Opposite Charges/Poles: What Form of Energy?

[adamg]

A positive and a negatively charged sphere attract each other and their kinetic energy increases. What form was the energy in orginally? Is it from the work done in separating the spheres originally? If so, when the spheres are separated, what form of energy do they gain from the work done in separating them? (Similarily with North and South poles of magnets?) Hope you can help, thanks *adam*

 

[HallsofIvy]

Yes, total energy is conserved. The energy the two spheres had before they started moving is "Potential Energy". You are correct, it is "from the work done in separating the spheres originally?".
It is exactly the same as if you lifted a 1 kg mass 1 meter in the Earth's gravitational field, giving it "mgh"= 9.8 Joules of (potential)energy. If you then drop that mass down 1 meter, all that potential energy is converted into kinetic energy: (1/2)mv²= 9.8 so
v²= 19.6 and v= 4.4 m/s. Exactly the same thing with charged particles or magnets.