# 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

@justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.

11. Jul 24, 2016

### Irene Kaminkowa

5.085⋅105 J/m3?

Regards to your teacher )

12. Jul 24, 2016

### justin15501

Well the way it works is my professor goes over this packet of problems (including this one) the day before the exam and leaves us to figure out the questions we got wrong on our own. And there's 165 questions in this packet. It's not a very ideal situation. If I get something wrong, it's not like I can go to him for help. I'm left to rely on the kind folks of this forum to help me however they can!