# why does not there is any change in velocity of a particle while enterring perp.. and leaving  a uniform magnetic field? (tracing a semi circle)  *****does the path is of uniform radius ?*****  if i project a particle sitting inside a field region  will it trace out a complete circleof same radius till there is field on?i just want 2 confirm my small doubts ....plez help me out

Saurabh Kumar
askIITians Faculty 2400 Points
9 years ago
The charged particle entering a magnetic field describes an arc which is at most a semicircle. If the span of magnetic field is limited, then there is no further bending of path due to magnetic force.
Let us consider a case in which a particle traveling in the plane of drawing enters a region of magnetic field at angle α.Motion of charged particle entering a magnetic field
Motion of charged particle entering a magnetic fieldWe should realize here that even though the charged particle enters magnetic region obliquely (i.e at an angle) in the plane of motion, the directions of velocity and magnetic field are still perpendicular to each other. The particle, in turn, follows a circular path. However, the particle needs to move in the region behind the boundary YY’ in order to complete the circular path.
But, there is no magnetic field behind the boundary. Therefore, the charged particle is unable to complete the circular path.