Halliday, Resnick & Walker — Magnetic Fields: Problem 61

61 An ion source is producing $^6\text{Li}$ ions, which have charge $+e$ and mass $9.99 \times 10^{-27}\text{ kg}$. The ions are accelerated by a potential difference of $25\text{ kV}$ and pass horizontally into a region in which there is a uniform vertical magnetic field of magnitude $B = 1.2\text{ T}$. Calculate the strength of the electric field, to be set up over the same region, that will allow the $^6\text{Li}$ ions to pass through without any deflection.