Three identical metal spheres are hung from a ceiling on rigid non-conducting rods. Sphere 1, sphere 2, and sphere 3 have charges 31 C, -46 C, and 54 C, respectively. Each sphere is separated by 15 cm. 1 2 3 (a) What is the net electric force on the middle sphere due to spheres 1 and 3? (Enter the magnitude only.) N (b) What is the direction of the net electric force? up down left right no direction (zero magnitude)