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Homework Help: Electrostatic fields

  1. Jan 17, 2012 #1
    Hello i have a problem that i need help with.

    Two fixed charges 'q' and '4q' are positioned along an axis with a separation d=5cm

    a) Calculate the forces acting on each charge

    - I understand that the the force eqn is F = qE
    - I also understand that i need to work out the Electric field before i can calculate the force
    - and the electric field eqn is E = (k*q)/r^2??

    what i dont understand is what are the values for the charges? There is no specific value for them, but they are written as 'q' and '4q'

    Can anyone please offer some clarity to this please.

    Thank you
  2. jcsd
  3. Jan 17, 2012 #2


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    Homework Helper

    You haven't been given the value of q but it certainly matters - the higher the q, the stronger the forces. So you will have a "q" in your answers.

    You can use E = (k*q)/r^2 and then F = QE or put it together and just use
    F = k*q*Q/r².
  4. Jan 17, 2012 #3
    Thank you for the response!

    ok so i understand now to use F = k*q*Q/r^2

    for the charge 'q' i have => (9*10^9)*q*Q/25 => (3.6*10^8)*q*Q

    would that be the final answer? Can i multiply q*Q to get Q^2? or should i just leave it as that?
  5. Jan 17, 2012 #4


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    Careful, you must convert your cm to meters. You don't want the two different Q's to appear in the final answer. Just put in "q" for one charge and "4q" for the other. You'll end up with a number times q².

    The question actually asks for TWO forces, so you should really say something about the force on the q and the force on the 4q and how they are different.
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