I like to think of myself as pretty smart and on the ball with the electrical stuff. But I am having trouble figuring out this one concept with power and it seems really simple. I am probably just over thinking the whole thing as usual. Hopefully someone can give me some insight.(adsbygoogle = window.adsbygoogle || []).push({});

In my Circuit Analysis (Elecrical Networks) class, we just covered power, P=IV, and all the stuff about how to figure out if an element is absorbing or delivering power. What we have been told is the following:

When current goes from + to -, Pabs = +VI and Pdel = -VI

When current goes from - to +, Pabs = -VI and Pdel = +VI

This seems pretty simple... you have 2A going into a element from + to - and the voltage on the terminals is 5V. (5)(2) = 10. Since the answer is positive, there is 10W power being absorbed.

Then we say that in the same case it is -2A. So, (-2)(5) = -10W. Here is where things start to get crazy. Current is going from + to - still... but then you can say that -2A from + to - is like 2A from - to +. This would then be correct with 10W Absorbed.

But when things really hit the fan is when we get a problem like this:

You have -2A going from + to - and the absorbed power is 6W. What is the voltage across the terminals?

Well, from P=IV we know that V=P/I. So, 6W/-2A = -3V. But if you have -2A going from + to -, then you have 2A going from - to +. So power abs needs to be -VI in this case....

I think I just confused myself again. Do you see my plight? I think I know it... it is just getting jumbled up in over thinking.

If you can answer the following questions, I would be grateful.

What is the convention to use to determine the power in an element and whether it is being absorbed or delivered?

How would you use the same convention to determine either Voltage or Current given the power and one or the other quanitity listed.

Thanks.

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# Simple Power Equations Gone Wrong!

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