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I Static Electricity -- how does it work?

  1. Sep 1, 2016 #1
    in my physics lab we did an expirement where we had to use an vernier electrostatic kit. So it consisted of placing a metal can inside a metal cage, and both were attached to an plastic disk, and below was a metal plate which was used for grounding.

    A black wire was connected from the cage to the metal plate, and a red wire was attached from the metal can to a charge reader, which was connected to a laptop to read the charges. I also had a blue wire in my wrist and it was also connected to the metal plate, for grounding.

    my question is this, we rubbed two charge seperators, one white and one gray, and we put the white seperator inside the can, resulting in a positive reading. And then i touched the metal can with my finger resulting in a negative reading (my teacher said that we did something wrong it should have a reading of 0, but that is not important, we only have to explain what happened).

    Then i released my finger from the can, and nothing happened. then i released the white charger from the metal can, and the reading become more negative, but why? where did the extra electrons came from at end phase?
     
    Last edited by a moderator: Sep 2, 2016
  2. jcsd
  3. Sep 1, 2016 #2
    I don't know the answer, but today I crossed a local river bridge, the footpath is made of some sort of synthetic rubber.
    On arriving at the far side I touched a metal post, part of the bridge, I got quite a severe shock.
     
  4. Sep 1, 2016 #3
    jajaja so let can you at least tell me if my explanation is correct?

    So I think that the white charge separator became negatively charged, which caused the electrons from the metal can to move to the ground (away from the negatively charged white separator) resulting in a positive reading from the metal can (more protons than electrons). then when I touched it, it became neutral (electrons came from the ground since I was wired to the metal plate which served as the ground). I release my finger and nothings happens. then as I removed the white separator, electrons came from the ground (electrons experienced a decreasing force as the negatively charged white separator moved farther), resulting in a negative charge. is that correct?
     
  5. Sep 2, 2016 #4
    When it comes to conductors, the general rule to remember is that when a charged item is put on another discharged item, and they are conductive, electrons and holes will move about to meet this criteria: Have the same potential, but also not create charge, if the net charge would be say 5 coulombs it'll stay 5 coulombs, conservation of matter for you. (for the purposes of this experiment, not applicable to certain fields)

    With the ground it is usually connected to the big oblate spheroid we call earth. The charge would spread so thinly it would be negligible. In fact the smallest you can ever detect would be +/- 1.6*10^-19 coulombs for classical experiments. Which for the purposes of such large scale experiments when compared to an atom is approximately 0.

    The electrons had to come from somewhere, if there are no places it could have come from, you should check your instruments, by using another sensor which you'll know for a fact is not faulty, see if they agree. If no such sensor exists, try a multiple of them and see if they agree on the values. If say one out of 10 can't agree with the other 9, it is possible that that one sensor is faulty.

    There is a possibility you were connected to the reference ground, but was not directly connected to mains earth, which could be at either a higher potential or lower potential. Was the reading 0 when you just set up the experiment and grounded the can momentarily before doing anything? The answer to that will be your clue.

    In our lab experiments we had cases where the equipment was faulty, sometimes we discovered it soon enough for us to change the equipment and take additional measurements. Only in one experiment I did to this day (and may haunt me for the rest of my life) we don't know why we did not see conservation of momentum as described.
     
    Last edited: Sep 2, 2016
  6. Sep 2, 2016 #5
    thanks!! when we set up the instruments, there was already a negative charge before doing anything. I assume that there was humidity, so that could be an issue!! thanks for the feedback =D
     
  7. Sep 2, 2016 #6

    sophiecentaur

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    It isn't exactly clear to me what your exact procedure was but you mention touching things half way through the process. That suggests to me that you were in effect, charging the object by Electrostatic Induction.
     
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