Past exam of the physics B course of the University of Cambridge / 2014 / Paper 1 / Section A / A3

For steady thin beams a distance $d$ apart, let ${\lambda }_1,{\lambda }_2$ be their laboratory charge per length. Their current magnitudes are $|{\lambda }_j|v$, giving force magnitudes per lengthThe electric force is repulsive and the magnetic force attractive for the parallel electron velocities. The net force remains repulsive for $v<c$.