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Insulators in electrostatics

  1. Jul 8, 2015 #1
    A negatively charged plastic rod will negatively charge a pith ball when in contact. My understanding is that the extra electrons transfer from the rod to the ball, thus giving the pith ball a negative charge.

    When an electrophorus is charged and the metal pan is put on top, it doesn't become charged. At least not until the metal is grounded by user. How is this different than the rod and pith ball? The pith ball isn't grounded, so why is there a transfer of charge?

    Is it because while holding the rod, the rod is grounded? But wouldn't that mean charge is flowing through the rod even though the rod is an insulator?
  2. jcsd
  3. Jul 8, 2015 #2
    The metal pan can hold vastly more charge than a pith ball. So a small charge transfer that would
    show a dramatic effect with a pith ball would show negligible effect with a much larger metal pan.
    Somewhat like a small hose filling a 1 gallon bucket or filling a large swimming pool.
  4. Jul 8, 2015 #3


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    The reason that a charged rod attracts an uncharged object is that the uncharged object becomes polarised by electrostatic induction. Unlike charges are displaced towards the rod and, being closer to it, are more attracted than the like charges. Once touched by the rod, some small local charge will be transferred to the surface of the ball, where the polarised molecules actually present some unlike charges right next to the rod.

    The action of an electrophorus is an extension of this because it is a conductor and many more like charges can flow to ground than just the few electrons that get onto the pith ball. The electrophorus doesn't actually need any contact for it to become charged by induction.
  5. Jul 8, 2015 #4
    Metal can't exchange charge with an insulator very easily. If a metal object touches an insulator it's really only touching over a very small area. The surfaces of objects are not perfectly smooth on a microscopic level. Most of the atoms on the metal surface are not actually touching the insulator.
    Pith on the other hand is a hygroscopic material that contains some amount of water. Water molecules can "crawl" inside microscopic gaps and form a connection. That water allows ions to move from the pith ball onto the surface of any object in contact with it.

    Another possible mechanism of charge transfer is ions moving through the air. That however will only happen to a significant extent if the field strength is high enough which may or may not be the case depending on how strongly the rod is charged and how small the ball is. While transfer through moisture can always happen as long as the air humidity is not so low that the pith dries out completely.
    Last edited: Jul 8, 2015
  6. Jul 8, 2015 #5


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    That is an interesting idea which I can't quite understand. Do you have a reference for your explanation? Are you saying that a pith ball would behave differently, depending on the humidity?
  7. Jul 8, 2015 #6
    The wikipedia article on the electrophorus explains that the microscopic contact between the metal disc and the insulator is very poor.

    About the moisture providing a contact that allows ions to be exchanged - it's more difficult to find reference material there. But the Zamboni pile offers a good example.
    Even though the paper in the pile is dry, the microscopic amount of moisture trapped inside is able to cause enough ions to be exchanged at the metal surfaces to produce a tiny current.
    There is also a simple experiment you can do to see the effect of moisture in hygroscopic materials on static charge:
    Take a piece of plastic foil and "glue" it to a wall by rubbing it.
    If the surface of the wall is wood or paper, the foil will only stick there for a short time. Probably less than an hour.
    If on the other hand the wall is made from metal and the room is dry, the foil will stick there for months or even years without falling off.

    edit - I found those two sources that describe ions "hopping" between adsorbed water molecules in cellulose.
    Last edited: Jul 8, 2015
  8. Jul 8, 2015 #7
    Thanks for the replies, I didn't know that small amount of localized charges could be transferred or about the water on the pith balls.
  9. Jul 9, 2015 #8
    I feel I should add some more comments. I already described two different ways charge could be transferred between the rod and the ball. Ions transferred through water trapped in the ball and ions transferred through the air.
    Pith balls can be made from many different materials, some of them are hygroscopic e.g. cork and some are not e.g. styrofoam coated with metal. So I guess that in most cases it's the air that becomes ionized and conducts the charge. But of course that will only work if the charge on the rod is strong enough to create a strong field that can ionize the air.
    Even before the ball receives any charge from the rod it already has a charge imbalance induced in it i.e. it becomes positive on one side and negative on the other due to the presence of the charged rod.

    The small size of the ball helps to concentrate the charge.
    And if the ball has irregularities on it's surface, especially sharp points, that will concentrate the charge even more and produce stronger fields that ionize the air more easily.
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