A velocity selector consists of a charged particle passing through crossed electirc magnetic fields. The forces exerted by these fields are in opposite directions, and only particles of a certain velocity will move in a straight line. In the following, disregard the magnitudes of the fields. In a velocity selector, the particles move toward the east and the magnetic field is directed to the north. What direction should the electric field point? Use the right hand rule the B-field force is into the page, so the elctric field force should be out of page. But we don't know the particle's charge how do we determine the direction of the electric field then?