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Electric current flow

Discussion in 'General Cisco Certifications' started by 0v3rload, Dec 12, 2005.

  1. 0v3rload

    0v3rload New Member

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    Hello,

    I am currently doing Cisco's Technical Skills and have stumbled across a tricky question...

    On the assignment they ask what the correct electric current flow is, if from :
    o + to -
    o - to +
    o + to - and - to +

    I am assuming they are asking about DC current as the symbol on the circuit chart resembles a battery.

    The reason why I am asking this is I have failed that assignment (only got 50% :( ) and got that particular question wrong: I chose + to -.

    I have been trying to figure by myself what the correct answer might be by googling it up, however, the more I do so the more I am getting confused...

    Can you please clear this out for me?

    Thank you very much!
     
  2. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    Electrons are negative
    Protons are positive
    Neutrons are neutral.

    Electrons are the only ones that move.
    They are attracted to the positive.

    8)

    Welcome to CF! Why not pop across to the New Members Forum and introduce yourself properly!
     
  3. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Simon is correct, but there's more.

    Firstly they are talking about DC, though the same does hold true for AC.

    Electrons (negatively charged particles) are on the outside of an atom, whizzing around much like the moon does to the earth. These can be attracted away by a stronger positive charge which leaves the atom they left positively charged and hence a good attraction for the next free electron and in more recent years, well since scientists have learnt more about the magic of electricity, we now know that current is actually a flow of electrons.

    So electron flow is from negative to positive but *conventional current* which is still often used as a model, flows from positive to negative. Conventional current was the basis of understanding prior to electron flow :biggrin

    So, you can't get that basic question wrong as they didn't stipulate whether they were talking about electron flow or conventional current :blink

    Here is a link http://www.mi.mun.ca/~cchaulk/eltk1100/ivse/ivse.htm
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  4. ffreeloader

    ffreeloader Terabyte Poster

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    Interesting....

    I took a two year college course back in the early 80's in which we were taught, and our text books all said the same thing, that electron flow is always from negative to positive. There is a way of looking at the flow in which it does flow from postive to negative but that is called "hole flow".

    An ion is a negatively charged atom. It has more electrons that protons and thus ends up with a negative charge. An atom with a postive change has fewer electrons than protons and so it has a positive charge. In the way current flows the electrons move from the negatively charged ion to the positively charged atom. Thus current flow (electron flow) is from negative to positive, not the reverse. It's the "holes" in the bands of electrons that flow from positve to negative because as an electron moves from a negatively charged atom to a positively charged atome the "holes" (missing electrons on the postively charged atom) appear where the electrons were on the atom they just left.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1
  5. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Correct, absolutely as I said we now understand this.

    I think Freddy you are confusing semiconductor physics with normal conductors.

    Semi conductors, like a diode or a transistor are made up of P type materials and N type materials joined together. Hence a transistor is either PNP or NPN. These materials are doped with something like arsenic (impurities) or some similar material that has a valency bond which is not quite the same as silicon (or germanium in the olde days) when these materials are created there is a bond between them that leaves a hole. One has say a capability of bonding with four other atoms and the doping material can only bond with say three other atoms. This is where the hole is created.

    Conventional current has always been described as flowing from positive to negative. It just makes some electronics easier to understand and teach. It is not an accurate description of what is really going on but people still use it today.

    Here is a definition of *hole* for you...

    Edit forgot the link oops http://whatis.techtarget.com/definition/0,,sid9_gci214516,00.html
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  6. ffreeloader

    ffreeloader Terabyte Poster

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    Nope. Not confusing it at all. Think about it. If electrons are flowing that means they are moving from one atom to another. That means that a "hole" is created on what was the negatively charged atom where the electron just left and a "hole" is filled on what was the positively charged atom onto which the electron just moved. That happens in all electrical current flow, not just in semiconductors.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1
  7. ffreeloader

    ffreeloader Terabyte Poster

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    Well, maybe outside the US it's always been taught that way, but I wasn't taught that and the text books we used didn't teach that.

    I don't have a degree in electrical engineering or anything like that, but I do have approximately 90 college credits in electrical/electronics theory so I do have a fairly good understanding of what's going on, and it's something that came very easily to me.

    It's news to me that electrical theory is taught using the reverse of what actually happens. I've never seen a text book that uses that method of teaching electrical theory. Why confuse people by teaching them the opposite of the physics that's actually happening? That makes no sense at all to me.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1
  8. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Freddy I honestly can't imagine how conventional current got missed somehow in your US text books. It is a glaring omission if this is true.

    I tried googling "conventional current" and came up with this...

    http://www.google.com.au/search?hl=...ff&q="conventional+current"&btnG=Search&meta=

    40500 hits!! I am 100% sure there must be some US sites with info pertaining to conventional current.

    I also delved in to refresh my memory. It was over thirty years ago that I studied electronics.

    I would now like to state that electricity in a copper wire or other metal conductor, is indeed a flow of electrons *but* in other non metalic substances it can be a flow of positively charged ions, going from positive to negative or a mixture of both.

    The following is from this good link http://amasci.com/amateur/elecdir.html

    It really doesn't matter a jot to anyone which way the current flows and the convention to use positive to negative, I believe came about so that electricity could be likened to the flow of water. It could be seen to flow from a high pressure to a low pressure or from a positive to a negative potential.

    Arrows that show the flow of current in circuit diagrams use the conventional current scheme.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  9. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Electrons orbit an atom and are only tenuously attracted to it. In many materials they can physically be brushed off causing the material to become statically charged.

    When a battery is connected to a simple circuit the positive electrode attracts electrons whilst the negative electrode feeds electrons into the circuit, those electrons repel or push there way through much like feeding ping pong balls into a tube, once it is full every time you pop one in at one end, one will pop out the other. So, although your moving hole model is okay, it does not fit as well as the moving hole model does for semi-conductors. Because as I said before, the holes are real. They are physically detectable as part of the structure of either P type or N type material.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  10. ffreeloader

    ffreeloader Terabyte Poster

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    I don't have a problem with that. But, "hole flow" is still "hole flow" even if it's not readily detectable when it's just electrons doing the moving. The principle is still the same.

    One other thing I don't like about the explanations of "conventional current" I've seen since you posted some links, and your analogy to water is is that emf isn't a push or a pull, it's a difference of potential, an attraction between positive and negative much like magnetism as electricity and magnetism are so similar.

    As to using the analogy of water flow to describe the relationship between emf, resistance, and current it doesn't need to take into account the direction of the flow of water to make sense. I've used it countless times to teach the principles of Ohm's law to the electrically challenged and not once did someone say that it wasn't understandable even though I've always taught electron flow direction as current flow direction. I guess you can say I'm just not a very "conventional" guy. :lol:
     
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    WIP: LPIC 1
  11. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    So, I think we both see things a little clearer now. I just wonder whether the OP does lol :twisted:
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)

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