I am stuck on my physics homework. I can manage the latter question, how ever when i work out the first one my working out has no rhyme nor reason (nor a right answer). Any help will be great. A small metal sphere on an insulating rod is placed on a top pan balance, as shown in the diagram below. An identical metal sphere carrying a charge of 2Q on an insulating rod is brought into contact with th sphere on the balance so each sphere aquires the same charge. When the two spheres are a separation of 68 mm, the top of the pan balance reading increases by 2.1 x 10 ^-3 N. Calculate the charge, Q, on each sphere. Explain why the electric field strength is zero at the midpoint between the two charge.