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Free Body Diagram and vectors

  1. May 19, 2010 #1
    Hello all,

    I just received back a unit test on fields that I wrote two or so months ago (teacher marks really slowly). I did pretty well on it. There is one question that I lost marks on. It isn't terribly difficult, but my teacher hasn't shown us the correct answers yet and I want to confirm what I did wrong.


    that is the scan.

    i put two Fq vectors towards the charged plates and gravity down. it was the last question on the test, so i thought the magnetic field was there just to throw us off and i neglected it (i had another reason to neglect it that i cant remember right now)
  2. jcsd
  3. May 19, 2010 #2


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    OK, well what do you think you did wrong?
  4. May 19, 2010 #3
    After looking at the question for a bit I realized that what made me not include the magnetic force on the fbd was because i think i am assuming that the particle is not moving, thus no magnetic force.

    i think what i did wrong is related to the fact that i assumed the particle is not moving. we never dealt with charged plates of the same charge throughout the unit so i dont really know where the particle would move. my instinct and what i remember from this unit, tells me that the particle would move to the right since it is closer to the right and thus the force between them is greater. in that case Fm would be into the page?? how would i show that on the fbd?
  5. May 19, 2010 #4
    Assuming both those plates carry the same charge, there shouldn't be any electromagnetic force on the particle if it is standing still. The opposite fields should cancel out.

    However, you would want to show that these forces cancel in your FBD.
  6. May 19, 2010 #5
    yeah i had two equal length electric force vectors (one to either side of the particle, towards the plates) and a gravity down. from what you said it sounds like im right.

    my problem is understanding if the particle is standing still. how do we know if it stands still? i assumed this when i answered the question but i dont really remember if i had any reasoning to it when i did it :)
  7. May 20, 2010 #6
    well according to another student this is the answer:

    http://yfrog.com/j6phystestj" [Broken]

    does that make sense?
    Last edited by a moderator: May 4, 2017
  8. May 20, 2010 #7


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    Honestly, no. I don't see how that could possibly be the answer, unless you've neglected to show us part of the question or something.

    For one thing, there is no indication given that the particle is moving. So the natural assumption to make is that it is at rest in the reference frame of the diagram, although ideally the problem should say so explicitly. Also, even if the particle were moving, in order for the magnetic force to be pointing down, the magnetic field at that location would have to be horizontal: either left-right or in-out of the page (or some combination of those), since the magnetic force is always perpendicular to the magnetic field. I don't see how a north magnetic pole in that position could generate a horizontal magnetic field.
    Last edited by a moderator: May 4, 2017
  9. May 20, 2010 #8
    thanks a lot.
    i guess i will speak with my teacher about it tomorrow to get an explanation. my answer was the same as the one shown, minus the magnetic force downwards. would that be right?

    EDIT: i showed the entire question btw
  10. May 21, 2010 #9


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    As far as I can tell, yeah.
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