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Name: Jennifer G.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 2001-2002


Question:
Dear Scientists:

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.


Replies:
Jennifer,

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 generator.

Dr. Ken Mellendorf
Physics Instructor
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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