capacitor -- Work done to charge it up If we imagine a capacitor as two conductors held a distance d apart from each other, we can find easily that the total charge and the potential are proportional: Q = CV To find the energy stored in a capacitor you can imagine taking a tiny chunk of charge and separating it from the one plate to the other. The work you must do is: Q/Cdq and you can integrate up to find the total work needed done. To me however, there is some subleties in this definition of the energy stored in it: Because, doesn't it matter from where on the conductor you take your negative charges and transport them to the positive one? Imagine we have a capacitor made of some conductors of weird shape. Generally the field between them can vary depending not only on the distance between them but also on where you are on the conductors. So the work needed for taking an electron from one and transporting to the other will vary depending on which electron you pick. What reference point on the capacitor should be used?