The force is q(VxB) It means that v is perpendicular to the force. Therefore the work done is zero. It means that change in kinetic energy is zero. Hence there is no change in the velocity of the particle.

 

Yes the path is of uniform radius provided the magnetic field is constant in that region.

 

Yes . R=mv/qb. If R(radius) is within the range of the field it will complete the circle.

 

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