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Very simple - noobish physic exercise.

  1. Nov 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Note that am a beginner in physics,hardy know any,so be gentle (:
    There are three particles in a line below.
    The question is where must we put Q3 so it will get 0 total power.
    Q1=6 mC
    Q2=15 mC
    Q3= -8 mC

    In a line of 2 meters like this:

    (Q3 is anywhere between q1 and q2.

    And that 3x is the two meter line.

    2. Relevant equations
    I don't know what exactly to use,and since you here are far more experienced you will figure it out.

    3. The attempt at a solution
    Couldn't even start it.sorry.

    Anyway,if someone has the patience to explain how this thing works it would be very appreciated.
    Help the newbie.
    Last edited: Nov 28, 2009
  2. jcsd
  3. Nov 28, 2009 #2


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    Gold Member

    OK, maybe someone else recognizes something in here, but I don't even know what course or field of science this is for.

    I see one and only one word that hints at a science field: the word 'power'. And that makes me wonder if maybe this is ... ahhhhhh ... an electrical circuit exercise?

    Am I close?
  4. Nov 28, 2009 #3
    oh sryyy forgot to mention that the Q's are counted in mC.
    fixed it.
  5. Nov 28, 2009 #4


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    Gold Member

    MC? A unit of air flow? Is this for a course in HVAC theory?
  6. Nov 28, 2009 #5
    No,its coulomb.
  7. Nov 28, 2009 #6


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    Homework Helper

    Well... we're not out to make you feel bad (honest!), but you still haven't really explained the problem at all (edit: as of before your last post, #5). The fact that you talk about "Q's counted in mC" makes me think you're talking about static point charges. So, first thing I want to ask: is this an accurate statement of the problem you're asking about?
    If that's what you're asking about, please confirm it so we can get on the helping you. (And you owe me a cookie for the effort :tongue2:)
  8. Nov 28, 2009 #7
    with extra chocolate for you ;P

    Ok,heres the deal.
    He said that there are 3 particles.
    Two with positive charge and one with negative.
    In a line of two meters. Q2 is on 0 point of the line ,Q1 is on 2m point of the line and Q3 is somewhere between.
    The question is: where do we put Q3 (0.01? 0.02?..1.99?) so Q3 wont have...resistance(?) something like that.
  9. Nov 28, 2009 #8
    mC is micro coulomb.
  10. Nov 29, 2009 #9
    so,any ideas? or this doesnt make sense at all?
  11. Nov 29, 2009 #10
    This poor guy doesn't know anything. He could have used the word energy, or force, or power, or probably a dozen other words and, to him, the statements would mean the same.
    I vote that we tell him we need the exact wording of any problem from his course that he wants to bring to us.
    I also think that he is probably feeling very discouraged right now and it'd be good to provide him with some certainties, one of which could be that PF guys are helpful.
  12. Nov 29, 2009 #11
    mC is generally millicoulomb, like the std prefix. Micro coulomb would generally be noted as uC, even if the u is a mu.

    To people trying to solve this, I believe Q3 goes in the position in which the electric fields from Q1 and Q2 are equal.

    E = (kQ)/(R^2)
  13. Nov 29, 2009 #12
    Dear Deicider
    I cannot answer the question you have asked because it is meaningless to me. Power is not a force which is exerted between charged bodies.
    In physics (and in every science) it is important to know exactly what each other is talking about, so we have definitions for words like force, power, energy... and we are careful to use the right word every time, lest our remarks be meaningless.
    If you are studying a formal course, what course are you studying, please?
  14. Nov 29, 2009 #13
    yes,its force not power :/
    the three particles are charged 2 positive and 1 negative.
    And i must find where exactly between the two positive charged particles i must place the negative so the force pressed on Q3(the negative) is zero.
    There is no energy and power i suppose.

    Ok i'll show you an example of what we did few days ago:
    k=9*10^9 N m^2/C^2=90 N cm^2/uC^2
  15. Nov 30, 2009 #14
    Am doomed :S
    Sry ,my native language is Greek and its difficult to 'translate' all these terms in the correct way.
  16. Nov 30, 2009 #15
    Im pretty sure this is just a simple coulombs law problem,

    Decider if you look up coulombs law you will find that it gives the force felt between two particles relative to there distance apart. So i think what you mean is where do i put Q3 so that it feels an equal force on both sides?

    Use coulombs law(look up the formula) for Q1 and Q3 and then Q3 and Q2 to find what distance from Q1 and Q2 you need to put the particle...hope this helps.

    Feel free to re-word this if any part is wrong PF :)
  17. Nov 30, 2009 #16
    Hello Deicider,as others have suggested it is probably a Coulombs law problem.I'm assuming Q1 and Q2 are both positive and Q3 is negative.When Q3 is placed between Q1 and Q2 it is pulled to the right with a force given by (k.Q1.Q3)/R1 squared and pulled to the left with a force given by (k.Q2.Q3)/R2 squared.R1=distance between Q1 and Q3 and R2=distance between Q2 and Q3(look up Coulombs law).When the two forces are equal in size the net force on Q3 is zero,this is when Q1/R1 squared=Q2/R2 squared.A bit of maths is needed to finish it.
  18. Nov 30, 2009 #17
    yess,this is the right way now that i remember,and venomx is somewhat correct but this is more detailed and accurate,thnxx.
    I'll try it now to see how it goes (:
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