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Conceptual Electric Charge Problem - No Math, but I don't quite get it

  1. Dec 17, 2006 #1

    Any help on this would be appreciated.

    1. If you bring a positively charged insulator near one of two metallic spheres that are in contact and then separate the spheres, what charge will the sphere further away from the insulator have?

    2. Relevant equations - none

    3. I believe the answer to this would be that the spheres remain uncharged because the insulator did not attract any electrons from sphere it was closer to. If for some reason the insulator did remove electrons from the sphere, it would become positively charged and when separated from the other sphere, it would pull electrons from it. So I guess my answer would be either no charge or positively charged. Does anyone understand this question better?

    Thank You.
  2. jcsd
  3. Dec 17, 2006 #2


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    Think about the distribution of charge on the two spheres while they are still in contact. What is going to happen if you separate the spheres while this distribution still exists and then take away the charged insulator?
  4. Dec 17, 2006 #3
    Ok so when the positive charged insulator is near the closer sphere, it will pull the electrons from both spheres toward the closer sphere and push the protons from both spheres toward the further one, and then when the spheres are separated, the sphere on the right will maintain a positive due to the surplus protons? Thanks once again for your help OlderDan. I think this is right.

    *edit - I just realized that the electrons would probably be the only thing actually moving - right?*

    And if I could trouble you just one more moment... I have a general question that is not anything specific on my homework. I don't quite understand the representation of an electric field's direction. I know it is based on the affect the field would have on a positive test charge placed at a location, but is the electric field's direction based on the movement it would cause in a positive test charge, or the reaction of the field to a positive test charge. In other words, I believe that if I had a Negative Electric field on the left, and and a positive test charge to the right, the Electric Field at the test charge would have a direction pointing to the left because that is the direction of Force that would be on the test charge. Is this correct? Thank you.
  5. Dec 17, 2006 #4


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    Electrons would be the only charges that actually moved, but that would still result in a net negative charge on the sphere nearest the insulator and a net positive charge on the farther sphere. The two spheres would wind up with equal and opposite net charges.

    The electric field from one charge does not "react" to another charge. The "test" charge responds to the field it is testing. This can be tricky because the test charge produces its own electric field, as all charges do, so the charge producing the field being tested experiences a force due to the test charge. Keep these effects separate in your mind. A charge all by itself is producing a field. The "test charge" concept is a "what if" concept. If you placed at test charge at some point in space, what force would it experience. A postive test charge would be repelled by a positive charge creating a field (call it a source charge) and attracted to a negative charge creating a field. For point source charges, the electric field is always away from positive charges and toward negative charges. The opposite would hold for a negative test charge.

    In your sphere problem, the positive insulator establishes an electric field pointing away from the insulator. The electrons in the nearby sphere are drawn toward that insulator. When the spheres are sparated there is an electric field in the space between them that points generally from the postive sphere toward the negative sphere.
  6. Dec 17, 2006 #5
    Ok I got it. Thanks so much for all your help tonight OlderDan. I really appreciate it. I better run to bed so that I can be at least a little bit awake for tomorrow!
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