The two fields , E and B are at right angles to each other and at right angles to the velocity of the charge. With this arrangement the force due to E and B are opposite to each other . If the magnitude of the force is equal , there will not be any resultant force . The charge will be undeflected. The velocity of this charge v=E/B. ( FE = FB)
If many charges of different velocities enter this region , you therefore can select a desirable velocity by fixing up a value of E and B. This process is called velocity selection.
