A 150-V battery is connected across two parallel metal plates of area 28.5 cm^2 and separation 8.20 mm. A beam of alpha particles (charge +2e, mass 6.64x10^-27 kg) is accelerated from rest through a potential difference of 1.75 kV and enters the region between the plates perpendicular to the electric field, as shown in Fig. E27.29 <IMAGE>. What magnitude and direction of magnetic field are needed so that the alpha particles emerge undeflected from between the plates?