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Why we call 'Passive' sign convention?

  1. Nov 23, 2008 #1
    Hello, It's my first time login.

    I'm not good at english.

    Of course, I'm not native speaker.

    I wonder why we call 'passve' sign convention.

    have a good day. :)
     
  2. jcsd
  3. Nov 24, 2008 #2

    berkeman

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    Staff: Mentor

  4. Nov 24, 2008 #3

    Redbelly98

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    I had never heard of passive and active sign convention before reading this. Now I've learned something new, thanks to you both!
     
  5. Nov 24, 2008 #4
    ugh, i don't like that link at all. :uhh:

    this is a recipe for failure.
     
  6. Nov 25, 2008 #5

    Redbelly98

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    Can we simply say:

    With passive sign convention, a positive current flows from the + terminal to the - terminal inside the device (examples: resistors, capacitors, inductors)

    and for active sign convention, it's the other way around (examples: battery cells and other power supplies)

    ???
     
  7. Nov 25, 2008 #6

    tiny-tim

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    Welcome to PF!

    Yes, but nobody's answered the original question, which was, why passive? :cry:
    oh, and hello coolpursuit! :smile:

    Welcome to PF!
     
  8. Nov 25, 2008 #7

    jim mcnamara

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    Proton - one thing to note about Wikipedia - what one person views at 1000 may not be what you see at 2300. Wikipedia is not the most reliable source for a variety of reasons.
     
  9. Nov 25, 2008 #8
    Re: Welcome to PF!

    i'm not exactly sure i understand the question. passive components are passive no matter which current convention you choose, but...

    i believe the important thing is that current into sinks (passives) is opposite the direction for current into a source. for example, you could choose to use electron current instead of conventional current and the current would change sign in both sinks and sources, but current would still be in the same direction as voltage for one, and opposing for the other.

    much of this goes back to the physics of the problem. if you draw a simple circuit with one voltage source and one resistor, the first thing you may notice is that Kirchhoff's Voltage Law forces the voltage across the sink and source to be opposite for the sum to be zero in the circuit. this circuit will also have two nodes, and Kirchhoff's Current Law will ensure that current is in phase with one voltage, and out of phase with the other.

    and in a more general sense, you can extend this type of network theory to other systems. Across and Through Variables are not limited to electrical, you could apply the same principles to mechanical systems, and solve with the same mesh and nodal analysis. and this probably goes back to why conventional current is used instead of electron current. it keeps things like power calculations in generalized through and across variable network theory consistent.
     
  10. Nov 25, 2008 #9

    tiny-tim

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    Exactly!

    I think what coolpursuit is asking (and I'd like to know, too :smile:) is why it's called a "passive" convention …

    passive components are passive (!), but what does that have to do with the sign convention for current? :confused:
     
  11. Nov 25, 2008 #10
    i have no idea why it's called a passive convention. why not call it source sign convention? why not label electrons positive? that's why i say i don't understand the question. conventions are conventions, and are chosen at convenience. some choices really don't matter, as long as we all agree on the definitions.
     
  12. Nov 25, 2008 #11

    tiny-tim

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    Why not indeed! :biggrin:

    But if it was called that, coolpursuit and I wouldn't be asking why … it would be obvious!
    If we asked why negative rather than positive (for electrons, or for charge generally), the answer is that there are only two choices, and the choice was only arbitrary as between those two, in the sense that the word "positive" has no prior meaning … it was undefined. But "passive" does have a prior meaning … it describes a type of component, the opposite of "active" components.

    So why is the convention associated by name with those components?
     
  13. Nov 25, 2008 #12

    Redbelly98

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    It just makes sense (to me anyway) that we would use passive sign convention for passive components (like resistors), and the opposite -- active sign convention -- for active components (like batteries).
     
  14. Nov 25, 2008 #13

    tiny-tim

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    Yes, but doesn't the passive sign convention apply to both?

    Or am I missing the point of the convention? :confused:

    (i'm not a very conventional goldfish :rolleyes:)
     
  15. Nov 25, 2008 #14
    that's just the thing, tho. if we choose electron current (by convention), then the signs for current change in both active and passive components.
     
  16. Nov 25, 2008 #15

    Redbelly98

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    Hi Tim.

    No, for batteries active sign convention (opposite to PSC) is used instead.

    With passive sign convention, a resistor has positive current when the voltage is positive: current flows from + to - within the resistor.

    With active sign convention, a battery has positive current and voltage when connected to a resistor. Current flows from - to + within the battery.

    If you like, I could provide a couple of drawings showing this, but I hope you can understand it from my description so far.

    p.s. I'm thinking that the whole issue of negative charge flow, and electron flow being opposite to conventional current flow, is not really relevant here.
     
  17. Nov 25, 2008 #16

    uart

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    I don't really like that wiki article either. Example 1.3 is just plain wrong. In this example the resistor is supplying power to the circuit and they state the opposite.

    **Ok of course it can't actually be a "resistor" then but that's unimportant to me. Whatever it is it's clear that the device with voltages and currents as given is suppling power and not as they state absorbing it.
     
    Last edited: Nov 25, 2008
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