An electron starts from rest and is accelerated through a potential difference of 500 V. It then enters a region of uniform magnetic field of magnitude $4\times10^{-4}$ T, moving at $30°$ to the field lines. Find the radius of the circular part of its helical path. (Take mass of electron $m=9.11\times10^{-31}$ kg, charge $e=1.6\times10^{-19}$ C.)