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Electrostatics - Charged ball tied to a ceiling

  1. Aug 29, 2008 #1
    a ball with a mass of 100g is tied to a ceiling on a 1m cord and is charged with a positive charge of 3*10^16 c, what angle will the cord lift to if it is placed next to an infinite board charged with a density of 5*10-6 c/m^2 (positive), the board is placed at a constant angle of 45 degrees to the floor.

    first i broke up the vectors of powers working on the ball,

    Y+
    Fcos45
    Tcos(alpha)
    --------
    Y-
    mg
    -------
    X+
    Fsin45
    -------
    X-
    Tsin(alpha)
    --------
    now i compare

    Fsin45=Tsin(alpha)
    T=Fsin45/sin(alpha)

    mg=Fcos45+Tcos(alpha)
    mg=Fcos45+(Fsin45/sin(alpha))*cos(alpha)
    mg=Fcos45+Fsin45*cot(alpha)

    cot(alpha)=(mg-Fcos45)/Fsin45

    F=E*q
    E=2(pi)(density)(k)
    F=2(pi)(density)(k)(q)
    F=0.27(pi)


    once i put al the values in, i get that alpha=32.37 degrees but the answer in my text book is not that, can you ssee where i have gone wrong
     
  2. jcsd
  3. Aug 29, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: electrostatics

    Your work looks OK to me, so recalculate that angle (just redo the arithmetic when you plug in your values). Looks like you've somehow found the complement of the angle, instead of the angle.
     
  4. Aug 30, 2008 #3
    Re: electrostatics

    youre right, the answer in the book is 57 degrees, do you see where i have gone wrong
     
  5. Aug 30, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Re: electrostatics

    As I said, you must have made an arithmetic mistake. (I confirmed your formula and, using the same formula and values as you have, got the correct answer.) I suspect you mixed up cotangent with tangent.
     
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