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Potential divider textbook example. Confused.

  1. Jul 23, 2016 #1
    DSC_0034.JPG DSC_0035.JPG Hi,

    I am just going through a textbook on electronics and have come across an explanation that uses potential dividers to work out potential values for resistors. I was wondering if anyone is able to explain how the author got the bit after "which can be written as": R1=(4.9/10.1)*R2

    How did he rearrange it to get this? Any help would be appreciated. Thank you! (I have added the pictures from the text book.)
  2. jcsd
  3. Jul 23, 2016 #2


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    The same current flows through R1 as R2. The voltage across R1+R2 = 15V
    The voltage Vout across R2 = 4.9V
    So voltage across R1 = 15V – 4.9V = 10.1V
    The ratios R1 / R2 = 10.1V / 4.9V

    Therefore R1 = ( 10.1 / 4.9 ) * R2
    So maybe the second equation cannot be rewritten as R1 = ( 4.9 / 10.1 ) * R2
  4. Jul 24, 2016 #3

    jim hardy

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    That's algebra not electronics


    solve for R1 ?

    There are several paths to get there
    i would

    1. divide both sides by 4.9
    2. multiply both sides by (R1 + R2)
    3. subtract from both sides R2

    and when you do that you'll find author's mistake
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