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B Charged insulator touches a conductor

  1. Jul 30, 2017 #1

    Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?

    I think that by simply touching the sphere by insulating rod , the metal should not acquire any charge as the charges on the rod are immobile unlike a conductor .

    But if the rod is rubbed against the metal sphere , then by friction sphere might acquire charge .

    So , is the above text correct when it states that the metal sphere acquires a net charge on being touched by charged plastic rod ?


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    Last edited: Jul 30, 2017
  2. jcsd
  3. Jul 30, 2017 #2
    Although the plastic rod is not a conductor, the static charge is carries has a very high potential, thousands of volts) relative to the neutral sphere. Under these conditions, simply touching the plastic rod to the sphere will deposit some of that charge. Moving the rod across the metal sphere can deposit more of the charge - not just because of the friction (which may or may not work), but simply because, as you noted, the electrons on the plastic are immobile except at the very high potentials.
  4. Jul 30, 2017 #3


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    I think I'll vote no. I guess a little bit of the charge could transfer at the point of contact, so strictly they could be correct. But if it doesn't transfer enough charge to pick up a bit of paper, then they've not fulfilled their whole claim.

    It's not how we get a decent charge on a conductor using a charged insulator.
  5. Jul 30, 2017 #4
    Thanks for replying .

    Do you think what I have written in OP is theoretically correct ?

    By induction ?

    Is it because rod might not be a perfect insulator ?
  6. Jul 30, 2017 #5


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    Not because the rod is not a perfect insulator.
    In order to become charged in the first place, it must be possible to move charges to or from the insulator. I think that only happens over a very short range - points of contact. Charging an insulator usually involves rubbing a soft insulator over the surface of the other insulator, so that many different points do come in contact.
    So touching the metal sphere to the insulator would collect charges only from the point of contact and very near by. I don't know how near by. I expect, as Scott said, it depends on the potential. A highly charged insulator would induce a big charge on the surface of the sphere, so that there was a strong electric field between them capable of attracting charges from a longer range.

    Induction captures all of the induced charge, so must charge the sphere more than just contact.here. Charging by contact must reduce the field and make it no longer strong enough long before the induced charge has been neutralised.

    BTW I'm not expert in this area.
  7. Jul 30, 2017 #6


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    That's the key word in this business. Read up on Induction and see how you can charge the metal ball most effectively. If you try just 'wiping' charge from the rod onto the metal ball, your results will not be impressive.
  8. Jul 30, 2017 #7
    Do you mind explaining why do you think charges on rod have very high potential ?
  9. Jul 30, 2017 #8


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    Not an awful lot, because the Potential on the sphere would soon equal that of the local piece of the rod and no more charge would be transferred 'physically'.
    Because the charges are very localised; they cannot move around. Small parts of the surface are like very tiny Capacitors and the Formula V=Q/C applies (Q is charge, C is Capacitance to Earth and V is the Potential with respect to Earth.
    If you just touch the metal ball, some charge will transfer but the Capacitance of the ball is relatively high - so V will be correspondingly Low.
    I suggest you read about Inductive Charging and that will tell you how you can get a much higher potential on the ball. I remember having the Electrophorus being demonstrated to us at School - pretty impressive demo.
  10. Jul 30, 2017 #9


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    I think you're reading my comment differently than I intended.
    Before the sphere contacts the rod, the field from all the charges on the rod is influencing the electrons in the sphere and causing a charge separation. With say a 10cm sphere held say a mm from the rod, the charges on the opposite side of the sphere are much further from the rod than the attracted charges on the near side.
    If the sphere were earthed, I'd expect the charge on the near side to balance the field from the rod. You won't get the same charge as the total charge on the rod, but maybe towards half? If the sphere is not earthed, then the opposite charges on the opposite side can't escape. So they will compete to attract the charges away from the rod. But not much, because they are further away.
  11. Jul 30, 2017 #10


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    If you take an electroscope (gold leaf) and bring a charged rod to it, rubbing some charge off. The leaves will part because the scope is polarised; top cap + (say) and leaves -. They will fall almost together when the rod is removed. There is not much charge left on the scope. If you bring the rod near (touching or not - it doesn't make much difference), the leaves will part (polarisation). You then Earth the scope and the leaves collapse. Removing the rod sends the leaves wide apart again - showing that the scope has been left with a lot of charge. So the charging by direct transfer is not very effective but the charging by induction is impressive.
    This link shows what happens. Not sure if this is in context with your comments Merlin but it's relevant, I think.
  12. Jul 30, 2017 #11
    You seem to be implying that you cannot appreciably charge an electroscope by conduction. I don't think this is the case. Charging by induction may be the more effective method, but I remember charging an electroscope by conduction just this last year in a lab class, and my text book agrees with me on that point.

