A doorknob is a spherical metal conductor with diameter 5.0 cm and a static charge of -1.5 nC spread over its surface. What is the magnitude of the electric field 1.0 cm in front of the doorknob's surface?

1. Because the doorknob is a uniformly charged sphere, treat all its charge as concentrated at its center. 2. The radius of the doorknob is half its diameter, 2.5 cm, so the field point is r = 2.5 cm + 1.0 cm = 3.5 cm = 0.035 m from the center. 3. E = k|Q|/r^2 = (8.99 x 10^9 N m^2/C^2)(1.5 x 10^-9 C) / (0.035 m)^2 = 1.1 x 10^4 N/C. 4. Since the charge is negative, the field points toward the doorknob rather than away from it, though the question only asks for magnitude. 5. Option A uses 2.5 cm instead of 3.5 cm as the distance from the center, forgetting to add the 1.0 cm gap to the doorknob's radius, which is the usual slip in this kind of problem. _Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.3 Electric Field_