AP Phyisics C E & M: Two Thin Concentric Spheres

[flannabhra]

Homework Statement

Two thin, concentric, conducting spherical shells, insulated from each other, have radii of 0.10 m and 0.20 m, as shown above(Graphic Above). The inner shell is set at an electric potential of -100 V, and the outer shell is set at an electric potential of +100 V, with each potential defined relative to the conventional reference point. Let Q_i and Q_o represent the net charge on the inner and outer shells, respectively, and let r be the radial distance from the center of the shells. Express all algebraic answers in terms of Q_i, Q_o, r and fundamental constants, as appropriate.

Homework Equations

I am having trouble with part (e), the graph. I understand the basic shape of the graph but I am confused as to why the value of E at 0.10 m is -4000N/C.Note: This is an old AP question from the 2012 AP Physics C E & M test.

Thank you for your help!

The Attempt at a Solution

I can't show my attempt to graph..

 

[jbsteele]

but I understand that the graph should look like a parabola with E = 0 at r = 0.20 m and E = -4000 N/C at r = 0.10 m. I do not understand why the value of E is -4000 N/C at r = 0.10 m.