confusion with getting Ct with a series circuit

Discussion in 'General Electronics Chat' started by james7701, Feb 18, 2016.

  1. james7701

    Thread Starter New Member

    Jan 5, 2016
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    ok so, the first problem i was able to apply the formula to get my answer but, the 2nd problem i don't understand why im not getting the answer. (which is 320 )
    any suggestions as of different approach to this?
     
  2. #12

    Expert

    Nov 30, 2010
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    The answer isn't 320 as far as I can tell. I see 470 uf in series with 1 uf, and the answer to 2 caps in series is always less than the smallest capacitor. I see .9978769 uf on my calculator.

    Still, I prefer a method that can do more than 2 capacitors, so don't get too invested in this formula. It's right for 2 caps, but life serves up different problems eventually.;)
     
  3. james7701

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    Jan 5, 2016
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  4. james7701

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    Jan 5, 2016
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    but, why are they showing an answer of 320?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Unit issues

    470 nF is equal to 0.47 uF

    Or, better to calculate it all out in nanoFarads instead of microfarads (uF)

    So, 1uF = 1000 nF in series with 470 nF

    Try again
     
  6. #12

    Expert

    Nov 30, 2010
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    It's the units labels that went wrong. You have to convert 1 uf to 1000 nf to get the formula to work.
     
  7. GopherT

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    Nov 23, 2012
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  8. james7701

    Thread Starter New Member

    Jan 5, 2016
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    so, i do 470÷1000 and 471÷1000?
     
  9. #12

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    Nov 30, 2010
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    (470 x 1000) /(470+1000)
     
  10. GopherT

    AAC Fanatic!

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    Take reciprocal of c1 + reciprocal of C2

    Then take the reciprocal of that total to get your answer
     
  11. #12

    Expert

    Nov 30, 2010
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    That's the formula I was trying to think of in post #2. You can add up the 1/x's all day long, then 1/x the result to get the answer.
     
  12. james7701

    Thread Starter New Member

    Jan 5, 2016
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    so, it's best to change the units? and than proceed?
     
  13. james7701

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    Jan 5, 2016
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  14. WBahn

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    Mar 31, 2012
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    Because that's the correct answer.

    If you track your units, you would have seen where you went wrong.

    <br />
\frac{470 \, nF \, \times \, 1 \, \mu F}{470 \, nF \, + \, 1 \, \mu F}<br />

    Now when you look at the denominator you don't see 470 + 1, you see two things that can't be added together that way because they don't have the same units.

    <br />
\frac{470 \, nF \, \times \, 1 \, \mu F}{0.470 \, \mu F \, + \, 1 \, \mu F}<br />

    NOW you can add the denominator

    <br />
\frac{470 \, (nF)(\mu F)}{1.470 \, (\mu F)}<br />

    Now the uF cancel, leaving you with 320 nF.

    Always, always, ALWAYS track your units!!!
     
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