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Question about charge

  1. Sep 23, 2007 #1
    2.1 Two objects, A and B, are initially uncharged. A negative charge of 4 nC flows from A to B. What is then the charge on each object?

    My problem with this is that
    a) the two objects are uncharged; to me "flow" of charge indicates current - I can understand the two objects becoming charged by static, but why on earth would a current flow between two uncharged objects?

    b) A negative charge of 4 nC... I know A will be left with a positive charge of 4 nC, and B will be left with a negative charge of 4 nC. So my instinct would be to say that A has a charge of 4 nC and B has -4 nC... however, it says a negative charge of 4 nC flows! This means A is left with -4 nC (because 4 nC has left it... but it was actually a negative charge that left it, so -4 nC is actually positive! I hope you're following this.)

    So, what will the charges be?
     
  2. jcsd
  3. Sep 23, 2007 #2
    a) Don't understand your point here. How does "being charged by static" not involve a current? Charge movement implies current, even if equilibrium is quickly reached.

    b) If A loses 4 nC of negative charge (let's say electrons), it will have a net charge of +4 nC. B will pick up the electrons and have a -4 nC net charge.
     
  4. Sep 23, 2007 #3

    Astronuc

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    Staff: Mentor

    a. Just assume a voltage is applied temporarily between A and B.

    Conservation of charge means a - charge removed from A and transferred to B must leave behind a corresponding + charge at B.

    Your instinct is correct.

    4 nC is the magnitude. Instead the problem could have stated a charge of -4 nC is transferred from A to B, instead of saying a negative charge of 4 nC.
     
  5. Sep 23, 2007 #4
    Ahh, right. So charge is a scalar quantity, then?
     
  6. Sep 23, 2007 #5

    Astronuc

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    Staff: Mentor

    Yes. Charge is a scalar. The electric field and electric force are vector quantities.
     
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