# Finding Electric Energy Density

## 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

$$u = \frac {1} {2} * e_{o} * E^{2}$$

## The Attempt at a Solution

$$u = \frac {1} {2} * 8.85*10^{-12} * 7.30^{2}$$

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?

## Answers and Replies

Dick
Science Advisor
Homework Helper
First you want to know what the value for E is in that situation. What formula might you use for that?

First you want to know what the value for E is in that situation. What formula might you use for that?
The only equation from this chapter that I could find that would be relevant and give you E would be:

$$V_{ab} = E*d$$

$$E = \frac {V_{ab}} {d}$$ ?

Dick
Science Advisor
Homework Helper
It's the electric field around a POINT charge. It HAS to be one of the first formulas you encounter. Hint: it's an inverse square law. Keep looking.

It's the electric field around a POINT charge. It HAS to be one of the first formulas you encounter. Hint: it's an inverse square law. Keep looking.
E = sigma / e_o ?

Dick
Science Advisor
Homework Helper
E = sigma / e_o ?
Nope. E=??/r^2. What's the numerator?

Nope. E=??/r^2. What's the numerator?
ooh $$E = \frac {KQ} {R^{2}}$$

I was thinking about this one but I thought you just meant in this chapter.

Q = 7.30 * 10^-19 ?

Dick
Science Advisor
Homework Helper
ooh $$E = \frac {KQ} {R^{2}}$$

I was thinking about this one but I thought you just meant in this chapter.

Q = 7.30 * 10^-19 ?
You don't have units on Q. I can't tell you if it's right. It's given as 7.30 nC. How many coulombs is that?