The 'zero total charge' option.


This option usually gives improved accuracy for the calculated potentials and fields.  

Its default value is 'on', unless the program asks for it to be disabled, and then an action is still available to the user, see below.  

It  is accessed in databuilder\segments\advanced options.

With this option the total charge Q of the whole system of electrodes is made zero by applying a constant potential offset Voff  to all the electrodes. This offset is added back again to all potentials before they are displayed. The user will therefore not be aware that the offset is being applied.

The option is not used by the program if there is only one applied voltage, nor if the voltages are antisymmetric with respect to reflection in any plane.

When the total charge Q of the system is zero then the monopole field is also zero and the potential at large distances d falls of as1/d2 or faster.

Suppose for example that all the electrodes of an unenclosed system are given the same positive voltage V.  Then although the potential at the electrodes is V, it tends to be less than V between the electrodes.  In effect the fall-off of potential outside the system is also present to some extent inside the system.  This 'penetration' is minimised by adding the offset Voff to make Q zero, thus minimising the external field.  Therefore a zero value of the total charge Q usually gives the best results in the Boundary Element Method.  These remarks do not apply of course to a system that is completely enclosed.

Therefore when the program states that the zero total charge option should be disabled, then improvements to the fields should be expected by enclosing the system, preferably by an electrode at the mean voltaage of the inner electrodes, as far as that can be estimated. 


Times when the option should not be used:

(1) When potentials, fields or rays outside a system of electrodes are being investigated, because otherwise the outer potential will tend to a non-zero constant and the outer field will be zero or small. In practice it is usually inadvisable to have an 'open' unbounded system (that is, it is better to enclose the system with extra electrodes) if rays are to be traced in the external region.

(2) When the field in or near a grounded region is critical but a large voltage exists elsewhere in the system, because the voltage offset could then be large and so perhaps harmful.

(3) When there is a cathode and the potentials near the cathode are critical (which they usually are) and the anode voltage is very large compared with the potential differences near the cathode.

 For oscillatory time-dependent voltages the potential offset is calculated for the basci voltage v0 and is not re-calculated as the voltages change.

The zero total charge option is not possible for user-defined time-dependent voltages, nor when the voltages are related to each other.


The option can be disabled in databuilder/segments/.