Potassium ions (charge -e, mass mK = 39mp) are accelerated from rest at plate A through a voltage difference of V = 5 kV to plate B. Some of the ions pass through a small slit in plate B to enter into a region of uniform magnetic field (the shaded region in the diagram). mp is the mass of a proton, 1.67 x 10-27 kg. 

(a) Which plate is at a higher potential, A or B? Calculate the speed of the K-ions when they reach the slit in plate B. (b) When the ions enter the region of the constant magnetic field, they experience a force that is perpendicular to their velocity. Using Newton's Laws, what must be the magnitude and direction of this force if these ions are to follow a curved path of radius r = 10 cm? (c) Find the magnitude and direction of the magnetic field needed for the force you found in part (b).