# Modeling of resistance in a rectangular sheet with COMSOL

#### sunshine09

Hi,

I have got a simple problem which makes me struggle.
I have a rectangular thin conductive sheet (2D) of width w, height h
and conductivity suspended in air.
I would like to calculate with COMSOL/Femlab the electrical resistance between one of the corners of the conductive sheet and any other point on the sheet?
I selected the 'conductive media DC' mode in 2D. I am not sure what
boundary condition I should choose and how to apply a potential to one
of the corners.
Since the sheet is suspended in air I thought of putting isolating
boundary conditions on all 4 sides, but then how will I apply the
potential to the corner? Somehow it does not give meaningful results.

Your help is greatly appreciated.

David

Related Electrical Engineering News on Phys.org

#### marcusl

Gold Member
I don't know COMSOL so I can't help with your numerical simulation. I can tell you that the potential becomes singular at the infinitesimal points where your are introducing/removing current. Suggest using finite-sized electrodes to make the numerical solver better behaved. A small circular electrode on the plate and a semi-circular electrode at the corner, both small compared to the rectangle but enclosing multiple meshes, should do it.

#### Nick Bruno

you should be able to set the potential along the boundaries. Make sure youve got a ground somewhere on ther, and have you tried a point condition?

#### sunshine09

@Marcusl: Thanks for the suggestion. makes sense to me.

@Nick: Yes, it is possible to set the potential along a boundary. I could for example define a small square electrode whose boundary I set to a potential of say 5V. I am also able to set the border of the big rectangle to isolating. Now I still need to get a ground somewhere, but where. I think if I want to calculate the resistance between one corner and a point (X,Y) then I have to set the corner to 5V and this point (X,Y) to ground and then somehow calculate the current. But this calculation I would have to code in a script and repeat 10000 times, if I assume a 100 x 100 grid.
I am wondering if there is a faster solution.
I have looked through all Femlab examples and could not find the solution.

Thanks

#### Nick Bruno

Try and contact Mina with Comsol on their website. shes a big help.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving