Download There are two ways to figure out whether power is positive

There are two ways to figure out whether power is positive (dissipated by the element) or
negative (generated by the element). Approach 1 is to look at the problem intuitively
(which is what I focused on in lecture) Is positive current entering through the positive
terminal? Then P is positive. Is positive current entering through the negative terminal?
Then P is negative. Be aware that P is also negative if negative current enters through the
positive terminal or if positive current heads away from the positive terminal (refer to
Figure 1.15b in the textbook).
Approach #2 is to take a more rigorous treatment of power, i.e.
P = Iab Vab
or P = Iba Vba
where Vab is the voltage from point a to point b, Iab is the current from point a to point b,
etc.
Consider how this applies in the following problem (which we worked out at the end of
class):
–2 v
A
2v
B
-2 A
Using the intuitive approach, the positive terminal should be assigned to B and the
negative terminal to A. (since B has a higher potential/voltage). The current is actually
flowing from left to right. So since positive current is entering the negative terminal,
power is being generated, i.e. P = -8 W.
Using the rigorous approach, Vab = Va – Vb = –2 – 2 = –4 v. Iab = 2 A, so P = Vab Iab = –4
* 2 = –8 W. Alternatively, Vba = Vb – Va = 2 – (–2) = 4 v. Iba = –2 A, so P = Vba Iba = 4 *
–2 = –8 W.
I don’t care which method you use. Just get the right answer. You can always use the
other method to check your work.
Try this technique on the following:
1v
0v
A
B
1A
(answer: P = + 1 W)