Electric Potential find the potential at its center

[lemaire]

Homework Statement

A thin plastic rod 15 cm long carries 4.2 nC distributed uniformly over its length.
(a) If the rod is bent into a ring, find the potential at its center.
(b) If the rod is bent into a semicircle, find the potential at the center (i.e., at the center of the circle of which the semicircle is part).

Homework Equations

V = KQ/r potential at a point.

The Attempt at a Solution

when bending the rod, how to find the distance r.

 

[cepheid]

when bending the rod, how to find the distance r.

If a 15 cm rod is bent into a circle, then it will be a circle 15 cm in circumference. What is the radius of a 15 cm circumference circle?

 

[bchandler]

Circumference=2(pi)r