Just a quick question. Let A and B be two points. Electrical work is defined as the amount of energy it takes to move an amount of charge Q through a potential difference VB-VA (for our purposes here, we will assume that the voltage values are measured with respect to an Earth ground) and is calculated as the product of QV. Now to find the instantaneous power, you would take the derivative of QV with respect to time. Why is it that most texts tend to throw out the second term of this product and just keep the result of the calculation as P=VI? Are there physical grounds for so many texts falling back on this assumption? Note to admins: Yes this question is of a mathematical nature, but I'm trying to see if there are physical grounds for most texts assuming that the Q(dV/dt) term can be thrown out other than just assumng that V is constant (which is not always the case, but doesn't stop text writers from still assuming the P=VI form of the law anyway).