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Please help. My mind ceases to function.

  1. Feb 12, 2005 #1

    Consider the circuit shown in Figure P28.9. (R = 33.0 .)

    The question ask me to find the current flowing through the 33 ohm resistance and also the potential difference between point a and b.

    I consider the 10, 5 and (33+5) to be parallel; 33 is in series with 5.

    So the equivalent resistance found by me is 13.06 ohm, thus the total current is 1.91A. Then I apply the current division method and found that the current flowing through the 33 ohm was 0.154A. This answer was wrong!

    Somebody please help me. I am quite depressing now. What's wrong with my answer?
  2. jcsd
  3. Feb 12, 2005 #2
    13.06 is right as is the 1.91A

    Your current through the R leg is wrong though. How exactly did you find that value?
  4. Feb 12, 2005 #3
    First I sum up the 10 and 5 resistance which are parallel. Then i can treat the circuit as if I have 2 resistances in parallel which are 10||5 = 3.33 ohm and (33+5) = 38 ohm. Then, I apply the current division law to find the current flowing through into the 38 ohm.
  5. Feb 12, 2005 #4
    Here's another approach(KVL which I'd use this for simple circuits):

    You have 1.91A going through the series 10 ohm resistor thus that resistor is dropping 19.1V.

    The parallel legs of the circuit will all have the same voltage drop(by KVL) which will be 25-19.1=5.9V

    so, [tex]V_R=5.9V/38=0.154A[/tex]

    I'm sorry, I used a total resistance of 52 through the parallel elements(adding in ones head can lead to wrong answers).

    Your answer appears to be right.
  6. Feb 12, 2005 #5


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    Science Advisor

    Any chance you miscopied the value R=(33 ohms)? Could it be R=(3.3 ohms)?
    Also, what was the answer provided? It may give a clue to the problem here.
  7. Feb 12, 2005 #6
    I have again submitted the answer. The potential difference appears appear to be correct. But the current is still wrong. What's wrong again? I only have one last attempt only. Please help.
  8. Feb 12, 2005 #7
    I have double checked the value, it's really 33.0 ohms. It's an online assignment of mine, so there is no answer provided.
  9. Feb 12, 2005 #8


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    Science Advisor

    Well, 3 people here have all calculated exactly the same results!!
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