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Homework Help: Net forces on a charge particle

  1. May 22, 2014 #1
    1. The problem statement, all variables and given/known data

    I have attatched a snapshot explaining the question

    2. Relevant equations

    ## \vec{F} = \frac{kq_{1}q_{2}}{r^2}\hat{r} ##

    3. The attempt at a solution

    First I will calculate the force on the left charge due to the other two

    ## \vec{F}_{net} = \frac{-kq^2}{d^2}(-1,0)+\frac{kq^2}{(2d)^2}(-1,0) = (\frac{3kq^2}{4d^2},0) ##
    Which means the left positive charge will feel a force pulling it inwards, and through symmetry I can say that the right positive charge will be pulled inwards due to the force of the other charges.
    Also for the negative charge; ## \vec{F}_{net} = \frac{-kq^2}{d^2}(-1,0)+\frac{-kq^2}{d^2}(1,0) = 0 ##
    Based on this i would choose option D.

    I was wondering if I have come to the right conclusion, if my calculations are correct, and if the way I approach the problem is correct. Thank you.

    Attached Files:

  2. jcsd
  3. May 22, 2014 #2


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    You need reassurance ? PF isn't really meant for that.
    Or do you want some tips for this kind of (low didactical quality) questions ?

    First you have to check if only one answer is right, or that perhaps more than one (or, wow, none) is correct.
    Here it clearly says ONE, so no problemo.

    Second: try to be economical: if no calculations are needed, don't make them. Same charge further away = less F suffices. Or: twice as far, quarter the force.

    Then: A and B are clearly nonsense. So is C (no calculating needed).
    Elementary, Watson: D is left and it passes scrutiny.
    Double check and move on to the next exercise.

    [edit] and yes, your calculations are correct...
  4. May 22, 2014 #3
    Well actually, this was a question on my physics test. And yes it was the easiest low quality question, which is why I posted it in the "introductory physics" section, just in case you missed that.

    Anyway the point is, in the marking key, the solution was listed as E. So I wanted to see what the PF community thought before I made an appeal. But thanks for your comments anyway.
  5. May 23, 2014 #4


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    Ah, you saved a nice bombshell for later.

    I'll add a tip: read all answers before embarking on choosing something. More or less as a reminder to myself (I missed E completeley :redface:, a reason to be modest instead of feeling insulted...).

    Make your appeal ! And don't mention PF, it really isn't meant for that -- but please tell us how it went !
  6. May 23, 2014 #5


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    My understanding of the rules is that this kind of question is perfectly acceptable.
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