How Is Work Calculated When Changing Orbital Radius in an Electric Field?

[jperk980]

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!

 

[mysqlpress]

TE= KE+PE
you forgot calculate the PE
increase the radius = change of TE

 

[Shooting Star]

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!

Show your calculations briefly so that we can take it up from there.