Help, I am hopelessly stuck.

Discussion in 'Homework Help' started by Jhughes707, Jul 10, 2019.

  1. Jhughes707

    Thread Starter New Member

    Jul 10, 2019
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    I wasnt given the resistance to R2, what I wrote next to it in the photo is where I ended up. I think the method I used to get there was far too complex and probably wrong. Even though I found the Power dissipation through that resistor was correct at 1.6w. Is there a more elegant way to find the missing resistance? Any help is appreciated.

    Oh, I used: 1/((1/Rt) - (1/R4) - (1/R3) - (1/R1)) = -17.86 + 65 = 47.14

    Then I used: It x R2 = 28
    Then: I (28) / V (45) = 1.6w

    And 1.6w is what's listed as the answer. But this just seemed like a lot to do.
     
  2. CharlesWMcDonald

    Member

    May 16, 2019
    218
    70
    I hate to tell you this but you have it completely wrong. I will give you the answer but you need to follow the calculation to learn how to do it yourself.

    1. Find the resistance of the parallel combination of resistors. We do this by determining the total resistance of the circuit and subtracting the value of the series resistor R4.
    upload_2019-7-10_20-30-19.png
    2. Now determine the value of R2 using the reciprocals.
    upload_2019-7-10_20-31-40.png
    3. Determine the power dissipation in R2 using the square of the voltage across R2 divided by the resistance. We have to find the voltage by subtracting the voltage across R4 from the total voltage.
    upload_2019-7-10_20-38-12.png

    This is obviously a little more complicated than your attempt but it is accurate. I used SMath Studio to calculate the values. I highly recommend this program for anyone learning electronics: https://en.smath.com/view/SMathStudio/summary
    If you get SMath you can open the attached file and play with the values. I had to put it in a zip file.

    Good luck with your studies, electronics is fun and rewarding.
     
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  3. Jhughes707

    Thread Starter New Member

    Jul 10, 2019
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    Oh my gosh thank you so much! I'm self teaching and it was driving me nuts trying to find the correct way to do this. Again, thank you and thanks for the link to SMath. My sanity remains intact.
     
  4. CharlesWMcDonald

    Member

    May 16, 2019
    218
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    What books, web sites or other materials are you using?
     
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  5. Jhughes707

    Thread Starter New Member

    Jul 10, 2019
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    Basic Electronics Theory 4th edition by Delton T Horn, Khans academy and that last picture I attached is a course overview from my coworker. He brought it back from a $1500 class structured for a test we want to pass for a specific position. So, those and whatever else I can find like TinyPic flash card app and the web.
     
  6. CharlesWMcDonald

    Member

    May 16, 2019
    218
    70
  7. RBR1317

    Senior Member

    Nov 13, 2010
    464
    87
    You mean "elegant" as in using fancy formulas? Or the elegance of stark simplicity? All that is necessary for this problem is the simple application of Ohm's Law to find the voltage & current for each resistor.
    20190711.png
     
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  8. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,787
    1,920
    Look up there ^ at the 'education' link. There is lots of useful stuff in there.
     
  9. Jhughes707

    Thread Starter New Member

    Jul 10, 2019
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    Wow, what a great resource. I skimmed volume 1 last night before bed, and I'm heading home from work now, so I'll be able to get some study time in. Really cant thank you enough for this.
     
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