Question

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

Answer 1

Answer: (B)

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.