A thin, non-conducting spherical shell of radius r carries a uniform charge density and a total charge Q. A small, circular disc is removed from the shell, leaving a hole of radius b of radius b (b < < < r). a) What is the magnitude and direction of the electric field at the center of the hole? Hint - how is a hole like on object of negative mass? The plug Is replaced. Use the result of a) to calculate the force acting on the plug. c) From this, calculate the electrostatic pressure that tends to expand the sphere.