Past exam of the physics course of the year 1 of the natural sciences course of the University of Cambridge / 2021 / Section B / 10 / d / Solution

For injected velocity $\mathbf{v}=v\hat{\mathbf{x}}$, the Lorentz force isFor nonzero electric charge and nonzero operating current, the velocity selector condition isThe requested nonrelativistic kinetic energy is consequentlyBoth electric current and electric charge cancel. Reversing the sign of $q$ reverses both component forces and preserves their cancellation. The dimensions are consistent because $R/{\mu }_0$ has units of speed, while $w/d$ is dimensionless. The assumed nonrelativistic particle regime requires $Rw/({\mu }_{0}d)\ll c$; the problem supplies no values with which to evaluate a numerical energy.