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Which way does Electricity Flow?

Discussion in 'Electronics Homework Help' started by c131frdave, Apr 23, 2014.

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  1. c131frdave

    c131frdave

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    Oct 4, 2013
    Okay, I hate to ask this, but I must. All my life I've been taught that the "hot" side of a battery, the positive terminal, is where the electricity comes from- and it flows to the negative. But then I took a PLC course and the instructor insisted that electricity in a DC circuit flows from negative to positive. He says if something is negatively charged, it will want to balance out, so the electron, which is negatively charged natively, will flow towards the positive side.

    If the instructor is correct, then my whole concept of what a diode does goes straight in the trash can.

    So I googled it- and much to my surprise, this question has been asked over and over, and you'll get opposing explanations right after another! So which is it? In a DC circuit, electricity flows from + to -, or - to +? If - to +, why is the diode marked backwards in a schematic?
     
  2. Ratch

    Ratch

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    Mar 10, 2013
    I can answer that one for you. First of all, the word "electricity" is too generic and has too many different meanings. In this case, if you want it to mean "charge carriers", then be specific and say charge carriers. In wires, the charge carriers are primarily electrons, so they are going to flow from the negative terminal of a voltage source (repulsion) to the positive terminal (attraction). With holes in a semiconductor or positive ions in electrochemistry, the opposite is true.

    When some folks try to do calculations, they really get wrapped around the axle badly because they first try to determine the polarity of the charge carriers and use that for their calculations. DON'T DO THAT. Always use the Conventional Mathematical Method when doing calculations. This method always assumes that:

    1) Positive charge carriers are mathematically leaving the positive terminal of a voltage source, even when you know damn well they are not physically doing so.

    2) A negative charge carrier traveling in one direction is mathematically equivalent to a positive charge carrier going in the opposite direction and vice versa.

    Do your calculations using assumption 1) above, and if you really, really need to know the direction of the physical charge carrier, then it is the same as the calculated direction if it is positive and opposite if negative.

    You have observed that semiconductor manufacturers mark their devices assuming that positive charge carriers flow from positive voltages. The arrows on diodes, transistors, and polarity markings representing current meter deflection are all marked this way.

    Ratch
     
    Last edited: Apr 23, 2014
    Arouse1973 likes this.
  3. c131frdave

    c131frdave

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    0
    Oct 4, 2013
    Makes sense! Thanks.
     
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