    So while I'm not arguing that charging by induction can (potentially) leave more charge on a conductor, if the OP is asking whether a conductor can be charged by a charged insulator, yes it can.
  13. Jul 31, 2017 #12


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    I have not said that no charge is transferred. It's just a small amount and I gave a reason for that - I repeat:
    If you have an insulator with charge all over it, the only way that any charge can transfer will be at a local level. A small area of the insulator that will be in contact with the ball will be at a high potential. It has a minuscule Capacitance. Touching it against the ball (much greater Capacitance) will share the charges between the two (C's in parallel, effectively) That will charge the ball with more or less all the charge on that patch of the insulator. But how much charge will that be and what Potential will the ball acquire? Measurable but not very significant. Charge on other parts of the insulator surface doesn't get to the conductor.
    Thing is that you are actually doing no work on the system and the only energy available is the Potential of the original tiny bit of charged insulator. When you use Induction, the initial polarisation of the conductor requires very little energy. But separating the discharged conductor from the insulator involves Energy input from the experimenter. So you get a large, induced charge at a high potential as you increase the distance.
    Google will give you many videos examples to support what I say and I have done it myself many times with a school electroscope.
    No "appreciable" charge transfer.
  14. Jul 31, 2017 #13
    I understand what you're saying about induction being able to create a much more significant amount of potential on the conductor. But you can also charge an electroscope enough for the leaves to visibly separate and remain apart by touching it with one relatively small part of a charged insulator. Perhaps we're misunderstanding each other using terms like appreciable and significant.

    In regards to the OPs question (I actually studied from the same text his picture is from), I would say that in fact charge is transferred, at least enough to pick up small pieces of paper or visibly part the leaves in an electroscope. I agree that only the charge which is localized to the portion of insulator in contact would transfer, but experimentally that can be enough to continue with an introductory level demonstration like the OP was asking about.

    Last edited: Jul 31, 2017
  15. Jul 31, 2017 #14
    Hi Daniel ,

    Thanks for your comments . Glad to know that you have actually performed this same experiment in your lab class .In real world , some charge might transfer from rod onto the metal sphere .

    My concern was with theoritical aspect of the experiment . Why should charge transfer from an insulator owing to its property that charges on it are immobile unlike a conductor ? The demo could have used a charged conductor with an insulating handle to show charging by contact .

    Do you think that some very tiny charge is transferred from the rod as it might not be a perfect insulator ? I am going by the theoretical definition of insulators given in intro Physics texts where charges are considered immobile in an insulator .
    Last edited: Jul 31, 2017
  16. Jul 31, 2017 #15
    When I did this experiment last year we simply held insulators in our hands after charging them, both plastic and glass. As someone said above, it doesn't have to do with the object not being a perfect insulator. Being an insulator just means that electrons are not free to move within the solid structure, as they are in a conductor.

    Because in this case the charge exists in the form of ionized molecules on the surface of the insulator, the electrons will be attracted to the neutral matter in the sphere (or electroscope). So the insulator is still not allowing any charged particles to move about within it's structure. The electrons on the surface that makes contact (or very nearly makes contact) are repelled by their like charge neighbors and move onto the neutral conductor.

    You may be taking the statement "charges on an insulator are immobile" a bit to strictly. This does not mean that an insulator can never be discharged. Because charge is a conserved quantity and insulators can be discharged, it follows that it must be possible to transfer charge from an insulator to other objects.
  17. Jul 31, 2017 #16
    OK .

    Please note that I am bothered more with the theory of this demo rather than what happens practically .What should a student answer if the question asked in OP is given in a theory paper ?

    Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?
  18. Jul 31, 2017 #17
    I would answer that yes, it would acquire some charge. That is, provided the piece of the rod that touches the sphere has charge on it. Insulators do not spread the charge uniformly over their surface like conductors too, so it is possible that an insulator be considered 'charged' but that the charge is not on the portion touching the metal sphere. If you're being asked specifically for a yes/no (which would be kinda lame) then I would still say yes.
  19. Jul 31, 2017 #18
    Alright .

    Actually a similar question appeared in one of the Junior Science Olympiads of our country and the official answer key suggested d) as correct option . Please see the exact question in the attached image .

    I was okay with the question and it's answer until I read this textbook :smile:

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  20. Jul 31, 2017 #19
    @Daniel Sellers , I see you have edited post 13 and posted a video showing charging by conduction . The text in the video states "Electrons transfer when conductors touching each other " . Are you sure the rod in the person's hand is an insulator ?
  21. Jul 31, 2017 #20
    Hmm that is confusing. I assumed it was a conductor because, A) it appears to be plastic and more definitively B) he is holding an object in his hand which he uses to charge the electroscope by conduction, which would discharge the rod if it were a conductor. Perhaps part of the rod is a handle and the other end is a conductor? It appears to be a homogenous material. There is also this video.

    Although she does do at least a minimal amount of rubbing in order to charge the electroscope. Other videos explain with diagrams the same concept; charging the electroscope by touching it with a charged insulator.

    It's certainly possible I'm mistaken (and misremembering my class?) and, practically speaking, a simple touch of very limited area is not sufficient to conduct a significant charge to a conductor, but I'm not convinced that's the case. This second video demonstrates that, at least theoretically, some charge is transferred from the insulator to conductor.

    Edited "...touching it with a charged conductor" to "...with a charged insulator" above. Whoops, typos.
    Last edited: Aug 1, 2017
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