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Homework Help: Help with electric field

  1. Jan 14, 2016 #1


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    1. The problem statement, all variables and given/known data
    The picture below shows two stationary charges q1= +2mC and q2= -15 mC.
    and I need to find the electric field vector at field point P(0,4m). E= [px, py] .

    A little dipole is positions in P(0,4m) and is given with p= [px,py]=[2*10^-8 ,0] Cm.

    Find the torque that acts on the dipole because of the electric field and its potential energy

    2. Relevant equations

    3. The attempt at a solution
    I have found out that E1=4.5*10[6][/SUP] and E2= 1.7*10^6. I am not sure if the electric field E1 is negative or positive ? I think its negative because the vector goes downwards. And when I find the angle for E2 do I use arctan(8/2)?

    Attached Files:

  2. jcsd
  3. Jan 14, 2016 #2


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    Homework Helper
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    OK. But I would keep an extra significant figure while doing the calculations. Then round off to the appropriate number of significant figures at the end. Also, don't forget the units.
    Electric field is a vector quantity. It is not good language to say that a vector is negative. However, the components of a vector can be negative or positive. So, if the field points downward (along the negative y axis) then you would say that the y component of the electric field is negative.

    This does not look right. Are you trying to find the angle that E2 makes to the horizontal direction or the vertical direction? If you are constructing a right triangle to help find the angle, make sure you have the dimensions of the sides of the triangle correct.
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