A particle of positive charge Q is fixed at point P. A second particle of mass m and negative charge -q moves at constant speed in a circle of radius r1, centered at P. Derive an expression for the work W that must be done by an external agent on the second particle to increase the radius of the circle of motion to r2. (Use epsilon_0 for 0, r1 for r1, r2 for r2, and Q, m, and q as necessary.) I know the answer to be (Qq/8pi*episolon_0)(1/r1-1/r2) I know that i use the equation w=k*q1q2/r and i believe r to be (1/r1-1/r2). I do not understand how to get 8 though could someone please help!!!