Nortons theorem

Discussion in 'Homework Help' started by sunny1982, Jan 14, 2013.

  1. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    Hi people I am stuck on this peice of homework, I have to find the current flowing through the load which is 1Ω using nortons theorem, I have done it in kirchoffs but when im doing it in nortons its not the same answer. I get 0.407amps when i've done it in kirchoffs sorry about the writing as its scruffy but its only a rough copy, I just wanted to see where am I going wrong.
     
  2. WBahn

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    Mar 31, 2012
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    I can't tell what the polarity of E3 is because you have drawn it symmetrically.

    Have you verified that 0.407A (you didn't say what direction!) is correct by checking if it satisfies KCL and KVL everywhere?

    Depending on the polarity of E3, I get either 0.444A or 0.889A.
     
    Last edited: Jan 14, 2013
  3. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    Heres how I did it in kirchoffs law, from this you will see the polarity so that means i have done it wrong in kirchoffs so please can you check where ive gone wrong with kirchoffs please.
     
  4. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    you are right i accidently drew it symetrically the drawing above shows how its meant to be.
     
  5. WBahn

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    Mar 31, 2012
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    Remember that I1 and I2 are LOOP currents. The actual current in a given branch is the algebraic sum of all of the loop currents that pass through that branch.

    You need to track, and check, your units throughout your work. Don't just tack them on at the end. Units are a fundamental part of the quantity and tracking units is perhaps the single most valuable error detection and correction tool available to you. Most (not all) mistakes you make will screw up the units.
     
  6. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    I can't find where i've gone wrong:( you see i did these two tasks in kirchoffs and thevenin and they both gave me the same answers. But I don't understand why when i did the kirchoffs and norton why i couldn't get the same answers.
     
  7. WBahn

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    Mar 31, 2012
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    Why do you keep insisting that the only current through the load is I2?

    Your very first line shows that you understand that the current through the load is (I1-I2).
     
  8. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    well thats right isn't it I1-I2?
     
  9. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    so basically all my work i have posted on here is wrong? i know the nortons theorem is wrong?
     
  10. WBahn

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    Mar 31, 2012
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    Yes, the current through the load is (I1-I2), but you keep giving the answer as 0.407A, which is JUST the value of I2 (let's stick to the first problem only). Where is I1? Where is (I1-I2)?
     
  11. WBahn

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    Mar 31, 2012
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    No. You calculation of I2 is fine. But I2 is NOT the load current. You are only halfway done. You need to find I1 and then compute the value of the current in the load (AFTER you DEFINE what direction the load current is).
     
  12. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    Please help me sort this mess out. First of can you please clarify this please, this circuit i did it in kirchoffs and then in thevenins can you tell me if these are correct?
     
  13. WBahn

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    Not until you track your units. Go back and put your units in your work, from beginning to end, and I will look at it.
     
  14. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    i'm confused but here lets do this one circuit at a time, this is the first circuit and i've tracked the units this is done in kirchoffs law is this answer correct?
     
  15. WBahn

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    Mar 31, 2012
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    Let's see if we can check the answer from the answer itself.

    Your answer is 0.474A (474mA) flowing downward through the 30Ω resistor. If we define the bottom node (Node D-C-F) as 0V, this means that the voltage on the top node (Node A-B-E) must be 0V+(474mA)(30Ω)=14.22V.

    If the voltage on the top node is 14.22V, that means that the voltage across the 10Ω resistor (in the left brance) is 14.22V-20V=-5.78V (assuming top of resistor is the positive side). This, in turn, means that the current in this resistor is -5.78V/10Ω = -578mA flowing downward, so it is actually 578mA flowing upward, the same direction as I1, so I1=578mA.

    The current flowing downward in R2 is (I1-I2)=578mA-474mA=104mA making the voltage (top relative to bottom) across R2 equal to (104mV)(12Ω)=1.248V.

    This makes the voltage of E2 equal to 14.22V-1.248V=12.97V, which is close enough to 13V to write off the difference to roundoff error.

    So it would appear that your answer is correct.

    Hopefully you also see how you can go about checking your own answers. This is a vitally important skill to develop because, as a practicing engineer, you will almost never have someone to check your answers for you -- if there were, there would be no need to pay you to solve the problem in the first place. But checking that your answers are correct is still pretty crucial, so you have to find ways to do it yourself.
     
  16. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    fantastic. Thanks alot you have been a great help. I've used your formula now both of my kirchoffs law, nortons and thevenins are correct. The reason why I'm not paying attention to this much is because this is a electrical engineeering science unit in the HNC mechanical engineering. If was going to go in to electronics then I would be facing these situations reguarly. After handing this assignment in i don't think il ever use this in my life.

    The next question is kirchoffs and max power theorem.
     
  17. WBahn

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    Mar 31, 2012
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    Okay, but the concept still holds just as well in mechanical engineering as in electrical -- you can usually verify the correctness of an answer from the answer itself and tracking and checking units is critical.

    Imagine the reaction if a doctor's patient died because the doctor prescribed them 10g of some really powerful drug instead of 10mg. People would be up in arms and insisting that the doctor be charged with criminal malpractice because, as we all know, if a doctor makes a mistake someone could die and, therefore, we demand that doctors take extreme care to avoid making mistakes.

    What most people don't realize is that doctors almost always kill people one at a time, while engineers can, and do, kill people in job lots. History is rife with examples of engineering mistakes that killed people and that came down to an engineer being too lazy to track their units or ask if the answer made sense.
     
    Last edited: Jan 15, 2013
  18. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    I understand when you say it like that it makes perfect sense. Now I need help on this task. the values are in the table it doesn't make sense to me.
     
  19. WBahn

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    There are a number of ways to come at this, depending on what it is you are being expected to learn, specifically.

    What is it about the values in the table that don't make sense to you? You need to learn how to gather your thoughts and ask specific questions aimed at filling in specific gaps in your understanding.
     
  20. sunny1982

    Thread Starter Member

    Dec 27, 2012
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    No I understand that the value of the tables are the values to be used in the circuit. But I don't know how to start this one off as it said to use kirchoffs law and max power theorem, the bit i don't understand

    1) can you combine kirchoffs and max power theorem
    2) do I do it in kirchoffs law first then in max power theorem
     
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