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Point Charge

Posted Dec 21, 2011, 6:34 p.m. EST Low-Frequency Electromagnetics, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2a 10 Replies

Bahar
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Hello all

I want to define point charges (lets say some circles as positive Or negative charges) in Comsol. Can anybody give me some idea how i can do this? what I have in my mind is:

define a constant as Q and equal that to a number (like 1) and then place Q or -Q as the value in space charge density when defining material properties.

However I face in to some problems:

1- I cam only find surface charge density (and not space) in 3d.
2- when i choose Q as surface charge density it seems it doesn't work

anybody have any idea?

Bahar
10 Replies Last Post Dec 18, 2013, 12:08 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 22, 2011, 3:53 a.m. EST
Hi

I'm not sure I get it fully in my 4.2a "ES" physics I have,
for 3D:
Space, Surface, Edge and Point charge (density) nodes to apply,

and in 2D:
Space, Surface and Line charge (density) nodes for domain, boundaries and points,
the change in dimension is due to the default 1[m] depth of any 2D model (as 2D is in fact 3D but with a default "depth")

--
Good luck
Ivar
Hi I'm not sure I get it fully in my 4.2a "ES" physics I have, for 3D: Space, Surface, Edge and Point charge (density) nodes to apply, and in 2D: Space, Surface and Line charge (density) nodes for domain, boundaries and points, the change in dimension is due to the default 1[m] depth of any 2D model (as 2D is in fact 3D but with a default "depth") -- Good luck Ivar
Bahar
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Posted: 1 decade ago Dec 23, 2011, 3:49 p.m. EST
Dear Ivar

I used those option but it seems something is wrong!
Lets say I have 2 charges (one negative and one positive) and I want to use arrow plots to show that they are attracting to each other.

I assigned a sphere as a charge and on each sphere i define charge points with positive and negative values. is that correct?

But i can't see any results!!

Do you know any examples that they defined charges on simulation?


Dear Ivar I used those option but it seems something is wrong! Lets say I have 2 charges (one negative and one positive) and I want to use arrow plots to show that they are attracting to each other. I assigned a sphere as a charge and on each sphere i define charge points with positive and negative values. is that correct? But i can't see any results!! Do you know any examples that they defined charges on simulation?
Alexander Kuznetsov
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Posted: 1 decade ago Dec 23, 2011, 6:06 p.m. EST
One way to define charge is through potential. This model might be helpful: www.comsol.com/showroom/gallery/211/
One way to define charge is through potential. This model might be helpful: http://www.comsol.com/showroom/gallery/211/
Bahar
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Posted: 1 decade ago Dec 28, 2011, 2:13 p.m. EST
Thanks a lot!
That link and Ivar's descriptions helped me a lot and now the problem is solved.

Best
Bahareh
Thanks a lot! That link and Ivar's descriptions helped me a lot and now the problem is solved. Best Bahareh
Srujan Mishra
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Posted: 1 decade ago Feb 6, 2013, 3:21 p.m. EST
I am also trying to simulate a simple two point charges, with one +ve and other -ve charges. In 2D model, I tried applying surface charge density, space charge density and line charge (out-of-plane). I do not seem to be able to get it to work. Computing the problem always returns "failed to find a solution; returned solution is not converged".

It seems like we need to specify the space between the two points as "air".

Any suggestions, please? Thank you.
I am also trying to simulate a simple two point charges, with one +ve and other -ve charges. In 2D model, I tried applying surface charge density, space charge density and line charge (out-of-plane). I do not seem to be able to get it to work. Computing the problem always returns "failed to find a solution; returned solution is not converged". It seems like we need to specify the space between the two points as "air". Any suggestions, please? Thank you.

Attachments:

Srujan Mishra
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Posted: 1 decade ago Feb 6, 2013, 3:21 p.m. EST
I am also trying to simulate a simple two point charges, with one +ve and other -ve charges. In 2D model, I tried applying surface charge density, space charge density and line charge (out-of-plane). I do not seem to be able to get it to work. Computing the problem always returns "failed to find a solution; returned solution is not converged".

It seems like we need to specify the space between the two points as "air".

Any suggestions, please? Thank you.
I am also trying to simulate a simple two point charges, with one +ve and other -ve charges. In 2D model, I tried applying surface charge density, space charge density and line charge (out-of-plane). I do not seem to be able to get it to work. Computing the problem always returns "failed to find a solution; returned solution is not converged". It seems like we need to specify the space between the two points as "air". Any suggestions, please? Thank you.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 6, 2013, 3:25 p.m. EST
Hi

indeed you need to mesh the domain around your point charges, and give them some material info, but vacuum or air is not that much dfferent, but still its not the same as a "void"

By the way in 2D a "point charge", is in fact a long vertical wire along Z charged to a given value in Coulombs per meter, since 2D is a representation of 3D with an invariant along Z the out of paper direction

--
Good luck
Ivar
Hi indeed you need to mesh the domain around your point charges, and give them some material info, but vacuum or air is not that much dfferent, but still its not the same as a "void" By the way in 2D a "point charge", is in fact a long vertical wire along Z charged to a given value in Coulombs per meter, since 2D is a representation of 3D with an invariant along Z the out of paper direction -- Good luck Ivar
Srujan Mishra
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Posted: 1 decade ago Feb 7, 2013, 1:43 a.m. EST
Thanks, Ivar. Your advise was helpful. Attached is an image of what I wanted to achieve. Thanks!

Thanks, Ivar. Your advise was helpful. Attached is an image of what I wanted to achieve. Thanks!

Attachments:

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 7, 2013, 7:33 a.m. EST
Hi

you should manage, in 2D try adding under "point" the line charge and give a + and a _ sign that should work. I'm off line for a weeks time so I cannot make an example (I'm not carrying my COMSOL WS all over Europe ;)

--
Good luck
Ivar
Hi you should manage, in 2D try adding under "point" the line charge and give a + and a _ sign that should work. I'm off line for a weeks time so I cannot make an example (I'm not carrying my COMSOL WS all over Europe ;) -- Good luck Ivar
Emil Joseph
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Posted: 1 decade ago Dec 18, 2013, 12:08 a.m. EST
Hi,

I was interested in finding image force on a charged particle near a conducting rod in comsol. Therefore as an initial check i simulated a charge near a conducting plate and compared the resulting image field to the field from analytic solution of charge on a infinite conducting plane. The results in these two case doesnt seem to match even if i increase the size of the conducting plate so that it appprox. an infinite plate.

Can you help me solve this discrepancy? Am i doing something wrong here?

Thanks,
-Emil
Hi, I was interested in finding image force on a charged particle near a conducting rod in comsol. Therefore as an initial check i simulated a charge near a conducting plate and compared the resulting image field to the field from analytic solution of charge on a infinite conducting plane. The results in these two case doesnt seem to match even if i increase the size of the conducting plate so that it appprox. an infinite plate. Can you help me solve this discrepancy? Am i doing something wrong here? Thanks, -Emil

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