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Homework Help: A charged ball on a string between two charged plates

  1. Nov 25, 2014 #1
    1. The problem statement, all variables and given/known data
    A charged ball of mass 0.2[gr] and q=6E-9[Coulomb] hangs on a string between 2 plates that are 5[cm] apart. the ball makes 300 with the vertical. what is the voltage difference of the plates.

    2. Relevant equations
    Electric field: $$E[\frac{N}{Coulomb}]=\frac{V}{S[m]}$$
    The force: $$F=E\cdot q$$

    3. The attempt at a solution
    The horizontal force: $$F_x=mg\cdot \tan 30^0=2E-4[Kg]\cdot 10\cdot \tan 30^0=11E-6[N]$$
    The field: $$11E-6[N]=E\cdot 6E-9[Coulomb]\rightarrow E=1924.5[V]$$
    Seems small.
    ##E=\frac{V}{0.05[m]}\rightarrow 1924.5=\frac{V}{0.05[m]} \rightarrow V=96[V]##
    It should be 9440[V]
  2. jcsd
  3. Nov 25, 2014 #2


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    Homework Helper

    What is the direction of the electric force?

    You mean $$F_x=mg\cdot \tan 30^0=2\cdot 10^{-4}\cdot 10\cdot \tan 30^0=11\cdot10^{-6}[N]$$
    Use g=9.8 m/s2, and do not round off so much. And check the magnitude of the force.
  4. Nov 25, 2014 #3
    Thanks, i solved
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