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

Options

A. kinetic energy changes but the momentum is constant
B. the 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

B

Explanation

Due to Lorentz force, `vec(F) = q vec(v) xx vec(B)`. When a charged particle enters a field with its velocity perpendicular to the magnetic field, the motion is circular. V constantly changes its direction (but not the magnitude). Therefore its momentum changes its direction but its energy remains the same.