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Homework Help: Voltage and Capacitance

  1. May 22, 2007 #1
    1. The problem statement, all variables and given/known data

    A small object of mass m carries charge q and is suspended by a thread between the vertical plates of a capacitor. The plate separation is d. If the thread makes an angle [tex]\theta[/tex] with the vertical, what is the potential difference between the plates.

    2. Relevant equations

    [tex]\Delta V=-Ed[/tex]
    [tex]C\Delta V=q[/tex]
    E = kq/r^2
    C = e0*A/d
    3. The attempt at a solution
    I drew a free body diagram, but since the length of the thread isn't given, it doesn't appear to help.
    I'm having a hard time finding a relationship between theta, m, d, and q. Any hints?
  2. jcsd
  3. May 22, 2007 #2


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    Staff: Mentor

    What is the relationship between the voltage on the plates and the electric field between them? What is the equation for the force on the charged mass due to the electric field? What is the force of gravity on the mass?
  4. May 22, 2007 #3
    Got it now, thanks
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