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Electric field doesn't change when changing the material parameters
Posted Feb 27, 2017, 4:36 a.m. EST Low-Frequency Electromagnetics Version 4.2 6 Replies
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Hi,
I'm new in COMSOL (version 4.2). I'm trying to simulate the electric field in a cylinder that contains an electrode of 2 poles and an isolation material between them. I have used Electrostatics physics from AC/DC module. The boundary conditions I have used are:
- Zero charge: in all cylinder boundaries. (Because I want zero current in all external boundaries)
- Charge conservation: in all domains by defect.
- Initial values: 0V in all domains by defect.
- Terminal: for each pole (on positive (1V) and the other negative (-1V)).
- Ground: for isolation material.
The cylinder is an isotropic and homogeneous medium. And the parameter COMSOL ask me to introduce is the relative permittivity. To see the electric field I have used streamlines and, to measure the electric potential in a specific zone I have used a 3D line.
The problem I have is that it doesn't matter the value of the relative permittivity of the material I introduce because the electric field and the electric potential never change. In a first simulation the relative permittivity of the cylinder material was 1, and in a second simulation it was 1000.
Can anyone help me and telling me what I am doing wrong?
And I have a another question: is it possible to simulate the electric field but only taking into account the electrical conductivity? I know I can use electric currents physics to simulate the electric field too, but it also ask me to introduce the relative permittivity of the material, and I only know the electrical conductivity.
Thank you in advance,
Carmen
I'm new in COMSOL (version 4.2). I'm trying to simulate the electric field in a cylinder that contains an electrode of 2 poles and an isolation material between them. I have used Electrostatics physics from AC/DC module. The boundary conditions I have used are:
- Zero charge: in all cylinder boundaries. (Because I want zero current in all external boundaries)
- Charge conservation: in all domains by defect.
- Initial values: 0V in all domains by defect.
- Terminal: for each pole (on positive (1V) and the other negative (-1V)).
- Ground: for isolation material.
The cylinder is an isotropic and homogeneous medium. And the parameter COMSOL ask me to introduce is the relative permittivity. To see the electric field I have used streamlines and, to measure the electric potential in a specific zone I have used a 3D line.
The problem I have is that it doesn't matter the value of the relative permittivity of the material I introduce because the electric field and the electric potential never change. In a first simulation the relative permittivity of the cylinder material was 1, and in a second simulation it was 1000.
Can anyone help me and telling me what I am doing wrong?
And I have a another question: is it possible to simulate the electric field but only taking into account the electrical conductivity? I know I can use electric currents physics to simulate the electric field too, but it also ask me to introduce the relative permittivity of the material, and I only know the electrical conductivity.
Thank you in advance,
Carmen
6 Replies Last Post Feb 28, 2017, 11:54 a.m. EST
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Posted:
8 years ago
Hello Carmen,
You are not doing anything wrong. The electric field does not depend on the relative permittivity in this case. One way to see this is to write down on paper the equation for electrostatics as well as the boundary conditions for your model. When you do that, you will see that since the permittivity is spatially-independent it can be taken out of the divergence, which leaves you with an equation for V that does not depend on the relative permittivity. Same thing for your BCs: the permittivity does not appear in them. So in this situation V is independent of the relative permittivity, and as a result so is E, the electric field (-grad(V)).
I do not understand your other question. Either your material is conductive and you should use the Electric Currents interface, or it's not and you should use the Electrostatics interface. The two equations are used to simulate different physical phenomena. Maybe I misunderstand your question or the phenomenon you are trying to simulate?
Best regards,
Jeff
You are not doing anything wrong. The electric field does not depend on the relative permittivity in this case. One way to see this is to write down on paper the equation for electrostatics as well as the boundary conditions for your model. When you do that, you will see that since the permittivity is spatially-independent it can be taken out of the divergence, which leaves you with an equation for V that does not depend on the relative permittivity. Same thing for your BCs: the permittivity does not appear in them. So in this situation V is independent of the relative permittivity, and as a result so is E, the electric field (-grad(V)).
I do not understand your other question. Either your material is conductive and you should use the Electric Currents interface, or it's not and you should use the Electrostatics interface. The two equations are used to simulate different physical phenomena. Maybe I misunderstand your question or the phenomenon you are trying to simulate?
Best regards,
Jeff
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Posted:
8 years ago
Hello Jeff,
I have developed the equations of the electrostatics physics, as you said, and now I understand why the electric field doesn't change in a homogeneus medium, but it does in a heterogeneous one. So, thank you a lot.
In the second question, I didn't specified anything about what I want to do really, so sorry for it. I'm trying to simulating the electric field generated by an electrode with four poles in the spinal cord. Before asking in this forum, I didn't really know the difference between electrostatics and electric currents physics to simulate the electric field, but now I do. Hence, as the spinal cord is made of different conductive materials, I'm using the electric currents physics. And the question was about if I could avoid introducing the relative permittivity because it is difficult to find the value of that parameter of the different materials of the spinal cord. I have seen the equations of the electric currents physics and it seems that it is not possible, because I think it needs the relative permittivity to calculate the electric displacement and, with it, the electric field.
I hope I have explained myself right.
Best regards,
Carmen
I have developed the equations of the electrostatics physics, as you said, and now I understand why the electric field doesn't change in a homogeneus medium, but it does in a heterogeneous one. So, thank you a lot.
In the second question, I didn't specified anything about what I want to do really, so sorry for it. I'm trying to simulating the electric field generated by an electrode with four poles in the spinal cord. Before asking in this forum, I didn't really know the difference between electrostatics and electric currents physics to simulate the electric field, but now I do. Hence, as the spinal cord is made of different conductive materials, I'm using the electric currents physics. And the question was about if I could avoid introducing the relative permittivity because it is difficult to find the value of that parameter of the different materials of the spinal cord. I have seen the equations of the electric currents physics and it seems that it is not possible, because I think it needs the relative permittivity to calculate the electric displacement and, with it, the electric field.
I hope I have explained myself right.
Best regards,
Carmen
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Posted:
8 years ago
Hello Carmen,
As long as you plan on solving the Electric Currents in a stationary way, only the conductivity is needed. The relative permittivity is needed when you do a time-dependent study, see attached screenshot.
Best,
Jeff
As long as you plan on solving the Electric Currents in a stationary way, only the conductivity is needed. The relative permittivity is needed when you do a time-dependent study, see attached screenshot.
Best,
Jeff
Attachments:
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Posted:
8 years ago
Hi Carmen,
this resource: niremf.ifac.cnr.it/tissprop/htmlclie/htmlclie.php
may be useful for your further work on body tissues.
Cheers
Edgar
--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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Posted:
8 years ago
Hello Jeff,
Thank you very much. Your replies have been very useful.
Best regards,
Carmen
Thank you very much. Your replies have been very useful.
Best regards,
Carmen
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Posted:
8 years ago
Hi Edgar,
Thank you a lot for the resource! I think I will need it in the future.
Best regards,
Carmen
Thank you a lot for the resource! I think I will need it in the future.
Best regards,
Carmen