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Homework Help: Question about Electric Fields and Charges

  1. May 22, 2016 #1
    1. The problem statement, all variables and given/known data
    So I was given some questions as practice, and I cant seem to do them. Here is a link to the questions:

    http://i.imgur.com/ZmkqCAV.jpg (the answers are in square brackets)

    Ive only been able to calculate Q3 and Q2 so far. I would really especially like help with Q1. Ive been stuck on it for ages.

    2. Relevant equations
    Ive been told this would be helpful: http://i.imgur.com/nmrsk2E.jpg

    3. The attempt at a solution
    Ive been stuck on Q1 for ages. Ive been playing around with F = k q1q2/r^2 but thats not getting me anywhere.

    For Q2: I used E = kq/r^2 for q1 and q2. Subtracting the answers gave me -225x10^3, which is somewhat equal to 2.3x10^5. Ive got no idea on how to get the j vector component though.
    Edit: I just got the j vector. Pythag and then the formula above got it.

    I have no idea where to begin for Q4

    ANY help appreciated. Thank you
    Last edited: May 22, 2016
  2. jcsd
  3. May 22, 2016 #2

    Simon Bridge

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    Lets take them one at a time:
    Q1: you have the correct equation.... you need to use the fact that force is a vector and Q seen a net zero force. However, it may give simpler maths if you use the electric field equation instead.
    Work out the force on Q due to each charge ... just using symbols to start with.
    What does the force add up to?
    How do you write that in maths?

    Q2: the electric field is a vector - which way does the electric field of q3 point?

    Q4: you need to look at your notes ... you will have an equation about the torque on a dipole.
  4. May 22, 2016 #3
    Thank you, i really appreciate it. Am I right in assuming the Force from Q to q1 = Q to q3? Or Q to q1 - Q-q3 = 0?
  5. May 22, 2016 #4

    Simon Bridge

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    The question says it is the force on Q due to the other three that is zero.
    You can use your understanding of forces (or electric fields) to work out the directions.
    i.e. a positive charge to the left of Q contribute an electric field pointing to the right and F=QE
    The actual value of Q does not matter (you'll see) so you can assume it is positive.
  6. May 22, 2016 #5
    Ok, so the forces (Q to q1) + (Q to q2) + (Q to q3) = 0?
  7. May 24, 2016 #6

    Simon Bridge

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    That is what the problem statement says all right - well done.
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