Question

A charged particle moves through a magnetic field perpendicular to its direction. Then

• A. kinetic energy changes but the momentum is constant
• B. The linear momentum changes but the kinetic energy is constant
• C. both momentum and kinetic energy of the particle are not constant
• D. both momentum and kinetic energy of the particle are constant

Answer 1

Answer: (B)

When a charge particle enters a magnetic field at a direction perpendicular to the direction of motion, the path of the motion is circular. In circular motion the direction of velocity changes at every point (the magnitude remains constant).
Therefore, the tangential momentum will change at every point.
But kinetic energy will remain constant as it is given by $\frac{1}{2}m{v}^2$ and $v^2$ is the square of the magnitude of velocity which does not change.
In this question, we could not infer that where the momentum mentioned here is tangential or angular.