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Electric field and electric charge, whats going on in the diagram.

  1. Jul 8, 2011 #1
    1. The problem statement, all variables and given/known data

    This picture shows two points charges, q1 = +5 nC, and q2 (unknown), fixed in place on the x axis. At the location marked P, the net electric field due to both charges is zero: EP = 0 N/C.

    Based on the information shown in the picture, what can you conclude about the magnitude and sign of the charge q2? Explain your reasoning without calculations.

    I've attached a pic.


    2. Relevant equations



    3. The attempt at a solution

    E1 = -E2 at P because the two fields must point in opposite directions at that position and have equal magnitudes at that position. (as discussed in the tutorial) Also E1 is pointing left from P and E2 is pointing to the right from P. Finally, q2 is definately a positive charge because E2 is pointing in the opposite direction (to the right) while E1 is pointing left, like charges repel.

    after looking at it again i'm not so sure i'm correct. Why must they be equal in magnitude and opposite in direction? the fields are equal and opposite just describes the equation E1 = -E2, but i'm not sure why its true at P and looking at q1, which is a positive charge. Therefore, the field E1 created by q1 at the point directly away from q1.

    ..I don't seem to be understanding whats with the diagram. I just can't put it into words I guess. I think I see whats happening just can't explain it, can someone please break this down simply and clearly, i think i must be over thinking it.
     

    Attached Files:

  2. jcsd
  3. Jul 8, 2011 #2
    I think what you want to do here is imagine that you are a little positive charge positioned at P. You know that E at a point is defined as the force per unit charge experienced by a positive charge in the field at that point.

    Think about what would happen if you were a positive charge at P and the only other charge in the diagram was q1? What sort of force would you experience?

    Now think about what would happen if q2 was negative. What forces are working now? Can you imagine any position between q1 and q2 that would put you into equilibrium (i.e. all the forces would balance out?) This should give you an idea about the sign of the charge on q2.

    As for the magnitude of the charge - I think without knowing the distance from q1 to q2 you won't be able to solve this exactly. But by looking at the position of P on the diagram you should be able to say something about the size of q2 compared to q1.
     
  4. Jul 13, 2011 #3
    Okay, here is my response, in turn I've also listed a response from a tutor, i don't understand what else there is to say about this diagram. Please fix my response. I have been back and forth thinking about what else to say, I really have no idea what else there is to say about this diagram.

    The E fields cancel each other out at point P. Both charges are the same polarity. The charges emit E fields at each other. Q1 emits a field to the right, while Q2 emits a field to the left which at point P there is a collision of field lines of equal strength. Also, since the E field is Zero at P and is closer to the Q2, then Q1 is greater in charge magnitude.
    Since E fields cancel at P, and Q1 is much farther than Q2, the right charge must be smaller because E fields get weaker with distance and Q1's E field is strong enough to counter the effects from Q2's E field, and Q1's E field can do this from a greater distance than Q2 has to do it from.

     
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