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Homework Help: Line charge within a charged shell (basket case)

  1. Jan 26, 2017 #1
    1. The problem statement, all variables and given/known data
    An infinite line of charge with linear density λ1 = 6.8 μC/m is positioned along the axis of a thick insulating shell of inner radius a = 2.5 cm and outer radius b = 4.8 cm. The insulating shell is uniformly charged with a volume density of ρ = -667 μC/m3.

    1) What is λ2, the linear charge density of the insulating shell?


    2) What is Ex(P), the value of the x-component of the electric field at point P, located a distance 8.1 cm along the y-axis from the line of charge?

    0 N/C

    3) What is Ey(P), the value of the y-component of the electric field at point P, located a distance 8.1 cm along the y-axis from the line of charge?

    This is where I'm stuck.

    2. Relevant equations
    Gauss' law ∫E⋅dA=Q_enc/∈
    I know there will be integrations in there but due to a week of snow days the instructor hasn't covered any of that, so I'm not sure if there is something else I'm missing, and I'm not finding any good material that I can extract something from.

    3. The attempt at a solution
    The first section to the homework was similar, but with a conducting shell. I figured that out with the help of Michael Van Biezen videos, but I don't get how to use charge densities, or how to integrate what.

    E∫(πR2)L = ∫dQ/∈ (I'm using ∈ for epsilon naught)
    dV=∫(from a-b)πL(Ra2-Rb2)
    EπR2 = (λπL∫(Ra[/SUB2]-Rb2))/∈
    E = (λL∫ab(Ra2-Rb2))/R∈

    That's what I have written on my paper. I'm just totally lost and desperate at this point. Like I said my teacher is expecting way too much without much instruction, and I'm sorry for short comings, but I just really need help.


    Attached Files:

    Last edited: Jan 26, 2017
  2. jcsd
  3. Jan 26, 2017 #2
    Ey(P) = 2*k(this is 9.0E9)*(Charge density of outer shell * area of outer cylinder + lambda of line) all divided by radius (.081m) let me know if this works out for you
  4. Jan 26, 2017 #3
    ******length of one is not even needed disregard that
  5. Jan 26, 2017 #4
    I got:

    Ey(P) =( 2(8.99810^9)(-667π(.0482-.0252))/.081
    = -780964101685
    and it was incorrect.
  6. Jan 26, 2017 #5
    what you are trying to do in this problem is act like the system is a point charge so take the lambda you found in the beginning and add it to the lambda given multiply it by two and k and then divide by the radius (0.081)
  7. Jan 26, 2017 #6
    Ok so (((6.8x10^-6)+(-3.518x10^-6))2(8,99x10^9))/.081

    Got it. Thank you so much! I think I'll have to work out that integral backwards.
    I'm asking this before I've figured out how to arrive at the formula you gave me, but why can I treat it as two point charges? The x components of the shell cancel, and y components < 0 (along y axis), is it just that the y-comp fields end up averaging to λ1? Does anyone know of a good link where I can learn more about this?

    I use Flipit physics, and I find it difficult to follow at best, and like I said we're cramming 3 sections into 1 week, where we'd normally be doing half that, and the teacher hasn't been able to give us much help.

    Again thanks Marchcha for the help.
  8. Jan 26, 2017 #7
    Hey as a user of flipit physics I feel your pain. You treat it as two point charges because it is outside of the conductor.
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