Q1. An open-pit coal mine consumes 96 hectares of land, down to the depth of 26 m, each year. What volume of earth, in cubic kilometres, is removed in this time?
Q2. Much of the material making up Saturn's rings is in the form of tiny dust grains having radii on the order of 10-6 m. These grains are located in a region containing a dilute ionized gas, and they pick up excess electrons. As an approximation, suppose each grain is spherical, with radius R = 1.2 x 10-6 m. How many electrons would one grain have to pick up to have a potential of -370 V on its surface (taking V = 0 at infinity)?