A magnetic field exists in the region and in the region where is a positive constant. A positive point charge moving with a velocity where is a positive constant, enters the magnetic field at . The trajectory of the charge in this region can be like [2007]

(1)
Use the vector form of and in the formula to get the instantaneous direction of force at and .
In the region force must be in the direction i.e., -direction, so vertically upward. And in the region , in -direction vertically downward.