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# Re: [FEMM] Doubt in BELA

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- Subject: Re: [FEMM] Doubt in BELA
- From: David Meeker <dmeeker@ieee.org>
- Date: Tue, 17 Feb 2004 17:35:38 -0500
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`nicholas lim wrote:`

> `Dear Meeker,`

>     i found some doubt in BELA, so i hope u can explain to
>     me. The doubltful that i found is in the femmplot the
>     electric field intensity is (V/m)and how we define the
>     distance(m) which is using to calculate the E.

> `thank you.`
> `Regard,
> Nicholas
> `

`Electric field intensity is defined as:`
`E = - grad(V)`
`The solver solves for V, so the program has to take derivatives of V to get E. There isn't any one distance that is used to calculate E. To get a smooth E field (e.g. for plotting purposes), the program obtains nodal values of E for each element and interpolates linearly between the nodal values to get the E field at other points inside the element. Nodal values of E are determined by identifying a best fit plane that most closely (in a least-squares sense) passes through the node of interest and all other nodes that are neighbors of that node (i.e. all nodes that are in elements that also contain the node of interest). The gradient of that best-fit plane is then taken as the nodal value of E. However, the program does subtly different things in the plethora of special cases associated with boundaries, corners, interfaces between materials, isolated charges or line conductors, etc, etc.`
`Dave.`

- References:
  - [FEMM] Doubt in BELA
    - From: nicholas lim <virusxmenz@yahoo.com>

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