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Basic question on 2 battery system and potentials between points

Discussion in 'Electronics Homework Help' started by chopnhack, May 2, 2014.

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  1. chopnhack

    chopnhack

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    Apr 28, 2014
    I started reading "Practical Electronics for Inventors" and had some questions pop up. I checked the errata first because I got all of the other sample questions correct in determining the voltages between different points. Applying the same logic did not yield the same result as the book showed so I wondered what was different about this model and why I got it wrong or did I?

    In this example, the book lists volts between points A and D as -21V.

    pg18.jpg


    I had two thoughts:

    1. Since the all grounds are tied together: the 9V is technically connected in series with the 12v, shouldn't this yield a positive 21V when measured between points A and D?

    2. The batteries are not really connected in series, but they both have a connection to ground so its more like a reference point. Point A would be positive 12v and point D would be -9v, so a net of +3V

    I am not in a class, just trying to self teach myself as much as I can, thanks for any assistance!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The difference is 21V. The polarity depends on whether you use A or D to be your reference against which the other is measured.
     
    chopnhack likes this.
  3. chopnhack

    chopnhack

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    Apr 28, 2014
    Thanks Steve! I feel like the book was misleading since they did write it as V (AD) and not V DA. At least I am not going crazy yet! Only 180 more pages of theory to go in this book :):eek:
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
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    Jan 21, 2010
    VAD essentially means "Place the black probe of the multimeter on A and the red one on D. If you do that, you'll get -21V".

    The black lead is the reference to which the voltage on the red lead is compared.

    I agree that it seems misleading because A is more positive then D so the voltage between A and D should be positive? Unfortunately, while that is a sensible reading of the problem as expressed in words, it is not the correct mathematical approach. because A has a higher potential than D, the voltage goes down as you go from A to D so the difference is negative.

    Don't get too stressed. As you do calculations like this the most common error is getting things exactly around the wrong way. If you had a heap of similar and related questions, the final answer would be correct except for the sign. And you wouldn't be the first person to go "oops" and change the sign in the last step :D
     
    Arouse1973 and chopnhack like this.
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