= Solution
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 <length>
$$
f_E=\frac{|\lambda_1\lambda_2|}{2\pi\epsilon_0d},\qquad
f_B=\frac{\mu_0|\lambda_1\lambda_2|v^2}{2\pi d},\qquad
\boxed{\frac{f_E}{f_B}=\frac{c^2}{v^2}.}
$$
The <electric force> is repulsive and the magnetic <force> attractive for the parallel electron <velocities>. The net <force> remains repulsive for $v<c$.
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