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## Homework Statement

Two charges concentric spheres have radii of 10.0 cm and 15.0 cm. The charge on the inner sphere is 4.00 x 10 ^-8 C, and that on the outer sphere is 2.00 x 10^-8 C. Find the electric field (a) at r = 12.0 cm and (b) at r = 20.0 cm.

## Homework Equations

I know that this is a fairly simple problem, but am somewhat confused on which numbers I use. Using this equation:

what should I be using for "Q"? At r = 12.0 cm do I use 4.00 x 10^-8 and at r = 20.0 cm do I use 2.00 x 10^-8? Or am I adding the two charges together to get a total charge, since both charges affect each point.

Thanks!

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