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physics

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Two small pith balls, each of mass m = 11.2 g, are suspended from the ceiling of the physics lab by 1.7 m long fine strings and are not moving. If the angle which each string makes with the vertical is è = 30.4°, and the charges on the two balls are equal, what is the magnitude of that charge (in µC)? (Please note that the unit of charge is micro-Coulomb here - some browsers might not display the Greek letter mu correctly and show it as an m.)

  • physics - ,

    If the charged ball is suspended be the string which is deflected by the angle α, the forces acting on it are: mg (downwards), tension T (along the string - to the pivot point), and F (electric force –along the line connecting the charges).
    Projections on the horizontal and vertical axes are:
    x: T•sin α = F, ….(1)
    y: T•cosα = mg. ….(2)
    Divide (1) by (2):
    T•sin α/ T•cosα = F/mg,
    tan α = F/mg.

    Since
    q1=q2=q.
    r=2•L•sinα,
    k=9•10^9 N•m²/C²
    F =k•q1•q2/r² = k•q²/(2•L•sinα)².

    tan α = F/mg =
    = k•q²/(2•L•sinα)² •mg.
    q = (2•L•sinα) • sqrt(m•g•tanα/k)=
    =(2•1.7•0.5) •sqrt(0.0112•9.8•0.577/9•10^9) =
    =4.5•10^-6 C =4.5 μC.

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