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# Electric problems

**The question relates to the use of the capacitance wizard. There appears to be two methods of calculation of capacitance?**

Wizards are designed to calculate capacitance of the single charged body (using charge C=q/U and energy C=2W/U/U approaches). Both approaches should yield the same result.

**I have not found a way to plot continuous electric field lines.**

There are no E-lines in QuickField. E-vector plot might be useful.

**How to load a conductor with a voltage source for current flow analysis?**

In current flow problems currents flow in the plane of the model. So the current sources and voltages are specified on edges.

**I am trying to simulate the potential distribution and electric field along an insulator string, knowing all the relevant dimensions of my insulator and my intention is to simulate the conditions when this insulator has a dry surface and a DC potential is applied. Is it possible to model this using QuickField, what are the limitations in this example?**

The geometry should be compatible with 2D: plane-parallel or axisymmetric. I believe insulators can be modeled as a body with rotation symmetry. The surface conductivity can be modeled by the thin layer with specific conductivity.

Disc insulator

**How to indicate number of turns for a model coupled with a circuit for current transformer... since you can not change conductivity and make it zero. How you show in the model?**

There are two options:

1. Draw each turn as is. Then you can calculate induced voltage and attach electric circuit.

2. Replace the winding with equivalent current. You can calculate the magnetic field, but there is no way to attach external circuit.

QuickField.com > Support > Glossary> Winding modeling

**I have two metal electrodes in the dielectric, and I have to show the tangential field with Quickfield.**

To show tangential field: View > Field Picture settings > Et or En. The same you can see on XY plot along path or in the table.

**I can plot many parameters but not charge density. Am I overlooking something, or is this a limitation of the software?**

Maxwell's law says Integral (D*ds) = q. So Dn = dq / ds. The normal component of the electric field displacement is what you need.

**The plates of the capacitor are parallel but unequal in size. The trouble I am having has to do with unequal charges on the two plates. How could I calculate capacity?**

Please specify the +q and -q on plates and measure the voltage difference U. The capacitance is C = q / U.

**I want to have a 120 V/m constant strength in an edge which represents the earth and I don't know how can I get it?**

We can consider the Earth as a charged conducting spherical body. The E vector points out from the center of the sphere. So it is normal to the surface of the Earth. For this case you can specify Dn = epsilon * En. We usually use U=0 condition to model the earth.

**What value of epsilon should I input the metal electrode?**

In electrostatic problems there is no electric field in conductors. You do not have to label it and build the mesh there. Please specify the potential on conductors boundary instead.