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## Homework Statement

Find the electric-field energy density at a point which is a distance of 10.0 cm from an isolated point charge of magnitude 7.30 nC.

## Homework Equations

[tex] u = \frac {1} {2} * e_{o} * E^{2} [/tex]

## The Attempt at a Solution

[tex] u = \frac {1} {2} * 8.85*10^{-12} * 7.30^{2} [/tex]

This was the equation we learned in class, and the online program say's its wrong. From the book it says that factors of A and d cancel out because of geometry, though I'm second guessing now, since the book says the equation is only in a vacuum, but this problem doesn't state either or?