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A spherical raindrop has a charge Q = 3*10^-11. The potential at the surface is V = 500 V (V = 0 infinitely far away).

a)Determine the radius of the drop.

My first thought was to consider the raindrop as a point charge and then integrate the electric field from this point charge from a distance that equals the unknown radius R to infinity and, finally, equate this to 500 V.. but I'm not sure how to do this or if it's even right...

Another, identical, raindrop is brought in touch with the first one.

b)Determine the magnitude of the repelling force between the raindrops just before they touch each other.

Here, again, I would consider the drops as point charges and then calculate the Coulomb force when the distance between them equals 2R. Can you do that even though the charge distribution is in reality homogenous?

The two raindrops is now brought together and forms a single, spherical raindrop.

c)What is the potential at the surface of this new raindrop?

Here I would integrate the electric field from the 'old' radius to the radius of the new drop and then subtract this from the 500 V.. Again, I have no idea if you can do that...

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# Homework Help: Tricky exercise (electricity)

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