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Electric field stregth between charges

  1. Feb 2, 2012 #1
    1. What is the magnitude and direction of the electric field strength midway between a +60uC charge and a -30uC charge, if the charges are 2.0m apart?

    2.E=kQ/r^2

    3.
    E1=Kx-30u/1
    E2=-2.75x10^5

    E2=kx60u/1
    E2=5.4x10^5

    Etotal=E1 + E2 ----> 2.7x10^5 N/C towards the - charge (my book says i have this right)

    The problem that i have is that i came across a question that's almost the same but different answer.

    1. What is the magnitude and direction of the electric field strength midway between a 75uC charge and a -25uC charge, if the charges are 2.0m apart?

    Now i did everything the same as the first one and came to

    E1=kx-25u/1
    E1=-2.25x10^5

    E2=kx75u/1
    E2=6.75x10^5

    Etotal=E1+E2 ----> 4.5x10^5 N/C towards the - charge (my book says its wrong)

    Book answer : 9.0x10^5

    which would work if i took away the (-) from -2.25x10^5 and added them up but thats a different way of doing it from the first so i'm confused as to which way is right.

    The only difference in the questions that i see is there's no (+) in front of the 75uC

    Thanks for any help you can give me
     
  2. jcsd
  3. Feb 2, 2012 #2

    SammyS

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    Your answer to the first problem is wrong too. The book is also wrong if it agreed with you.

    Does the electric field due to a positive charge point towards it, or point a way from it?

    Does the electric field due to a negative charge point towards it, or point a way from it?
     
  4. Feb 2, 2012 #3
    The electric field would point away from a positive charge

    The electric field would point towards a negative charge
     
  5. Feb 2, 2012 #4
    (-)-------(m)---------(+)
    <----- ......<-------

    So you would just take out the negative and add it up? Negative is only referring to direction in this case?
     
  6. Feb 3, 2012 #5

    SammyS

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    Yes.

    or they're both negative.
     
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