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Currents in Circuits.

Discussion in 'General Electronics Discussion' started by IanPSwift, Sep 23, 2011.

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  1. IanPSwift

    IanPSwift

    6
    0
    Sep 17, 2011
    Quick theoretical question. Say there is a circuit (in this particular case AC, but I believe DC would apply as well) with N resistors placed in series and or parallel. Can anyone offer a theoretical (perhaps using a water pipe or other some such metaphor if you feel so compelled) explanation for why the current through the first resistor in the circuit is equal to the source voltage divided by the TOTAL resistance? (As opposed to using some form of a current divider). Please feel free to use KVL and KCL if this helps you explain it, as I have been working with both of them.
     
  2. shrtrnd

    shrtrnd

    3,763
    485
    Jan 15, 2010
    I don't understand how you can't understand that.
    I don't feel compelled to use it, but you brought-up the water pipe metaphor.
    You've got a water pipe with water running throught it. Down the line you've got other
    pipes branching off the line.
    The water current through the first pipe is going to be the same as the
    combination of all the other branches off of the first pipe.
    Any restrictions (resistors) in the branches are the varying resistances to the overall
    water flow (the current). So the water pressures in the branches (voltage) can change
    in the branches (including the first resistance, the input pipe (or resistor) itself).
    But the overall current of the pipework (circuit) will remain the same in total, (the voltage drops) are the different branches of the pipe system, restrictions in the branches.
    If you further simplify it:
    The amount of water going into the pipe system is going to be the exact same amount
    of water coming out of the ends of the pipe system, no matter how many of those there
    are. The pressure changes (voltage changes) in the different branches of the pipe
    system are due to the restrictions (voltage changes) of the different branches. But
    the amount of water (current) flowing through the pipe system, will remain constant.
     
  3. IanPSwift

    IanPSwift

    6
    0
    Sep 17, 2011
    thank you, that was very helpful. It was one of those things where I could have seen it making sense either way and just wanted to ensure I understood the theory.
     
  4. shrtrnd

    shrtrnd

    3,763
    485
    Jan 15, 2010
    I really oversimplified it. In circuit theory and practical application, one of your most valuable tools, is reliably knowing your current. It tells you which devices to use in your designs. As a repair tech, the biggest issue I find is circuit damage due to excessive
    current through a device. To save money in parts, manufacturers want engineers to
    design as close to acceptable as possible, in parts. That means saving money by
    buying parts with marginal current handling capability.
    I guess you may not be interested, but I wanted to rant on this pet peeve of mine.
    If you're cutting it close in a design and a 1/4W resistor will probably work, consider
    using the 1/2W, just so the device doesn't have to be repaired by me later when the
    current through the resistor exceeds the expected value.
     
  5. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    That's good advice shrtnd :)
    I'm a good deal more conservative yet, and prefer my active acomponents over-rated by a factor of 10 if possible.
     
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