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Weird plate capacitor

  • Thread starter shomey
  • Start date
  • #1
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Homework Statement



I have a capacitor which i just like a plate capacitor, but instead of the upper electrode being a plate it is a point charge Q. the lower plate is held at a fixed potential V=0.

I am supposed to calculate the capacitance of the capacitor.


The Attempt at a Solution



I've used the image method with a negative charge (-Q) at the same distance from the lower plate (forcing V=0 on this plate).
now I wanted to calculate the potential difference between the "plates", but it is obvisously inifinite since as we get closer to the point charge the potential grows like 1/x^2.
and thus C = Q/delta_V = 0
what am I missing?

thanks :)
 

Answers and Replies

  • #2
30
0

Homework Statement



I have a capacitor which i just like a plate capacitor, but instead of the upper electrode being a plate it is a point charge Q. the lower plate is held at a fixed potential V=0.

I am supposed to calculate the capacitance of the capacitor.


The Attempt at a Solution



I've used the image method with a negative charge (-Q) at the same distance from the lower plate (forcing V=0 on this plate).
now I wanted to calculate the potential difference between the "plates", but it is obvisously inifinite since as we get closer to the point charge the potential grows like 1/x^2.
and thus C = Q/delta_V = 0
what am I missing?

thanks :)
someone? even if you are not sure I will be glad to hear what you think...
 
  • #3
Dick
Science Advisor
Homework Helper
26,258
618
I looks to me like you are right. If one of the plates is an ideal point, you are going to have a really hard time shoving any charge into that capacitor. I'll vote for C=0.
 
  • #4
30
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I looks to me like you are right. If one of the plates is an ideal point, you are going to have a really hard time shoving any charge into that capacitor. I'll vote for C=0.
thanks.
I've wrote in email to my lecturer asking about it.
I'll let you know how it goes...
 
  • #5
30
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thanks.
I've wrote in email to my lecturer asking about it.
I'll let you know how it goes...
Okey so my lecturer had answered to use a small radius spherical electrode instead of a point charge.
actually I'm pretty confused, so I'll try to work on it for a little while and let you know if I made some progress and/or have any questions.

thanks for the help!
shomey
 
  • #6
30
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Okey so my lecturer had answered to use a small radius spherical electrode instead of a point charge.
actually I'm pretty confused, so I'll try to work on it for a little while and let you know if I made some progress and/or have any questions.

thanks for the help!
shomey
I solved the question, it's not that difficult after all - just a lot of calculations.
solution is attached if you are interested :) if you see an error i would love to know about it..

now - supposed I know the capacitance and the force felt by the upper electrode, and I know all dielectric coefficients eps_k, could I derive the dielectric layers' lengths??

I can see only two equations, thus missing (N-2) to solve the problem, but I was told it is possible...
 

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  • #7
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I solved the question, it's not that difficult after all - just a lot of calculations.
solution is attached if you are interested :) if you see an error i would love to know about it..

now - supposed I know the capacitance and the force felt by the upper electrode, and I know all dielectric coefficients eps_k, could I derive the dielectric layers' lengths??

I can see only two equations, thus missing (N-2) to solve the problem, but I was told it is possible...
I've made progress, I'm stuck with 6/8 equations, could you think about anything I might have missed??? thanks!
 

Attachments

  • #8
30
0
I've made progress, I'm stuck with 6/8 equations, could you think about anything I might have missed??? thanks!
I have the feeling I got the first two sections wrong...
something about my dealing with the dielectrics.
could you have a look and tell me what you think?
 

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