the electron actually would go on the
water molecule, and it didn't seem as though there was a place that made
sense, unless it had enough energy to move to a higher orbital. Anything
you can contribute to this discussion would be appreciated.
The electron does not have to become part of a water molecule for humidity
to help electrons leave the generator. The polarized molecules are
attracted to the charged globe, but water droplets are good conductors.
Extra electrons can travel through the droplets without joining any specific
molecule.Passing into and through the water droplet allows the charge to
leave the generator. It can continue to jump from droplet to droplet,
finally reaching the positive end of the generator. The "difficulty"
involved in making the trip depends almost exclusively on the paths between
the drops. If 50% of the distance is spent inside water, the charge
build-up is no more than what you would get on a dry day with the ground
wand at half the distance. If 90% of the trip is in water, it's as if the
distance traveled is only 10%. much less energy is lost in the path on a
wet day, so much less electric field is needed to push extra charges off the
Dr. Ken Mellendorf
Illinois Central College
The charge built-up on an object remains if the object cannot discharge
or reduce its charge by interacting with other media of opposite or lower
charge. In humid -as opposed to dry - air, water molecules provide a medium
of higher electrical conductivity for the discharge to take place.
Electrons move relatively freely in the water droplets as they do in a pool
of water. The electrons will not go "on the water molecules." They just
wonder about in between molecules.
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
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Update: June 2012