Floating Potential
The Floating Potential node is used when modeling a metallic electrode at floating potential. It applies a constant voltage V0 on the boundary (for domain features, this is the boundary enclosing the selected domain), such that the total normal electric displacement field D equals a specific charge Q0:
where ∂Ω represents the boundary and n refers to the surface normal. The constant boundary voltage implies the tangential electric field equals zero; the electric field will be perpendicular to the boundary:
In case of Q0 = 0 (the default case), the boundary will behave as an unconnected, neutrally charged, good conductor under electrostatic conditions (a floating equipotential). Although locally the displacement field may vary, the total electric displacement field entering or leaving the boundary equals zero.
If the floating potential touches a point, boundary, or domain feature that is not floating (a Terminal or Ground feature), the floating potential will acquire that feature's potential. If the floating potential is set to a certain charge, or connected to a circuit, it behaves like a terminal.
Floating Potential
The Floating potential identifier text area shows the unique identifier for the floating potential feature. It is used to identify the global variables created by the node, such as the voltage. The Floating potential group check box in the Settings window for Floating Potential controls how potentials are assigned to boundary segments in the feature’s boundary selection. If this check box is not selected, a single potential is applied to all boundaries in the selection. If the check box is selected, each group of contiguous boundaries in the selection is given a unique potential. This simplifies the setup of models with many floating electrodes. The values of the potential at each group of boundaries are then made available in postprocessing, collected in a vector variable.
The following options are not available if the Floating potential group check box is selected. Select a specification for the Electric charge — choose User defined to specify a total Charge Q0 deposited on the surface. Select Circuit to connect the floating potential to an Electrical Circuit. Enter a value for the Contact work function Φc (SI unit: V). The default is 4 V.
The Electrical Circuit interface requires a current from the Floating Potential, so the Circuit terminal type can only be used in Time Dependent or Frequency Domain studies, in which the current can be computed as the time derivative of the charge.
Initial Values
The initial values subsection is used to provide initial values for the internal state variables in the Floating potential. These settings can be useful to provide an initial guess to the nonlinear solver or the initial value in a time-dependent study.
Enter an Initial value for voltage Vinit.
The Initial value for charge is available for the Circuit excitation. It specifies the initial charge on the floating potential when the circuit is connected. Enter an Initial value for charge Qinit.
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.
Settings and Properties Windows for Feature Nodes in the COMSOL Multiphysics Reference Manual