A particle in uniform but non-constant magnetic field Recently I was going through a book on magnetism and it said that " If a moving charged particle enters into a region of uniform magnetic field from
outside, it does not complete a circular path".My views about this statement is that it is wrong.Is this statement correct in case of uniform but non constant magnetic field.If it's wrong can I get an example. 
 A: 
in case of uniform but non constant magnetic field.If it's wrong can I get an example.

This has a contradiction. A non constant magnetic field by the meaning of words cannot be uniform  in space and time.
There should be at least a step function.
A: The first view is right. 
The trajectory of the charged particle will start deflecting from its otherwise straight line path as soon as it enters the region of uniform magnetic field. Hereafter the particle will move in a circular arc. This arc must be tangential to the initial straight line path at the point of entry. If you draw such a circular arc, you will find that a part of the full circle is outside the region of magnetic field.
Regarding the second view, the particle’s path can be fully contained in the region of magnetic field by gradually increasing the field strength and reducing the arc diameter since r = mv/Bq that is r is inversely proportional magnetic field strength. Speed does not change in the process.
A: *

*If the movement is not perpendicular to the magnetic field, the path is a helix. 

*Beside Pont 1, the electron emits photons during its deflection, looses kinetic energy and spiral inside as long, as the kinetic energy is not exhausted.

*The emission of photons is accompanied with impulses to the electron, this, the electron moves along the spiral path, made of tangerine slices. 
To get a more or less circular path one need time to time an accelerating electric potential. This is, why particle accelerators have both, magnetic field devices and electric field devices. 
Making an uniform magnetic field with vanishing step by step over time magnetic field strength could be used to move the electron in a circular path until it comes to the standstill. 
