## Electrostatic Potential in Air-Filled Frame

This example shows how to find the electrostatic potential in an air-filled annular quadrilateral frame by using the unified workflow.

The PDE governing this problem is Poisson's equation

–∇ · (*ε*∇*V*) =
*ρ*.

Here, *ρ* is the space charge density, and *ε* is the
absolute dielectric permittivity of the material. The toolbox uses the relative permittivity
of the material *ε _{r}*, such that

*ε*=

*ε*

_{r}*ε*, where

_{0}*ε*is the absolute permittivity of the vacuum. The relative permittivity for air is 1.00059. Note that the permittivity of the air does not affect the result in this example as long as the coefficient is constant.

_{0}Assuming that there is no charge in the domain, Poisson's equation simplifies to the
Laplace equation: Δ*V* = 0. For this example, use these boundary conditions:

The electrostatic potential at the inner boundary is 1000 V.

The electrostatic potential at the outer boundary is 0 V.

Create a finite element analysis model for electrostatic analysis. Include a geometry of a frame.

model = femodel(AnalysisType="electrostatic", ... Geometry="Frame.STL");

Plot the geometry of the frame with edge labels.

`pdegplot(model.Geometry,EdgeLabels="on");`

Specify the vacuum permittivity value in the SI system of units.

model.VacuumPermittivity = 8.8541878128e-12;

Specify the relative permittivity of the material.

```
model.MaterialProperties = ...
materialProperties(RelativePermittivity=1.00059);
```

Specify the electrostatic potential at the inner boundary.

model.EdgeBC([1 2 4 6]) = edgeBC(Voltage=1000);

Specify the electrostatic potential at the outer boundary.

model.EdgeBC([3 5 7 8]) = edgeBC(Voltage=0);

Generate the mesh. This assignment updates the mesh stored in the
`Geometry`

property of the model.

model = generateMesh(model);

Solve the model. Plot the electric potential distribution using the
`Contour`

parameter to display equipotential lines.

```
R = solve(model);
u = R.ElectricPotential;
pdeplot(model.Mesh,XYData=u,Contour="on")
```