Which one among the following properties of a proton may change while it moves freely in a magnetic field?

When a proton moves freely in a magnetic field, it experiences a magnetic Lorentz force given by F = qvB sinθ. This force is always perpendicular to both the magnetic field and the velocity of the particle. Because the force is perpendicular to the displacement, the work done by the magnetic field is zero. Consequently, the kinetic energy and the speed of the proton remain constant. However, since the force acts as a centripetal force, it causes the particle to change its direction of motion, often resulting in circular or spiral paths. Velocity is a vector quantity defined by both magnitude (speed) and direction; therefore, while the speed remains unchanged, the change in direction means the velocity changes. The charge and mass of the proton are intrinsic properties and remain invariant in this context [1].

Sources

1. [1] http://www-thphys.physics.ox.ac.uk/people/FelixParra/CollisionlessPlasmaPhysics/notes/lecI_magnetizedmotion.pdf