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Electrostatic Induction / Conduction Question

  1. Feb 13, 2011 #1
    So, when I was learning about static electricity and induction and conduction, there were several labs done where a pith ball or electroscope could be manipulated through induction when a charged object was placed near it.

    In the case of the little pith ball, a negatively charged piece of PVC was placed near it, which attracted the ball as electrons in the ball were moved over and it became positively charged on one side and all that jazz. Then when they touched, the ball became negatively charged as well, and was repelled rather forcefully away from the PVC, because of conduction.

    So, at first I figured that only metals could do things like that, but then there was a lab which involved neutral pieces of paper being charged by induction, jumping to the PVC or whatever, and then being charged by conduction, and leaping back off.

    Now I'm confused, because the electrostatic charge on the TV for example doesn't spit out dust like that, but it does spit out the paper as expected (I tested it).

    Anyways, I guess my question is, does anybody know what determines the electrostatic conductivity of a material?
  2. jcsd
  3. Feb 15, 2011 #2
    I'll just bump this if that's okay.
  4. Feb 16, 2011 #3
    Bumping again, lol.

    Does anybody have any clue about this?
  5. Feb 16, 2011 #4
    All materials have some electric conductivity (ie some charges that are free to move around), which even if small , can be enough to initiate the movement and seperation of charges via the electrostatic induction mechanism and then the attraction.

    When they attracted and finally come in contact then charges move from one material to another, but this is one point that i am not sure, that is if all kind of materials can exchange charges or for some materials this cannot happen even when they are in contact and are charged.

    Also once they come in contact they appear other contact forces like van der waals forces, which i believe is the explanation why the dust stays on TV (due to strong van der waals forces) while paper is being repelled (not so strong contact forces).
  6. Feb 16, 2011 #5
    Hmm, from my limited knowledge of the Van der waals forces, I always thought that they occurred between the same types of unbonded molecules.

    If Van der waals forces are the cause of the dust sticking, though, I assume that they wouldn't in any way be proportional to the electrostatic charge on the TV or the dust, would they?
  7. Feb 16, 2011 #6
    Another thing, that I forgot to mention.

    If you give a rubber balloon an electrostatic charge, and put it to the wall, it'll charge the polar molecules of the wall through induction, but no conduction will occur.

    Now, as I expect and as my high school physics textbook says, this is because the wall and the rubber are insulators.

    But last time I checked, it also said glass was an insulator, and that's what the TV screen is made of! I'm also fairly certain that paper is an insulator too. But, regardless, apparently conduction is occurring.

    What gives?
  8. Feb 17, 2011 #7
    Van der waals forces can be between any type molecules. They can have strength comparable to the electrostatic force, some glues and adhesives utilize van der waals forces.

    About insulators and the exchange of charge, no material can be a perfect insulator. Maybe the dust is better insulator than the paper and the glass, so some small conduction occurs between glass and paper but much smaller between glass and dust.
  9. Feb 17, 2011 #8
    That's interesting, because when I held a thin strip of paper (from a newspaper) close to CRT TV screen, the paper was attracted, but not repelled afterwards. When I wiped the dust from the screen and gave it a new try I got the same result, but the attraction seemed smaller. Not sure how to interpret it.

    I imagine the electronegativity of the substance matters to. Some materials are more prone to become positive, others to become negative.

    Air humidity also has an effect on static electricity, though I'm not sure exactly why and how.
  10. Feb 17, 2011 #9
    Van der waals forces between paper-dust-glass. Dust works kind like a glue.
  11. Feb 17, 2011 #10
    Hmm. The TV I used didn't have any dust on it, and the paper was attracted then fell/fluttered right off after making contact, and sometimes being noticeably repelled.

    It's winter where I live, and we aren't having a lot of humidity.
  12. Feb 17, 2011 #11
    But that doesn't explain why, after wiping off the dust, the paper wasn't repelled, as in my experiment.

    I think the discrepancy between my experiment and his may be the magnitude of the static charge present on the TV. When I carried out my experiment, the TV had been in use for at least an hour (probably more) playing videogames.

    Shoestring, perhaps you should try your test again, after the TV has been on and in use for a while?
  13. Feb 17, 2011 #12
    Yes, I'll try it again. I tried picking up small pieces of the same paper with a piece of plastic that had been rubbed against some fabric. A couple of them did jump off after a while, but most of the didn't. I guess it means that some charges transferred to (or from) the paper, but not enough to make all of them be repelled. The paper pieces appeared polarized, because one edge stuck to the plastic and the other seemed repelled. That made the area of contact between the plastic and paper very small.
  14. Feb 18, 2011 #13
    Interesting... did the pieces which stuck by only one corner ever repel?

    I'm thinking that perhaps it's the low contact area of dust that causes it to resist sufficient charge by conduction to be repelled.
  15. Feb 20, 2011 #14
    I'm not sure, but one of the pieces that remained (for as long as I had patience to wait) seemed attached only by a few fibers. I had thorn the pieces from a newspaper, so their edges were quite rough. It sound plausible to me that a small contact area slows down the transfer of charges to the attached particles.

    I think the attachment of polarized paricles will depend both on the strength and the gradient of the elctric field. The polarization itself should be proportional to the electric field strength, and the attraction on the dipole will depend on the gradient of the electric field.

    So, if the gradient is big, and the conductivity and area of contact are low, there should be a good chance that the particles remain attached for a longer time.
  16. Feb 20, 2011 #15
    I think you're right.
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