What is the surface charge density on this face of the penny?

The equation for surface charge density is η = Q/A. Since the electric field is just above the face of the penny, the penny can be modeled as an infinite plane of charge Eplane = η/(2ε0). Combining these equations will result in Eplane = Q/(A⋅2⋅ε0). Rearranging this equation to find the surface charge density will be (Q/A) = (Eplane⋅2⋅ε0). However, this will be the surface charge density for both sides of the penny. In order to find the surface charge density of only one side of the penny, divide Eplane⋅2⋅ε0 by 2.

Source https://www.physicsforums.com/threads/what-is-the-surface-charge-density-on-the-face-of-the-penny.904118/

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -