Q.

A magnetic field B→=B0j^, exists in the region a<x<2a and B→=-B0j^, in the region 2a<x<3a, where B0 is a positive constant. A positive point charge moving with a velocity v→=v0i^, where v0 is a positive constant, enters the magnetic field at x=a. The trajectory of the charge in this region can be like              [2007]

1  
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Ans.

(1)

Use the vector form of B→ and v→ in the formula F→=q(v→×B→) to get the instantaneous direction of force at x=a and x=2a.

In the region a<x<2a force must be in the direction i^×j^ i.e., +z-direction, so vertically upward. And in the region 2a<x<3a, in -z-direction vertically downward.