Hi, 1. The problem statement, all variables and given/known data A system, with cylindrical symmetry and charge density ρ(r) = ρ0e-r2/a2, where a is a given constant, is given. The system moves at a constant velocity V in the z^ direction. V<<c. The charge density in the lab's reference frame is approx. equal to that in the rest reference frame. I am asked for the electrical and magnetic fields (magnitude and direction) everywhere. 2. Relevant equations 3. The attempt at a solution I have managed to find the electrical field to be equal (2πρ0a2/r)*(1 - e-r2/a2) r^. I have confirmed that my answer is correct. Now, what I don't quite understand is why couldn't I equate Lorentz force to zero ("constant velocity") in order to find the magnetic field? This would yield (2πa2ρ0/r)*(c/V)*(1 - e-r2/a2) [itex]\varphi[/itex]^, whereas the correct answer, as asserted by the book, is (2πa2ρ0/r)*(V/c)*(1 - e-r2/a2) [itex]\varphi[/itex]^! What is the reason for this discrepancy? By using Ampere's Law, I was able to solve it correctly, yet would like to understand why couldn't I simply have applied the requisite condition on Lorentz force to find the magnetic field? I'd truly appreciate your help.