1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes. Charge is a scalar. The electric field and electric force are vector quantities.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook