Need help with this problem

Discussion in 'Homework Help' started by duncker, May 11, 2014.

  1. duncker

    Thread Starter New Member

    May 11, 2014
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    Hi, I hope someone can help. I've been stuck on this problem for 4 hours. The answer is in the back of the book but I don't see how they came to it.
    And the text book has no example problems similar to it.
    I'm supposed to find the voltage across each component in the circuit.
    So as you can see in the attached picture, R1 (220Ω) is in series with the parallel combination of L(1.0mH) and R2 (1500Ω). I can deal with thr R2 and L to find Z for that part of the circuit. I'm just having trouble figuring out what to do with R1.
     
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  2. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Do you understand Kirchhoff's voltage law. That is key to solving your dilemma.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    R2 is in parallel with XL. What is the equivalent combined resistance?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    You need to show your work (your best attempt) so that we can see what you have right and where you appear to be going astray.
     
  5. duncker

    Thread Starter New Member

    May 11, 2014
    7
    0
    I got 224 Ω for R equivalent. But it doesn't seem right. The book says VR1 should be 7.92V and of course the voltage across L should be the same as R2 . Book says they should be 20.8V. So lost as to how that adds up to 25V.
     
  6. duncker

    Thread Starter New Member

    May 11, 2014
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    ok...I'm on it
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Why doesn't it seem right? If you can articulate that, then perhaps that will help you see what you are overlooking.

    Remember that they are phasors and therefore add trigonometrically. So the sum of the two could have a magnitude anywhere between roughly 13V and 29V depending on the relative angle of the two.

    Show the steps you used to find the R equivalent, because it is pretty clear that you are NOT taking the phase information into account.
     
  8. duncker

    Thread Starter New Member

    May 11, 2014
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    This is how I got R equivalent. What did I do wrong?
    Thank you guys for the help.
     
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  9. WBahn

    Moderator

    Mar 31, 2012
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    As I suspected, you are messing up with the phase information.

    Think about what your equivalent circuit would be if R2 wasn't there (or was another inductor). Then, your approach, you would end up with 0Ω instead of 224Ω. But you would know that that made no sense because you WOULD develop some voltage across the inductor at any non-zero frequency.

    Now, before we can go much further we need to get a feel for the level of math you are bringing to the table. Are you working with impedances and reactances using complex numbers (i.e., real and imaginary components) or strictly with magnitudes and angles and a ton of formulas on how to combine them?
     
  10. duncker

    Thread Starter New Member

    May 11, 2014
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    Yeah, about that. I pretty much stopped at Math 98. And I muscled through it. I tend to do better when I see an example through to the end. I don't like making excuses but I don't understand a single sentence my teacher says due to his thick accent. I've been doing ok with just the textbook and youtube but I'm really stuck on this one. Also he got a different answer than the book which just really threw me off. See this was one of last weeks homework problems. But I got an exam tomorrow and if I could just see how this is solved even with different values I think I can do it.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    Let's see if this will help. Try to answer each question one by one. Get as far as you can. Then we will go through them one by one and get you back on track as needed. Remember to show your work.

    Let's call the parallel combination of L and R2 the impedance Z.

    Q1) What is the resistance Z?

    Q2) What is the reactance Z?

    Let's call the series combination of R1 and Z the impedance Ztot.

    Q3) What is the resistance of Ztot?

    Q4) What is the reactance of Ztot?

    Q5) What is the impedance of Ztot?

    Now let's use the source and Ztot to find the total current.

    Q6) What is the magnitude of the total current flowing out from the source?

    Q7) What is the phase angle of the total current flowing out from the source?

    Now let's use that to find the voltage drop across R1.

    Q8) What is the magnitude of the voltage drop across R1?

    Q9) What is the phase angle of the voltage drop across R1?

    Finally, we can use this to find the voltage across L and R2.

    Q10) What is the sum of the voltage rise due to the source and the voltage drop due to the current in R1 (remember, we need both magnitude and angle).
     
  12. duncker

    Thread Starter New Member

    May 11, 2014
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    Now I'm all confused....I don't learn very well this way...just not my style. I learn better by seeing someone working out a similar problem. Thank you anyway and sorry if I wasted your time.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Sorry I couldn't help. Don't worry about wasting my time -- you didn't. But I'm concerned that you might be wasting your time by choosing this career path. It seems like it might not be a good match for how you generally function. Something for you to consider and reflect on.
     
  14. duncker

    Thread Starter New Member

    May 11, 2014
    7
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    Career path? The degree I'm shooting for is just a piece of paper to me. I got the job already, I just took advantage of the free degree program my work offered last year. They are having teachers come to the company and teach there. See management changed the requirements for the job title for the new hires. Now they need a AS degree in electronics before they can even apply. I got the job 7 years ago but when they said "free degree" I thought "why not?". I build, repair, and maintain photolithography machines for a semiconductor fab. I work alongside people with PHDs (Engineering department) that couldn't fix a sandwich let alone a machine. So you must understand, everybody's DNA is different.Some people work better on paper, somehow I have a gift for troubleshooting problems on real machines. I'm a hand's on kinda guy (which is why I would hate to be an engineer).Hence everybody learns differently. The way one "functions" does not determine their career options.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    My mistake. I assumed that you were trying to learn electronics so that you could use that knowledge. If it's just a free piece of paper to you, then have at it and good luck to you.
     
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