What is the sign convention for generated power?

Question

Generally speaking, generated power is denoted as negative, correct? If I understand correctly, this is just the convention.

Conceptually, I think of it kind of like a force, it has a magnitude and direction. In this case, the direction is either positive or negative, effectively.

If that is correct, is it necessary to write that a source generates -15 W (for example)? It seems redundant. I wouldn't write that a force is -10 N up, that just doesn't make sense. I would write that it is 10 N down. So similarly, isn't it sufficient to say that there are 15 W generated, or 15 W absorbed?

The reason that I ask is that I was doing a homework assignment and I was doing a power check on a simple circuit. When it came down to it, I had "15 Watts generated with 15 Watts absorbed, so the power check is satisfied." My professor marked me wrong, because I supposedly needed "-15 Watts generated." Is he correct in doing this? (I specifically talked to him, all my work was correct, it was just the sign.)

If he is wrong, what source can I use to show him why it's not supposed to be that way? I don't think my opinion will persuade him, I need something more substantial.

Answer 1

I think you are right and your professor is wrong, but as this is a subjective question I cannot offer a source for evidence.

One argument I would offer is that "15 W generated" and "-15 W absorbed" could be considered equivalent, whilst "-15 W generated" implies that power is absorbed; it could be considered a double negative and so is at best unclear.

Answer 2

Generally, you're not going to win this sort of student/professor battle in academia. If he's factually wrong, that's one thing, but if it's only a convention, just go with it for this class. The intellectual flexibility that you'll learn in the process is just part of the education, as is dealing with bureaucracy and doing things (taking classes) that make absolutely no sense at the time but might turn out to be useful later.

Sometimes the skills that aren't taught directly are just as valuable as the ones that are.

By the way, if you're doing a much more complicated problem than what you're looking at now, it may actually be useful to think of generated = negative absorbed or vice-versa because it makes the math easier or because certain components tend to switch functions a lot or for some other reason.