A charged particle is projected with velocity V in a region where both electric field E and magnetic field B exist  If the charged particle passes undeviated , then

If  E→,B→ and V→ are parallel, then magnetic force FB→=aV→×B→=0. So the particle will continue to move along E with increasing speed. If  E→,B→ and V→ are mutually perpendicular, then QE→+QV→×B→=0 ⇒v= E/B.