Question

A particle enters uniform constant magnetic field region with its initial velocity parallel to the field direction. Which of the following statements about its velocity is correct? (neglect the effects of other fields)

• A. There is change in both magnitude and direction
• B. There is no change
• C. There is change only in magnitude
• D. There is change only in direction

Answer 1

Answer: (B)

Magnetic force when the particle, initial velocity parallel to the field direction
$\overset{ \rightarrow }{F}=q\overset{ \rightarrow }{v}\times \overset{ \rightarrow }{B}=q\left|\overset{ \rightarrow }{v}\right|\left|\overset{ \rightarrow }{B}\right|sin \theta =q\overset{ \rightarrow }{v}\overset{ \rightarrow }{B}sin ⁡ \theta =0$
( $\because \, \, $ Direction of the velocity of the electron is parallel to direction of the magnetic field)
So, velocity is constant. Hence, there is no change in direction and magnitude of the velocity.