So we want to know what is the magnitude of the magnetic force F on charge Q=5.0*10^-7 C if the charge is moving with speed v=2.6*10^5 m/s perpendicular to the magnetic field B=1.8*10^-2 T. So the formula for the Lorentz force is F=Q*v*B, where Q is the charge, v is the speed of the charge and B is the magnitude of the magnetic field. If the speed of the charge is parallel with the magnetic field, the force is 0 N. I'm going to assume that the screen and also the magnetic field B is perpendicular to the speed of the charge. So we simply plug in the numbers in the formula and get the magnitude of the force F=0.0023 N = 2.3*10^-3 N
