I have just taken a physics test in my high school in which one question posed that a positively charged rod is held near two uncharged spheres being held by silk filaments. Sphere 1 is held by a dry silk filament while Sphere 2 is held with a wet silk filament.

edit: the ceiling is made of steel so the filament would be conducting the negative charge to the ceiling.

The second filament eventually dries. One part of a question was essentially asking if the second sphere was charged negative, positive, or neutral after the filament dried.

I reasoned that the second sphere would be positively charged because the filament would conduct the electrons from the sphere and evaporate, carrying the charge with it. This would leave the sphere with less electrons? Many people said the rod would induct electrons from the sphere because it was a conductor. But the sphere itself was not wet so how could it be a conductor?

  • $\begingroup$ Are the spheres insulating or conducting? Are the spheres hanging from the ceiling on the threads? What is the material of the ceiling? Note that pure water is not a good conductor, but water with impurities does conduct. Which type of water do we have here? In other words: you need to add a lot of clarification to your question. It's way too vague as it is. $\endgroup$ – garyp Feb 19 '15 at 15:17
  • $\begingroup$ "But the sphere itself was not wet so how could it be a conductor?" It could be made out of copper, for example. $\endgroup$ – The Photon Feb 19 '15 at 17:20

I'm sure the intention of this question was to test your understanding of charging by induction. The assumption of the question is that the wet filament would be a conductor and the dry filament an insulator. The positively charged rod attracts negatively charged electrons. Electrons will move through the wet filament onto the sphere. When the filament dries, these electrons are 'trapped' on the sphere, hence the sphere will then have a negative charge.


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