rlc in parallel, polar form

Discussion in 'Homework Help' started by ninjaman, Apr 7, 2016.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
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    1
    Hello,

    i have to find the total circuit impedance of an rlc parallel circuit. i used a method on here to find the current and used multisim to simulate the circuit and confirm the currents. so the individual impedances and currents are correct. but i have to use a different method shown in the course work. i have included it at the top of the scan.
    i think this is correct but would like a little help please

    thanks

    simon
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    If you want someone to walk through your work, then make it easy for them to walk through your work. Show the circuit, show the values, and track your units.
     
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
  4. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
  5. WBahn

    Moderator

    Mar 31, 2012
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    Why do you refuse to track your units?

    You won't even tack on the units that you want the answer to have to the final result.

    If I tell you the depth of a lake is 123, do you know if it is safe to scuba dive to the bottom of it?

    What happened to the j factors in the last two terms?

    How did you come up with 1/0.1 as the magnitude? Do magnitudes of complex numbers written in polar form simply add?

    In your handwritten work, you have a value for 1/R that you just tacked units of μΩ onto? Does it make any sense that 1/resistance can have units of resistance?

    Look at the currents you have as answers in Block 10 above. If those are correct, then clearly the total current is dominated by the 1 A ∠90°. Is that consistent with a total impedance of 10 Ω ∠0° (assuming that your '10' is really '10 Ω')?
     
  6. ninjaman

    Thread Starter Member

    May 18, 2013
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  7. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    i went back over it using a method off the internet. this seems to get the correct some answers, whether they are right is beyond me. any help would be great!
    also, some explanation of tacking on units would be good.

    thanks

    simon
     
  8. WBahn

    Moderator

    Mar 31, 2012
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  9. WBahn

    Moderator

    Mar 31, 2012
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    I think you need to take a step back and review how to work with complex numbers.

    A complex number is a number that is made up of the sum of a "real" part and an "imaginary" part. An imaginary number is merely a number that is a multiple of the sqrt(-1), which we associate with the symbol 'j' (or 'i' in mathematics).

    So Z such that

    Z = a + jb

    where 'a' and 'b' are real numbers is a complex number.

    The rules for working with complex numbers are the same as the rules of algebra for working with any thing else. Just think of 'j' as a variable that happens to have a somewhat strange, constant value.

    So if

    Z1 = a + jb

    and

    Z2 = c + jd

    then

    Z3 = Z1 + Z2 = a + jb + c + jd

    Z3 = (a + c) + j(b + d)

    Also note that

    Y = a / (jb) = (ja) / (jjb) = (ja) / (j²b) = (ja) / (-b) = -j (a/b)

    This is simply multiplying top and bottom by 'j' and then recognizing that, by definition, j² = -1.

    Look at how that impacts your work in Block #8.
     
  10. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello WBahn,

    thanks for responding. I am still fairly new to this. i did do two years of college, one day a week. my last college day was over a year ago. i have started a distance learning course at hnd (second year degree) level. this is the first assignment of the first of eight units. i have given up with the method that i used in my previous attempt and have hand written something else. i attached it above, two images 78 and 79. those are my new attempts. why do i like to ignore j factors? its a new hobby, thats why.
    i havent done maths for a while and last time i did it was with a lecturer of poor quality. he made me want to give up electronics, so for a year i did.
    i dont want to give it up but going it alone is hard. not tracking units and leaving out j numbers is something that some one with little background in this stuff does. i have never worked in electronics before and studied at hnc with a poor lecturer. i have read some about it. what i have done above is in rectangular form and then at the end i have changed it to polar form. i am going to scrap the other lot that i put on here as trying to understand what you are saying is a little hard. imagine i have aspergers with possible learning difficulties. units are numbers yes, i tried to do it all in polar form because i am required to provide the answer in polar form. i have read some stuff on this site about complex numbers and tried it that way and seemed to get the "correct" results. though i have to use the way the lecturer specified, i assume anyway. i havent spoken to him yet. hes on holiday until the 12th and i want to get on with it.
    can anyone recommend a good book that explains circuit analysis that would be good for a beginner, lite reading nothing too intense.

    thanks

    simon
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    Before you can hope to use complex numbers to facilitate circuit analysis, you have to become proficient at using complex numbers.

    So spend some time gaining that proficiency. Otherwise you are just digging yourself a big hole.

    Just because the answer is supposed to be in a particular form does not mean that the work has to be done in that same form. Rectangular form works well for adding and subtracting complex numbers, but it is a bit more troublesome for multiplying and dividing. Conversely, polar form is great for multiplying and dividing by is nightmarish for adding and subtraction (unless done graphically). So learn how to translate between the two and how to use the format that best suits the needs of the moment. At the end you can then just translate to whichever format is needed.
     
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