Passive Sign Convention

  • #1
From what I understand: P=+iv (current enters through the positive polarity of the voltage, implying that the element is absorbing power) and P=-iv (current enters through the negative polarity of the voltage, implying that the element is releasing or supplying power)

I remember reading that there are passive elements (resistors etc. which dissipate energy) and active elements (batteries etc. which supply energy). I recall that resistors only dissipate energy but the wiki entry here:

http://en.wikiversity.org/wiki/Passive_sign_convention

example 1.2 suggests that the resistor is supplying energy. How can this be explained?
 

Answers and Replies

  • #2
66
0
Dude screwed up.

One thing could be is that he's erroneously using "resistor" for any two terminal element, if someone stuffed in "element" I think one would be okay
 
  • #3
245
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This is wrong example. A resistor can only dissipate power and it can't never supply power.
 
  • #4
The writer of the article wrote "Find PTotal and Determine if this resistor is supplying power or dissipating it." How can a resistor be supplying power? I know from a physical point of view it is impossible for resistors to supply power but how do we explain the negative sign for power if the current enters from the negative terminal of a resistor?
 
  • #5
142
1
The writer of the article wrote "Find PTotal and Determine if this resistor is supplying power or dissipating it." How can a resistor be supplying power? I know from a physical point of view it is impossible for resistors to supply power but how do we explain the negative sign for power if the current enters from the negative terminal of a resistor?

as perfection pointed out, he tried to use a resistor as a generalized passive linear circuit element. resistors cannot store and release energy, of course, but other elements that are passive can. for electrical circuits, those passive elements are capacitors and inductors.
 
  • #6
245
0
I know from a physical point of view it is impossible for resistors to supply power but how do we explain the negative sign for power if the current enters from the negative terminal of a resistor?

The fact is current never enters the negative terminal of a resistor (in the sense you mentioned) and the power will never be negative.
 
  • #7
The fact is current never enters the negative terminal of a resistor (in the sense you mentioned) and the power will never be negative.

Ah, thanks. I think that's the key. I saw a diagram in my text book with the current going
i--->+[resistor A]- i---> -[resistor B]+ and I thought that the current passes through the negative terminal of resistor B first. But the actual voltage across resistor B is negative, indicating that the "+" side of resistor B is -V voltages above "-" side, hence "-" side has a higher potential than "+" side. Thus current goes from higher potential to lower potential through the resistor.
 

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