(A): V=10V, f=50Hz sin-wave.
(B) (C) (D): V=9V, f=50Hz square-wave.
Particles move away from the turning points if the angle >180. This also
indicate that particles move to the turning point if the angle
< 180. Black edges of (B), (D) are electrodes.
background: f=50Hz, square-wave, V=9V.