Chapter 22 - Magnetism - Problems and Conceptual Exercises - Page 793: 27

$3.1cm$

We know that $r=\frac{mv}{eB}$ $\implies r=\frac{m}{eB}{\sqrt{\frac{2K.E}{m}}}$ $implies\ r=\frac{\sqrt{2mK.E}}{eB}$ We plug in the known values to obtain: $r=\frac{\sqrt{2(1.673\times 10^{-37})(4.9\times 10^{-16})}}{(1.6\times 10^{-19})(0.26)}$ $r=3.1cm$