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Homework Help: Calculate charge on a body(in Coulombs)

  1. Sep 19, 2004 #1
    All right i have a simple problem but i just want to verify my results, so any help would be greatly appreciated. The problem is as follows:

    7.1* 10^12 electrons are added to a body that already has a negative charge of 0.2uC(the u is micro 10^-6). Calculate the final charge on this body.

    Now i have
    (7.1* 10^12)(1.602* 10^-19) + (0.2* 10^-6)

    which gave me 1.337 uC

    is this correct?
  2. jcsd
  3. Sep 19, 2004 #2
    whoops, i think it should have been:

    (7.1* 10^12)(- 1.602* 10^-19) + (0.2* 10^-6)

    which gave me 0.937 uC

    now im lost...please help.
  4. Sep 19, 2004 #3
    new info, 1C is equal to 6.24* 10^18 electrons
  5. Sep 19, 2004 #4
    ok, formula should be:

    (7.1* 10^12)/(6.24* 10^18) + (0.2*10^-6)

    = 1.337 uC which is the same as the first result i got...... 0_o
    i am going to go with this one.

    is this correct?
  6. Sep 19, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Check all of your negative signs. The problem states that the body already has a negative charge,

    Your magnitude looks right, but the sign is wrong.

    [mod hat]
    Moving the thread to homework help.
    [/mod hat]
  7. Sep 19, 2004 #6
    Thanks you were right about the sign, it was supposed to be -1.337 uC

    Also, sorry about the location of my topic i only put it there because the course i am taking is electrical engineering and there was a section that matched exactly. I just joined the forum and i already know i will be here every day to read up on the info i need. Thanks again for the help, i will keep all homework related problems here.
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