The problem statement, all variables and given/known data 2. A large uniform sheet of charge on an insulator produces a field of 1000 V/m directed towards the sheet, at a distance 2 mm in air. What is its charge in microcoulomb per square metre? Take care with the sign. 3. A spherical balloon of radius R = 2 m has a uniformly charged surface of total charge +Q. The electric field strength is 3.0 kV/m directed outward at a radial distance 4R from the balloon's centre. If the balloon is now is inflated so that its new radius is 1.1R, and its charge is doubled, what now is the field at the same radial distance 4R from the balloon's centre, in kV/m? The attempt at a solution 2. I think this is the correct method. From Gauss's Law I use 2EA = enclosed charge/epsilon0, giving enclosed charge/area = -1.77*10^-8. Converting that to microcoulomb/square metre gives -1.77*10^-14. I think it's right, but not 100% 3. Got no idea! Edit: I solved the first question.