7. A 15 g pith ball with a charge of -8.7 C is hanging on a thread 10 cm long. A second pith ball with a charge of 9.0 C is brought near which causes the hanging pith ball to move to an angle to 30 from the horizontal. How far apart are the two pith balls? (6 marks) 8. In a hydrogen atom the electron and proton both have a charge with a magnitude of 1.610-19 C. The electron has a mass of 9.1110-31 kg and orbits at a radius of 5.2910-11 m. How fast is the electron moving to prevent it from spiraling into the proton? (6 marks)