Electrical Potential Energy Problem

[Embermage]

Hello,

I've been trying and trying to do this problem for quite a while now, and I can't seem to get an answer which agrees with the book.

In a charging process, 10^13 electrons are removed from a metal sphere and placed on a second sphere that is initially uncharged. Then the electrical potential energy associated with the two spheres is found to be 7.2x10^-2 Joules. What is the distance between the two spheres?

I figured that one metal sphere would then end up with a charge of 10^13 electrons (so I multiplied that by 1.60x10^-19 to get the charge of all the electrons in the sphere). But there is no charge given for the second sphere! I was using the equation:

(q1*q1/r)Coloumb Constant (8.99*10^9) = Energy

Nothing seems to work. Even using the same value for q1 and q1 doesn't produce the correct answer, which the book feels is 0.32 meters.

I'm stuck... thanks so much for any help!

 

[cookiemonster]

Try making the assumption that the sphere that lost the electrons was initially neutral in charge. Don't forget to convert electrons to coloumbs twice, once for each charge!

cookiemonster

 

[Embermage]

Thank you very much... it worked like a charm.