Question

why a moving proton in a magnetic field experience change in momentum​

Answer 1

$\mathfrak{\huge{\green{\underline{\underline{Answer :}}}}}$

The magnetic force exerted will be perpendicular to the direction of motion of the proton. As we know when when force acting is perpendicular to the direction of moving charge, work done will be zero. It means kinetic energy does not change. The force is able to change the direction (velocity) of the proton but not its speed (magnitude). Thus momentum and velocity changes.