A solid copper sphere whose radius is 1.0cm has a very thin surface coating of nickel. Some of the nickel atoms are radioactive and emit an electron with each decay. Due to the geometry of the situation, half of these electrons enter the copper sphere, each carrying away a charge of -e. The nickel coating has an activity of 10mCi(millicuries) = 3.70 x 10^8 radioactive decays per second. The sphere is hung from a long, nonconducting string and isolated from its surroundings. How long will it takefor the potential of the sphere to increase by 1000V? While it is easy to work out the net charge of copper = the no. of -e decayed, and the energy gained by copper, I don't really know what they meant by potentials of the sphere in this case. Since potential has unit of Joules / Coulomb, does it mean that in this case, Potentials = Total Energy Gain / Total charge of the sphere??