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Electrostatic question

  1. Sep 1, 2016 #1
    1. The problem statement, all variables and given/known data
    hi, 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 read the charges. I also had a blue wire in my wrist and it was also connected to the metal plate. 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?


    2. Relevant equations


    3. The attempt at a solution
    what i think is that the white charger seperator became negative charged when i rubbed it with the gray one, and when i put the white seperator inside the can, the electrons from the metal can moved away from the metal can through the black wire and to the metal plate. So the metal can became positively charged, resulting in a positive reading. when i touched the metal can, my body transfered electrons to the metal can to neutralize it, which should have resulted in a 0 charge. then when i released my finger, nothing happened (that is easy to understand). but when i released the white charger, a negative reading resulted. but why did it became more negative? is it because more electrons moved to the metal can from the metal plate? help!!!
     
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  3. Sep 2, 2016 #2

    andrevdh

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    When you insert the white charger into the can it attracks electrons out of the charge sensor onto the can leaving the charge sensor with a positive charge, which is what it indicates. The white charge producer thus gets surrounded by negative charge on the inside of the can, we say a neagative charge is induced on the can. If you were to remove the charge producer from the can the electrons would return to the charge sensor resulting in it indicating a zero charge again.
     
  4. Sep 2, 2016 #3
    but then what is the point of grounding and putting a black wire from the can to the metal plate? isnt the white charger separator negatively charged, which when insidie the can, it will remove the electrons through the black wire by repulsion to the metal plate and then to me? can electrons move through space? or contact only? :[
     
  5. Sep 2, 2016 #4

    andrevdh

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    Grounding carries excess charge away. The can was not connected to the metal plate. It was connected to the charge sensor with a red wire. The surrounding cage was connected to the metal plate with a black wire. The two charged produces are charged oppositely when rubbed together. The white one becomes positively charged and the gray one negatively. Electrons can move through air, like in a lightning strike, but you need very special conditions for this to occur.
     
  6. Sep 2, 2016 #5
    ahhh thanks!! i like physics i will get a minor in it, even though it takes me days to understand a concept xD
     
  7. Sep 2, 2016 #6
    wait, so if the positively charge seperator attracted electrons from the charge reader (positive reading), then the electrons that are coming from the reader to the can will repel those electrons that are on the cage to the ground, right? so the cage will become positive as well? and when i touch the can with my finger, the electrons will go through my finger, neutralizing the can and thus a 0 reading. but i dont understand what happens when i remove the white seperator, resulting in a negative reading? is it because the ground will give electrons to the positively charged cage, which will cause the electrons from the can to be repelled by the electrons from the ground to the red wire, and thus a negative reading?
     
  8. Sep 2, 2016 #7

    SammyS

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    Can you give us a diagram of the experimental set-up?
     
  9. Sep 2, 2016 #8
    https://www.google.com/search?q=vernier+electrostatics+kit+induction&client=ms-android-att-us&prmd=vsin&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiYh9Oc9fHOAhXFYiYKHSVxBrgQ_AUICSgD&biw=360&bih=560#imgrc=R2_BQAVt3aBf4M%3A
     
  10. Sep 2, 2016 #9

    SammyS

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    It's not view-able .
     
  11. Sep 2, 2016 #10
    sorry!! here is a quick video, that is exactly what we did. but when i inserted the white charger into the can, it became neutralized (our magnitudes were wrong, but it doesnt matter).
     
  12. Sep 2, 2016 #11
     
  13. Sep 2, 2016 #12
    sorry, when i touched the can with my finger, it became neutralized. but when i release my finger and the white charger, it became a negative reading, why!
     
  14. Sep 5, 2016 #13

    andrevdh

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    The system started out neutralized - that is the can and the charge sensor, but when you inserted the white charge producer in the can it became "electron hungry" (don't say that in your explanation :wink:). That means that the positive charge producer induced a negative charge on the inside of the can and a positive charge on the outside and on the charge sensor. The positive charge on the charge producer and the negative charge on the inside wall of the can also become "locked together" (:wink:) . When you touched the can electrons flowed from you onto the outside of the can and the charge sensor thereby neutralizing the positive charge and induced on the system and the charge sensor now registers a zero charge, but now the system is not neutral anymore. It has a net negative charge, which the sensors registers when the charge producer is removed from the can, since the additional electrons are now free to distribute evenly throughout the system.
    P9050307.JPG
     
    Last edited: Sep 5, 2016
  15. Sep 5, 2016 #14
    jaja yes thank you!! i cant believe it took me 3 days to understand this simple concept!!! oh my goshhhhhhh
     
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