1) Imagine you have a clock but in each number you have a charge "q". If I have a test charge Q at the center, then by simetry, the net force on Q is zero. I imagined like, 1 cancels 7, 2 cancels 8, etc... But then, if I have a polygon with 13 sides, with a test charge Q, the net force also would be zero! How come? (This is the very first exercise in Griffiths book, 2.1) 2) Also, that very commom problem of two charges, apart from a distance "d" when I need to find the Eletric Field in a midpoint between these two chages placed on z axis. If the charges are +q and +q, I only consider the vertical component of the eletric field, but if the charges are +q and -q then I only consider the horizontal component. Why?? I don't understand why! I don't need help with the math per se, but I'm missing something about understanding simetry in Electric Fields... Thanks in advance for an explanation!