1. The problem statement, all variables and given/known data Q. A metal sphere of radius 20 cm carries a positive charge of 2.0 μC. a. What is the electric field strength at a distance of 25 cm from the centre of the sphere? b. An identical metal sphere carrying a negative charge of 1.0 μC is placed next to the first sphere. There is a gap of 10 cm between them. Calculate the electric force that each sphere exerts on the other. c. Determine the electric field strength midway along a line joining the centres of the spheres. 2. The attempt at a solution I've got 2.88x105 Vm-1 for (a). For (b), I've got -0.072 N, but the book gives the answer as a positive value, whereas the product of the charges gets you a negative value. I don't get why it's negative. That's one problem. The real problem I suppose I've got is at (c). I've been at it for two hours now, and I don't get the answer the book has. The book says it's 4.72 Vm-1 but I have absolutely no idea how to get that. What freaking formula am I supposed to use to get that? I mean I've got two charges, but the single formula I've got needs one value for charge. What value do I use?! Please help!