where v is the velocity (magnitude and direction) of the particle, and B(r) is the magnetic flux density at r. where in this case ˆl is the unit vector...where v is the velocity (magnitude and direction) of the particle, and B(r) is the magnetic flux density at r. where in this case ˆl is the unit vector in the direction of the motion; i.e., the direction of v. Note that the purpose of the dot product in Equation ??? is to ensure that only the component of Fm parallel to the direction of motion is included in the energy tally.