Option for finding electrostatic force on a body.

See also notes on importing charges and test3d29 and xmpl3d85.

This option is accessed from the bottom of the sheet /databuilder/segments/advanced options/.

Click on the option and enter two numbers that define the first and last segments of the body (which might be made up of more than one electrode). The net force on those segments will then be calculated and put at the end of the ray output file. This force is of course a vector and its components are given in SI units (Newtons).

The net force is obtained by summing (essentially integrating) over the forces on the individual segments. These forces are q*f, where q is the segment charge and f is the electrostatic field (at the centre of the segment) due to all the other segments, but excluding the self-field of the segment itself. The reason for excluding the self-field is that it is essentially zero at the segment (although it is finite and in opposite directions on the two sides of the segment surface) -or it could be said that a segment cannot ‘pull itself up by its own boot-straps’.

If you want to find the force between two bodies then find the forces F1 and F2 on each separately. If these bodies are isolated from other fields, then obviously F1 = -F2. On the other hand if an external field gives the same force on each body, then the force between them is (F1 - F2)/2.