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Homework Help: Finding electric field between two charges

  1. Jan 31, 2013 #1
    1. The problem statement, all variables and given/known data
    Two charges, Q1 = 4*10-8 and Q2 = 2.7*10-8 are 3.25 m apart.
    What is the electric field magnitude halfway between them?

    2. Relevant equations
    E = kQ/r2

    3. The attempt at a solution
    E1= k (4*10-8C)/(1.63m)2
    = 135 N/C
    E2= k (2.7*10-8C)/(1.63m)2
    = 91.4 N/C

    135 N/C - 91.4 N/C = 43.5 N/C

    Is this correct?
  2. jcsd
  3. Jan 31, 2013 #2


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    Yes. You will get somewhat different answers depending on how you round your numbers during the calculation.
  4. Jan 31, 2013 #3


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    Looks ok. Do you have a reason to doubt it?
  5. Jan 31, 2013 #4

    Simon Bridge

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    It is best you don't rely on someone else to tell you if you are right or not.
    Take this opportunity to practice :)

    There are only two ways you are likely to be wrong in this problem:
    1. you got the numbers wrong
    2. you got the physics wrong

    for 1 you just check your substitution
    for 2 you go through your reasoning to see if your answer is consistent with the way charges behave.
    When you start out doing this stuff it seems quite hard but you'll quickly get used to it.
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