# Homework Help: Energy density

1. Jul 23, 2016

### justin15501

1. The problem statement, all variables and given/known data
Two large non-conducting plates of surface area A=.025m^2 carry equal but opposite charges Q = 75microC. What is the energy density of the electric field between the two plates.

2. Relevant equations
I wrote the equations on my attempt. This was a multiple choice problem and the correct answer he gave us was 1.7*10^7 but he's been wrong before.

3. The attempt at a solution

Any help is appreciated!

2. Jul 23, 2016

Try again. A=.025 The rest looks correct.

3. Jul 23, 2016

### justin15501

The new answer I get is 508475 instead of 5085. I feel like I may be using a wrong equation or something? My teacher didn't really lecture on energy density at all so I'm a bit uncertain.

4. Jul 23, 2016

I think you got the rest correct. I think your teacher's answer of 1.7E+7 may be incorrect.

Last edited: Jul 23, 2016
5. Jul 23, 2016

### justin15501

My answer isn't even one of the multiple choices?

6. Jul 24, 2016

### Irene Kaminkowa

What is the distance between plates?

7. Jul 24, 2016

### Irene Kaminkowa

Energy density is
ω = W / V = E / (Ad)
where W - energy, V - volume, d - distance between the plates.

Capacity of the system (actually, we consider a capacitor)
C = ε0A/d

Energy
W = Q2/2С

Hope, it is enough to find the solution.

8. Jul 24, 2016

### justin15501

The problem doesn't say the distance between the plates.

9. Jul 24, 2016

### Irene Kaminkowa

But have it in your mind.
Derive the final formula.

10. Jul 24, 2016

11. Jul 24, 2016

### Irene Kaminkowa

5.085⋅105 J/m3?