I have been given the exercise above which relates to an article we are reading. I can calculate all the results but I can't interpret them physically. I think my major problem is that I don't see how self capacitance is a physically measureable quantity. For a recap the self capacitance is the mutual capacitance between a given conductor and a conducting sphere of infinite radius. a) In the first question I am asked to find the self capacitance of spherical aluminium grain of radius R and charge Q which is just 4[itex]\pi[/itex]ε0R. b) In the next question I am to assume that the grain is coated on each side by two separate oxide layers which both act as plate capacitors. This is also straightforward. c) In this question I am asked to find how much the voltage on the grain changes if we add an electron given the two situations a) or b). The big problem for me is that I do not understand what is physically meant by the voltage on the grain. For a plate capacitor the only sense of voltage I can make is the voltage between the two plates which I don't see as a voltage on the grain? And for the self capacitance I don't know what is meant by the voltage physically either since the voltage appearing in the expression for the self capacitance as said is a voltage difference between out conductor and a sphere of infinite radius. So what is meant by the "voltage on the grain"? And how do we measure it physically and how does the self capacitance enter?