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Cant seem to get the answer right (E&M HW)

  1. Jul 26, 2005 #1

    cronxeh

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    The charge per unit length on a long, straight filament is -95.0 µC/m.

    (a) Find the electric field 10.0 cm from the filament. Distances are measured perpendicular to the length of the filament. (Take radially inward toward the filament as the positive direction.)
    My answer: 1.07 MN/C

    (b) Find the electric field 29.0 cm from the filament.
    My answer: 3.11 MN/C

    (c) Find the electric field 120 cm from the filament.
    My answer: 12.9 MN/C


    Formula: 4*pi*8.99*10^9*distance*(-95*10^-6)

    On the side of caution I've used both + and - aforementioned answers and both seem wrong. Am I missing something here?
    I thought [​IMG] ?
     
  2. jcsd
  3. Jul 26, 2005 #2

    cronxeh

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    Also one more problem

    A conducting spherical shell of inner radius of a and outer radius b carries a net charge Q. A point charge q is placed at the center of this shell.

    (a) Determine the surface charge density on the inner surface of the shell. (Use Q, q, a, and b as necessary.)

    My answer: -q/(4*pi*a^2)
    (This is correct)

    (b) Determine the surface charge density on the outer surface of the shell.
    My answer: (Q+q)/(4*pi*r^2)
    (This is marked wrong!)

    Whats wrong about this answer? The WebAssign seems to think (Q+q)/(4*pi*r^2) and (Q+q)/(4*pi*a^2) for (b) are both wrong
     
  4. Jul 26, 2005 #3
    i don't understand how you get your formula. i get
    E=(-95*10^-6)/2*pi*R*ε0, as in formula in the image you gave link to.
     
  5. Jul 26, 2005 #4

    OlderDan

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    The radius of the outer surface is ?????.
     
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