I have a question, in one of my problems in the book(too complicated to post here), they ask for the terminal potential difference across a battery. But usually the formula is (Terminal Potential difference)= (emf)-(I X r) where r is the internal resistance. Like in one case, current is flowing from the positive terminal to the negative one and then through the resistor, but then apparently we have to use this formula to calculate the terminal potential difference: (emf) + (I X r)....why is there a plus sign? I'm just wondering can the emf ever be negative or is it only considered as a positive constant? Like when we measure terminal potential difference, do we usually start at the internal resistance and move to the negative to the negative terminal?