question_answer

If a charge particle enters perpendicularly in the uniform magnetic field, then:

A)  energy and momentum both remains constant

B)  energy remains constant but momentum changes

C)  both energy and momentum changes

D)  energy changes but momentum remains constant

Correct Answer: B

Solution :

                 Key Idea: Perpendicular   magnetic   force exerted due to magnetic field provides the required centripetal force to the particle for motion on circular path. When a particle with charge q enters a magnetic field B, the force acting on it, is \[F=qvB\]                 This provides the centripetal force to the particle.                 \[qvB=\frac{m{{v}^{2}}}{R}\] Since,\[\overrightarrow{v}\]is perpendicular to \[\overrightarrow{B}\] the particle is acted upon by a force\[\overrightarrow{F}\]which is perpendicular to velocity, it does not change the magnitude of the velocity (speed) it changes only the direction of velocity. Hence, energy is constant but momentum changes.