a -3.0 nc charge q is initially at rest a distance of 15 cm ( r2 ) from a 5.0 nc charge q fixed at the origin. naturally, the coulomb force accelerates q towards q, eventually reaching 10 cm ( r1 ). what is the work done by the electric field between r2 and r1? answer must be in the unit of nj (nano joule or 10-9j)