I need help to find an unknown Resistor value

Discussion in 'Homework Help' started by KirsDe, Jun 10, 2014.

  1. KirsDe

    Thread Starter New Member

    Apr 27, 2014
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    Rt is 1455. I=6.87 mA and R1= 1K ohm. R2 is the unknown and R3 is in parallel with R2 and R3 has a value of 500 ohms. Please help me with the steps to figure this out!! Thanks in advance!
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    Start by drawing out the circuit and labeling what you are given.

    In that process, you will also show us how it is connected as your description doesn't mention the relationship R1 has with R2 and R3...
     
  3. KirsDe

    Thread Starter New Member

    Apr 27, 2014
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    I cant seem to get this drawn and into message...blond moment...but the R1 is 1 K ohm and in series with the others.
     
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    When posting, click on the "Go Advanced" button. Scrolling down the page a bit, there is a "Manage Attachments" button. On the ensuing page, you can browse to a picture (your sketch) and upload it. Then close the upload window and write your post.

    If you'd like the picture in the body of your post, don't close the upload window right away. Scroll down a bit, and you'll see a link to the picture. Click on it and a new browser window will appear with the picture. Highlight the address in the address bar and copy it. Back in your post, there is a button to insert a picture. Click on it and a popup window will appear asking for the address of the picture. Paste in the address copied of the picture. Now you can close the upload window! Click on "Preview Post" and voila!

    If your having trouble getting it drawn, try the Windows Paint program. Or sketch it on paper and take a picture with your phone. Email it to yourself or otherwise get it on your computer, so you can attach it to your post.

    Is this helpful? I hate blonde moments; my daughter and I suffer from them.
     
    Last edited: Jun 10, 2014
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  5. KirsDe

    Thread Starter New Member

    Apr 27, 2014
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    [​IMG]

    Thanks djs fantasi to the quick assist. I never would have figured that one out!
     
    Last edited: Jun 10, 2014
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    As presented, it can not be solved.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    By the way.
    Where in the circuit Rt and It fit in?
     
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    I am assuming that Rt is Rtherevin or Rtot. Hence,
    Rt=R1+(1/((1/R3)+(1/R2)))
    1455=1000+(1/(.002+(1/R2)))

    One equation and one unknown. Eminently solveable.
     
    KirsDe likes this.
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    R2 is approx 5K1 ohms, 500ohms in parallel with 5K1 is approx 455 ohms
     
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  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Statements made by OP do not support YOUR assumptions at this time.

    When OP explains their circuit more fully, then you and I will re examine the problem.
     
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  11. hobbyist

    Distinguished Member

    Aug 10, 2008
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    step1) Total circuit current (IT) is in series with R1, so all the current flows thru it, causing a voltage drop (VdR1) of (IT x R1) where IT is total circuit current.

    step2) the voltage drop (Vd,R2//R3) across the parrallel combo of R2 // R3, is equal to (V - VdR1), where V is Supply voltage, and (VdR1) is the drop across R1 as calculated in step 1).

    step 3) Now divide R3 into voltage drop (Vd,R2//R3), to solve for branch current (I3).

    step 4) subtract this branch current (I3) from total circuit current (IT) to solve for branch current (I2) flow thru R2. because the total current (IT) flows through this parrallel combo, and branches thru R2 and R3.

    step 5) divide this branch current (I2) into voltage drop (Vd, R2//R3) to solve for value of R2.
     
    step 6) check your work by solving for the parrallel combo (R2//R3) then add this to R1 to see if it comes out to total circuit resistance as prescribed.
     
    Last edited: Jun 12, 2014
  12. BR-549

    Well-Known Member

    Sep 22, 2013
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