Power Sign Convention: Confused? Get Help Here!

[merry]

Hey guys,

So the sign convention for power is really confusing me. According to my knowledge, if a positive current is entering the positive terminal of an element, the voltage drops across the element and power is hence absorbed. Why does the question solution change the sign of the current and say that the power is negative though? I am really confused =[
Help me please!

Thanks!


Attachment:
page 7 from Basic Engineering Analysis 9th Edition, by J. D. Irwin and R. M. Nelms.

 

[Zryn]

Be careful of the polarity of your supply. In picture 'a' the current would flow from the positive of the supply to the negative, so the direction given being 4A would need to be reversed to make sense, thus making the current -4A.

 

[merry]

Is it because the potential increases as current passes through the source? Would it be the same if I changed the voltage to a negative 2?
I got confused because I assumed that the source would be supplying the current and hence the direction of the current makes sense, but I guess according to the passive sign convention, the voltage would then be negative (or the dir. of current would be reversed). Am I thinking the right way? @.@

 

[Zryn]

The supply (blue thing) in this case is acting as an element, because of the direction of current, and the element (square) is supplying the power. We assume by convention that the supply is supplying power to the element, however in this case the power turns out to be negative, indicating the opposite.

If you assume current goes one way, and you get an answer with negative current, it means the current is actually going the other way. This is similar.

Is it because the potential increases as current passes through the source?

If you think of the supply as an element, and you increase the current through it (keeping the resistance fixed) what happens according to Ohms Law?

Would it be the same if I changed the voltage to a negative 2?

If you changed the voltage (in question a) to -2 (and swapped the polarity signs) you would have (-2V)*(4A) and would get the same -8W.

Yes, you are thinking the right way! You can assume anything you like, as long as you be careful about sign convention and write down what is valid, not what validates your assumptions ;). In the end, if you have the maths correct and the assumption wrong, you will just get a negative value, indicating the opposite of what you assumed.

 

[merry]

ahh! =.= I assumed tht the blue thingy was defined as the element! and the box was the source/supply!

But it makes sense to think the same anyway xD Thanks so much for helping me out! I've been stuck at this question all day...

 

[kevs926]

so like both are considered elements,

the right element is supplying power on both cases and the left element (blue creature) is absorbing power, also on both cases