Electric Field Energy Density: Plates of Q & A

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Discussion Overview

The discussion revolves around calculating the energy density of the electric field between two large non-conducting plates with equal but opposite charges. Participants are attempting to solve a homework problem that lacks certain information, specifically the distance between the plates, which is critical for determining the energy density.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states the problem involves two plates with a surface area of A = 0.025 m² and charge Q = 75 µC, seeking the energy density.
  • Several participants express uncertainty about the calculations, with one noting a discrepancy between their answer and the teacher's provided answer of 1.7 x 10^7.
  • Another participant suggests that the teacher's answer may be incorrect, as their own calculations yield a different result.
  • Participants discuss the formula for energy density and mention the need for the distance between the plates to complete the calculation.
  • One participant shares a derived formula for energy density involving energy, volume, and distance, but notes that the problem does not specify the distance.
  • Another participant expresses frustration with the teaching method, indicating that they cannot seek help directly from the professor due to the nature of the homework packet.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct answer or the validity of the teacher's provided answer. There is ongoing uncertainty regarding the calculations and the missing information about the distance between the plates.

Contextual Notes

The discussion highlights limitations due to the lack of information about the distance between the plates, which is essential for calculating energy density accurately. Participants also express varying levels of confidence in their calculations and the teacher's guidance.

justin15501
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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


316fbdf3df98834d648106f0d95a8bd5.png

Any help is appreciated!
 
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Try again. A=.025 The rest looks correct.
 
Charles Link said:
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.
 
justin15501 said:
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.
 
Last edited:
Charles Link said:
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?
 
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.
 
Irene Kaminkowa said:
What is the distance between plates?
Irene Kaminkowa said:
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.
 
justin15501 said:
The problem doesn't say the distance between the plates.
But have it in your mind.
Derive the final formula.
 
  • #10
@justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.
 
  • #11
My answer is
5.085⋅105 J/m3?

Regards to your teacher )
 
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  • #12
Charles Link said:
@justin15501 Please let us know if your teacher agrees with your answer. I do think you computed it correctly.
Irene Kaminkowa said:
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!
 

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