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Anode vs. Cathode

Discussion in 'General Electronics Discussion' started by Gadgetastic, Jun 16, 2013.

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  1. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    Okay, here's another question that will show just what a newbie I am in electronics.

    I'm looking at a circuit where it is VERY important to know which output is cathodal and which is anodal. For background, I'm researching medical uses of trans-cranial direct current stimulation as an inexpensive replacement for meds.

    So, in tDCS a very small current (2mA) is delivered through the scalp to surface regions of the brain. Anodal has one effect, cathodal has another.

    If I have wires connected to a battery, one to the + terminal and one to the - terminal, which are then connected to electrodes (the actual circuit is somewhat more complex but you get the idea) which electrode will be cathodal and which will be anodal?

    I've checked online and in some books, but it still seems confusing and it's important I don't get this wrong, so this is my quadruple check. :)
     
  2. eKretz

    eKretz

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    Apr 8, 2013
    I use a mnemonic for this: catnap. Shorten it to CNAP and you have your answer. In electronics, it's Cathode Negative, Anode Positive. In chemistry when discussing ions, it is the other way.
     
  3. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    Thanks.

    It's confusing because in some books they say it's the other way around. One book even says "think of the T in cathode as a +. Ca+hode. Seems like no one can agree on this. Then there's the whole "conventional current" thing, which just makes it worse. :)
     
    Last edited: Jun 17, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, everyone agrees.

    It's just that they are exactly the opposite in a battery as opposed to outside a battery (I guess that's the simplest way of saying it)

    But just to confuse you, the anode of a diode is always more +ve than the cathode (when the diode conducts). However in a power supply, the cathodes are where you get +ve from.

    This is one of those things that rote learning of rules (and a lack of understanding) makes you think that nobody agrees on a definition.
     
  5. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    okay.
     
  6. shrtrnd

    shrtrnd

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    Jan 15, 2010
    We've had this discussion before.
    I'm outnumbered here, but I'm still a 'electron flow', rather than 'hole flow' guy, for
    current flow.
    (But I grudgingly agree, that everyone agrees about what the anode/cathode actually
    do).
     
  7. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    Okay, here's me being dumb... again.

    This seems to conflict with

    'splain please.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
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    Jan 21, 2010
    :-D

    The cathode is more negative than the anode, but it is more positive than ground.

    Thus, our positive voltage comes from the more negative end of the diode.
     
  9. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    Okay. Think I got it now.

    The reason I'm being so touchy about this is that cathodal tDCS has a much different effect than anodal. One increases neuronal activity, the other decreases it. So it's kind of important to know which electrode to position on the head and which one to put on the shoulder. :)
     
  10. Number

    Number

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    Jun 9, 2013
    Somewhat correct on this. Anode is where more anions are, and cathode is where more cations are found (in reference to the component). Anions have more negative charge, cations have more positive charge (again in reference to the component). The ground (Earth) itself will have a more positive & negative side as charges attract and repel each other. Think of the metal on your bread board. There is always a more positive or negative charge within any given section of metal/contact. Plastic for example can be more positive than metal but due to the atomic structure it either allows or does not allow electron movement/transfer.

    Chemistry 221 &...

    EDIT:
    This is also the reason why I suggest you view schematics or really anything electronics based, as electron flow, not conventional current. As the terms of anode/cathode, positive/negative get easily confused, but it is always true for electron flow. From negative to positive. Shorter pin to Longer pin.
     
    Last edited: Jun 18, 2013
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