Hello, I was reading Griffiths- Intro to electrodynamics, and I came cross some difficulties in understanding some cases which are not covered in the book well. I really appreciate your insight. Here are my questions; 1) If we bring a charge near a grounded infinite metal sheet, it will induce charge (we can find the potential using images method) . Now, can we say that the E field on the other side of the sheet is zero? basically does this sheet shield the E field on one side produced by the charge on the other side? It is obvious the div of V is non zero for the region that charge does not exist, but this solution is not for that region. 2) Under a uniform ext E field, if we place a non conducting sphere with total charge Q, then I would expect it to move along the direction of the field, is that right? 3) What happens if this is a metal sphere with total charge Q? will it also move? I know that it will redistribute its charge such that it will cancel the E field inside, but does that affect the over all motion? 4) How can be the E field inside a metal sphere be still zero if the ext E field is very strong that there is no charge to balance the field? 5) If the sphere is grounded, will it still make E field inside zero? and can it hold against any amount of ext E field? what happens if the field is very strong?