Electric Field Energy Density: Plates of Q & A

[justin15501]

Homework Statement

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.

Homework 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.

The Attempt at a Solution

Any help is appreciated!

 

[Charles Link]

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

 

[justin15501]

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

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.

 

[Charles Link]

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.

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

 

[justin15501]

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

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

 

[Irene Kaminkowa]

What is the distance between plates?

 

[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.

 

[justin15501]

What is the distance between plates?

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.

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

 

[Irene Kaminkowa]

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

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

 

[Charles Link]

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

 

[Irene Kaminkowa]

My answer is
5.085⋅10⁵ J/m³?

Regards to your teacher )

 

[justin15501]

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

My answer is
5.085⋅10⁵ J/m³?

Regards to your teacher )

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!