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Incredibly basic Coulomb's law question - am I being silly?

  1. May 12, 2015 #1
    • member reminded that problem statement should be text and not a graphic
    I've attached my workings... But is this correct? I can't see the fault with it. However I can't see how there will be no y-component to the force... Can someone a bit smarter than me tell me if I'm being really dim.

    17387568979_224c324095_b.jpg
    https://flic.kr/p/sutQu4][/PLAIN] [Broken]
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 12, 2015 #2

    Doc Al

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    Staff: Mentor

    What's the y-component of the force from B on C? Compare that to the y-component of the force from A on C.

    (I'm having trouble reading your diagram.)
     
  4. May 12, 2015 #3
    Apologies about the poor iphone photo/my diagram drawing skills. If you click on the link you can then zoom in and see it better. I am no good at typing things out - and it seems like a waste of time to do so to ask a question, so I just photo my work. But I do realise it's a pain.

    I have done this and found that the y-component for Fca and Fcb cancel each other out. Which is what I have a problem with. I have tried to draw out the what the ovreral force should look like... However I think I should get a y-component from the overall force... Noting here I am just using the paralellogram method and am not taking into account the magnitude of the charges...

    Hold on - are you saying I should look at the relative sizes of the charges and compare them to the y component using the angle... If so I've seen this done before however I have never done it myself. I always just use the algebra and maths to give me an answer. I usually get the right answer to drop out - however sadly you are always a little unsure if you have it right or not.
     
  5. May 12, 2015 #4

    Doc Al

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    Staff: Mentor

    Right.

    If you used the parallelogram method properly to add those vectors, you would get a net force on C that would have no y-component. You must take into account the magnitude of the charges as they determine the magnitude of the forces!

    You certainly must consider the relative charges to properly scale your forces when drawing your parallelogram.
     
  6. May 12, 2015 #5
    Thanks - I will do that in the future. I am currently revising for an exam where I only have around 5 minutes so answer both parts of that question including reading and checking time - so I can't do too much. But i do appreciate if I am going to do something I should do it properly.

    For the avoidance of doubt - and because I'm a bit dim. Is the answer correct? Thanks Sam
     
  7. May 12, 2015 #6

    Doc Al

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    Staff: Mentor

    Posting your problem as an image and not following the homework template is against our rules. (See: How to Ask for Homework Help)

    (My bad for not pointing this out sooner.)

    On an exam, I would not waste time with graphical addition of vectors.

    Type out your answer and I'll check it out.
     
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