Crossed E and B fields  "Velocity Selector"

     This physlet illustrates Crossed Electric and Magnetic fields (a "velocity selector").
     Assume there are two parallel horizontal plates (at the top and bottom of the window shown on the right) - thus there is a vertical electric field. If Ey is positive, the electric field points North (think of the top plate as being negative and the bottom plate being positive), if Ey is negative, the electric field points South (and the charges on the plates are reversed).
     There is a magnetic field perpendicular to the screen - if Bz is positive, it is out of the screen (and you see a "circle and a dot" as though you are looking at the tip of an arrow), if Bz is negative, it is into the screen (and you see the "cross" as though you were looking at the back end of the arrow .. the fletching!).
     Two charges enter from each side, with the magnitude of the velocity you give below. The Positive charges are green, the negative charges are Red.
     You can change the initial speed of the charges, the magnetic field strength and the electric field strength, the magnitude of the charges (the signs are already chosen as +/- on each side), and the mass of the charges.

Suggestions :
   a) Set Ey to zero to look at only B field effects
   b) Set Bz to zero to look at only E field effects