### Author Topic: electric field modelling for capacitor  (Read 29453 times)

#### sb2009

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• Posts: 4
##### electric field modelling for capacitor
« on: May 11, 2005, 06:54:55 am »
I have a parallel plate capacitor arrangement. Each of the plates is coated with a thin insulating layer. Through the gap between the two plates, a conducting fluid flows. If I apply an AC voltage accross the plates, how would the e-field be distributed accross the arrangement. Please let me know soon. Any help would be appreciated.

Also, i would welcome suggestions on how to model and simulate this arrangment (using which tool and how ?)

This is urgent.

Thanks
SB

#### Fu-Kwun Hwang

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##### topic165
« Reply #1 on: May 11, 2005, 08:43:57 am »
If the response time for the conducting fluid is fast enough so that the electric fiele is zero inside it all the time, then you just have a capacitor with very thin separation layer.
It does not matter using which tools to model. What is important is your assumption and physics model about the system.

#### sb2009

• Newbie
• Posts: 4
##### topic165
« Reply #2 on: May 11, 2005, 08:49:23 pm »
Thanks for the quick response. I am not sure about the response time of the conducting fluid (conductivity ~5 S/m). I'll be using salt water at high concentrations (~ 3.5%). I presume the response time is going to be moderately high. Well, in the event that the field in the gap where the salt water flows is zero, the separation between the plates should not ideally affect the capacitance of the capacitor. Am I right in making this assumption ?
Correct me if I'm wrong.

I would like to have a list of tools that may help me to model this. I have little clue about emag modelling softwares.

Appreciate your help
Thanks

#### Fu-Kwun Hwang

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##### topic165
« Reply #3 on: May 12, 2005, 08:00:15 am »
If the separation between the plate is much smaler than the width/height of the plate (ideal capacitor) and the field in the gap where the salt water flows is zero, then the capacitance is not affect by the separation between plates. Do not forget about the basic assumptions!

#### sb2009

• Newbie
• Posts: 4
##### assumption
« Reply #4 on: July 24, 2005, 12:29:54 am »
Hello Hwang
In saying that the gap between the coated plates does not matter (when the conducting solution flows through it) we are making an assumption that the solution is acting as a perfect conductor. However, if you compare the conductivities of the two, the conductivity of the solution is around 5 S/m while the conductivity of a perfect conductor is of the order of 10^10 S/m. Also, this solution has a dielectric constant of 78. Would it not contribute to any capacitance. The structure is:

M | I | sea water | I | M

where M - metal, I - insulation layer.
An AC voltage of low frequency is applied accross the metals.
Is it possible for you to give me an equivalent circuit model for this system ?

Any help will be appreciated[/img]

#### Fu-Kwun Hwang

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##### topic165
« Reply #5 on: August 04, 2005, 02:52:28 pm »
The response time for the solution : tau= permittivity /conductivity
For your case, if the frequency is less than GHz, the solution can be treated as a
perfect conductor. If the separation is much small than the dimension for each side of the insulator . You can use ideal parallel plate formula to calculate the capacitor of your system.
Or , just use a capacitor meter to measue it directly.