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Homework Help: Electric Potential and charge flowing between 2 spheres

  1. Jun 30, 2015 #1
    1. The problem statement, all variables and given/known data
    A large hollow metallic sphere A is charged positively to a potential of 100 V and a small sphere B to a potential of 50 V. Now B is placed inside A and they are connected by a wire. In which direction will the charge flow?
    2. Relevant equations

    3. The attempt at a solution
    A is at a higher potential, B at a lower potential. So charge should flow from A to B?
    Even if the potentials alter due to the placement of B inside A, A is still at a higher potential?
    I don't know the answer, so please confirm?
  2. jcsd
  3. Jun 30, 2015 #2
    Positive charge will always flow from high potential to low potential.
  4. Jun 30, 2015 #3
    I just googled this question, and the answer is the opposite, like I'd expected. Charge flows from B to A
  5. Jun 30, 2015 #4
    Notice how I said positive charge?...
  6. Jun 30, 2015 #5
    I like to think of these question with respect to positive charge. Then, when I know what direction positive charge moves, I can simply say: positive charge cannot move, therefore the negative charge must be moving in the opposite direction to make it seem as though the positive charge is moving.
  7. Jun 30, 2015 #6
    I'm fairly confused here, sorry. I'm just talking in terms of positive charges here, so don't worry about negative.
  8. Jun 30, 2015 #7
    By connecting the wire, you are creating a conductor. You know that the electric field anywhere in a conductor is zero. Therefore, the voltage must be constant all throughout the conductor you are now creating. This means that the voltage from the 100V sphere will be reduced and become equivalent all throughout the conductor. Since positive charge cannot move, you know that negative charge will flow from sphere B into sphere A.
  9. Jun 30, 2015 #8
    Oh okay. Thanks :)
  10. Jun 30, 2015 #9
    Since you seem to understand I have only 1 thing to add :)

    It took me a while to rectify that electrical current is positive charges flowing.
  11. Jun 30, 2015 #10
    Ahhhh, can somebody pleeeeasssseeee just tell me the answer to my question now?

    That's cute ;)
  12. Jun 30, 2015 #11
    All of the excess mobile charge will move to the surface of the outer sphere.
  13. Jun 30, 2015 #12
    I'll agree with Luke.Current will flow from A to B but the Negative charge will flow from B-A as negative charges move from lower potential to Higher potential.
    For example.Think of a Circuit normally the current flows from higher potential to lower potential (Positive Terminal to Negative Terminal) But if we think the electron would rather move from Negative Terminal to Positive terminal.(opposite charges attract).
  14. Jun 30, 2015 #13
  15. Jun 30, 2015 #14
    Answer is B-A.End of Story.
  16. Jun 30, 2015 #15


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    my2cts has the correct answer.

    The charge on the outer sphere is thus increased and therefore, so is it's potential.
  17. Jun 30, 2015 #16


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    Look below your post on the left hand side. There are labels:
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    Click on "Delete" and respond appropriately.
    [Moderator note: The posts in question were duplicates and have been removed]
  18. Apr 26, 2016 #17
    from B to A . since the negative charges will move to high positively charged plate
  19. Apr 26, 2016 #18


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    What @Luke Cohen , @Dennis Ngeno and @Sync are all missing is that the given potentials were for the case when A and B were separate (and presumably some distance apart). When B is placed inside A, still not connected, the potentials will change. When the two are connected, they become a single conducting solid with a spherical external surface. What do we know about the charge distribution for such? What does the field inside it look like?
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