Why does current direction change the sign of delivered power?

[DaVinci]

I tried this thread in the Electrical Engineering area and nothing. So, hopefully someone reading this can explain this problem I am having.

We know that current in one direction is equal to its negative in the opposite direction. We also know P=IV for power. We also know that when current is going from + to -, P absorbed is +VI and P delivered = -VI. When you have current going from - to +, Pabs = -VI and Pdel = +VI.

So, you have -3A going from + to - through an element. There is 6V across that element. P=IV = (-3)(6) = -18W. Since the answer is - and current going from + to -, Pdel = -VI so it is -18W delivered. (Is my train of thought the correct way to view this?)

Now, imagine the same problem. You would have 3A going from - to +. So, P=IV=(3)(6) = 18W. Since current going from - to +, Pdel = +VI, so you have 18W delivered.

The signs are different! The book says the answer is P delivered = -18W. In both cases I found that the power is delivered. But the sign is off in the second example leading to an incorrect answer...

Anyone know what the heck is going on?

 

[NateTG]

Have you considered that current has a direction, and could have a negative sign as well?

 

[DaVinci]

Yes. I said "We know that current in one direction is equal to its negative in the opposite direction."

The problem is EXACTLY that. Once you get the answers to the examples I gave, the sign is messed up and I cannot figure out why.

 

[stunner5000pt]

If a current is going from a negtive to the positive what is its sign??

look for your error in the solutions

 

[DaVinci]

The problem states -3A going from + to -.

This translates to 3A going from - to +, correct?

X amps going in one direction is equal to -X amps going in the opposite direction.

 

[stunner5000pt]

The problem states -3A going from + to -.

This translates to 3A going from - to +, correct?

X amps going in one direction is equal to -X amps going in the opposite direction.

for the sake of convention stick with the + to - and convert everything from + to - (i mean direction!)to avoid confusion in the future

 

[DaVinci]

If I do that then the answer is incorrect.

That is what I want to know. Why is the answer incorrect?

 

[NateTG]

If I do that then the answer is incorrect.

That is what I want to know. Why is the answer incorrect?

It appears that you're double correcting for the direction of the current.
Try always using Pabs=IV and Pdel=-IV.

 

[DaVinci]

I talked to my professor about this today...

The problem is the textbook has two wrong answers in it. His solutions manual for the text has the correct answers. So, all weekend I spent trying to make the numbers work and confusing myself. Now it is all clear since I found the book was wrong. :)