Once again, my textbook seems to do a poor job of explaining another concept just like I commented earlier here. Anyways, here is the relevant part of the textbook: http://www.mediafire.com/view/?cw5t2wr9vqqprnv To calculate p4 in example 1.7, voltage of 8V is multiplied by a current of -0.2I. Firstly, why is the current negative? Secondly, why do they substitute a value of 5A for I. Where did they get this value? I tried to do practice problem 1.7, but I don't seem to get the right answer for p3 when I use a voltage of 3V and a current of 0.6(3)A.