The black arrows represent the direction of the electric field produced by the particles.
The green arrows attached to the particles represent the direction of their acceleration.
Controls:
WASD or Arrow keys to move left, right, front and back.
Space and shift to move up and down respectively.
Hold right click to look around and scroll wheel to zoom in and out.
Left click to place a particle at the cursor.
Press the - key before placing a particle to place a negatively charged particle.
And press the + key (no need to hold shift) to place a positive a positively charged particle.
Press the P button to play/pause the simulation.