A V = 130-V battery is connected across two parallel metal plates of area 28.5 cm^2 and separation 8.40 mm . A beam of alpha particles (charge +2e, mass 6.6410^27 kg) is accelerated from rest through a potential difference of 1.70 kV and enters the region between the plates perpendicular to the electric field. What magnitude of magnetic field is needed so that the alpha particles emerge undeflected from between the plates?