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Homework Help: Electric potential of water droplets

  1. Jun 19, 2013 #1
    1. The problem statement, all variables and given/known data
    Twenty seven charged water droplets each with a diameter of 2 mm and a charge of 10-12 C coalesce to form a single drop .Calculate the potential of the bigger drop.

    2. Relevant equations

    3. The attempt at a solution
    i don't really have an idea from where to start.....please help...
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 19, 2013 #2
    Calculate the potential where? Or do you need to find it as a function?
  4. Jun 19, 2013 #3
    Well we know that the 27 drops are forming into a bigger drop. So now you have a big drop who's charge and radius is unknown - you have to calculate the potential of that. Try to find out the radius and total charge of the new big drop.
  5. Jun 19, 2013 #4
    The volume of bigger drop will be equal to volume of 27 small drops
    From here find the radius
    and charge is conserved , so add the total charge
  6. Jun 19, 2013 #5
    radius came out to be 10-3 m and finally potential came out to be 81V.....thanks for helping guys....
  7. Aug 27, 2013 #6
    How? The radius comes out to be 6x10-3m and not 10-3m. And the potential comes out to be 81/2 Volts and not 81 Volts. Can somebody please verify?

    => R3=23.33
    => R=6mm=6x10-3m

    =>Vbig drop=1/(4∏εo)xQ/R=9x109x27x10-12/6x10-3=81/2V

    Last edited: Aug 27, 2013
  8. Aug 27, 2013 #7


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    Staff: Mentor

    The problem specifies that the DIAMETER of the small drops is 2mm. The radius of the large drop should turn out to be 3 x 10-3 m , or 3 mm.
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