1. If the magnitude of an electric field in air is as great as 3 × 10⁶ N/C, the air becomes ionized and begins to conduct electricity. This phenomenon is called dieletric breakdown. A charge of 18 μC is to be placed on a conducting sphere. What is the minimum radius of a sphere that can hold this charge without breakdown?

2. If the electric field E is zero everywhere on a closed surface, is the net flux through the surface necessarily zero? What, then, is the net charge inside the surface?