# Homework Help: Finding Electric Energy Density

1. Mar 8, 2007

### stylez03

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

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

3. 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?

2. Mar 8, 2007

### Dick

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

3. Mar 8, 2007

### stylez03

The only equation from this chapter that I could find that would be relevant and give you E would be:

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

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

4. Mar 8, 2007

### Dick

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.

5. Mar 8, 2007

### stylez03

E = sigma / e_o ?

6. Mar 8, 2007

### Dick

Nope. E=??/r^2. What's the numerator?

7. Mar 8, 2007

### stylez03

ooh $$E = \frac {KQ} {R^{2}}$$