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Homework Help: Electronics Help

  1. Mar 27, 2005 #1
    Hi guys,

    Just a question with a current vs voltage graph (or any graph with voltage on the horizontal axis)...why does voltage sometimes go into the negative section?
  2. jcsd
  3. Mar 27, 2005 #2
    in I-V graphs voltage represents the potential difference . Usually current starts for a positive potential difference. For linear devices R = V.I
    There is no difference in current behaviour on reversing the polarities i.e. even for negative voltages, so the forward bias behaviour is same as reverse bias behaviour.

    For non-linear devices like diodes again the current flows for a particular positive voltage.There is a difference between forward bias characteristics and reverse bias I-V curves. In reverse bias we look for the current flow for negative voltages. In such case the direction of flow of current is opposite to the former case (forward bias i.e. positive voltages)

    but in some cases of semiconductor devices like FET family ,current is governed by two simultaneous voltages --Vds, drain-source voltage (potential difference between drain and source) and Vgs,gate source voltage. For a particular Vds, current can start even for a negative Vgs. curves we see a positive current flowing at a negative Vgs but the value of Vds is specified along with the device parameters.
  4. Mar 28, 2005 #3
    even easier than that........

    it's all relativity [no, not Albert E's kind...]... :smile:

    the graph or axis you refer to is a representation of voltage measured with respect to a reference, usually refered to as "ground."

    if you put a voltmeter across a flashlight battery, it's going to tell you how much more positive the voltage is on the positive terminal of the voltmeter, as compared to the voltage on the negative terminal of the voltmeter.

    in other words, if the positive terminal of the voltage-measuring tool is more positive than the other (negative) terminal, the voltage will read as positive, you'll be on the "positive side of the graph", and the needle or digits will move "up."

    if you reverse the leads to the meter, or reverse the battery, what was read as a "positive voltage" on the meter or the graph will now be indicated as a "negative voltage."

    yep, the meter didn't change, and the battery didn't change; the way they were connected to each other did, and the reference point from which the "zero-level" voltage was being measured (typically the "-" terminal of the meter) DID change.

    hope that helped a little.

    now, in real life, there are many electrical components, particularly things like diodes and transistors, that behave very differently when they have voltage applied to them in what's routinely called the "forward" or "reverse" directions.... and that's where the fun (and their usefulness) begins....

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