# Question #eda55

The force on a beam of particles is given by, $F = B \times q \times v$ where B is the magnetic field strength (often called flux density), q is the charge on the particles and v is their velocity (I’m deliberately avoiding vectors here, I don’t think you need them at this level.)
So you can see if q = 0 (or any other factor =0) the force must also be zero, hence if a deflection occurs ($F \ne 0$) the particles must have charge.