Animating field lines

The teaching of time-dependent electromagnetic phenomena could be made much easier if the evolution in time of the field lines could be visualized. In principle this can be achieved on a computer screen, but it is not a simple thing to do, since the process of drawing field lines is time consuming, and this rules out interactive animated presentations. In this work we restrict the problem to the representation of a superposition of uniform and dipolar fields and optimize the process making it extremely fast and precise. Special care is taken to ensure that the line density is everywhere proportional to the field intensity. The process is implemented as a Pascal procedure and exemplified by an animated representation of the field lines being expelled from a hollow conducting sphere which is suddenly immersed in a uniform field.