Question: A point charge is moving with speed 2 x 107m/s along the x axis. At t=0, the charge is at x= 0m and the magnitude of the field at x=4m is B0. The magnitude of the magnetic field at x= 4m when t= 0.1μs is: The equation and my attempt at solving it is in the attachment. --I also converted amperes to 1C/1s and I figured that theta is 90 degrees and the magnitude of the unit vector r is one, and thus the cross product of vector v and unit vector r is 2 x 10^7 m/s. ( 2 x 10^7 times one times sin(90 degrees).