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# why should for a cap. AREA>>>DISTANCE between the

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#### electronics_kumar

##### Advanced Member level 2
i read from the attachment post

there it is mentioned as area of cap should be greater than its distance between two plates...this is due to fringe effect..

what does really it means.....

Re: why should for a cap. AREA>>>DISTANCE between

Most of the capacitance theory depends on the assumption of 2 infinite planes. However in real world, we cannot have that. If the area is big enough compared to the distance between the plates, then we can use the assumption that we are dealing with 2 infinite planes. This will be true for the center of the plates, with the electric field going vertically from one plate to the other.

As you move out towards the edges, the assumption isn't true anymore. The electric field will begin to bend and the on the edge, the field actually curves out, making the 'fringe' like field.

Re: why should for a cap. AREA>>>DISTANCE between

what kind of sequences we are going to have by means of Fringe effect....

Re: why should for a cap. AREA>>>DISTANCE between

Hi electronics_kumar,

I didn't really find anyone who dealt with this problem, but I can propose you a method of thinking about this problem.

If you don't want to neglect the fringing effect try to draw the field lines using any graphical method (eg. Curvilinear squares) and obtain the capacitance from there. A very comprehensive approach to draw the field lines and use it in capacitance measurements is available at "Engineering Electromagnetics" by Hayt, 6th Edition.

Believe me, You'll like it!! . The results are really artistic and you can use this method further in the study of Semiconductor Devices if you want to visualise the electric field in short channel MOSFETs.

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