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please help missing resistance value

Discussion in 'Homework Help' started by bigtomoh, Mar 4, 2016.

  1. bigtomoh

    bigtomoh New Member

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    please help me with question 8
     

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  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi big,
    As this is homework I will just give you a hint.
    Lets do it step by step.

    What is the circuit resistance, if the current is 40Amps at 200Vdc.???

    E
     
  3. Les Jones

    Les Jones Well-Known Member

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    Google "ohms law" Show us how you have attempted to solve the the 4 parts of the question.

    Les.
     
  4. dave

    Dave New Member

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  5. bigtomoh

    bigtomoh New Member

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    200/40 =5 ohms
     
  6. Les Jones

    Les Jones Well-Known Member

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    That is correct for the value of the 3 resistors in parallel. I can see two ways to proceed. 1/ You now know the value of the 3 resistors in parallel. You know the value of two of the resistors so you can work out the value of the resistance of those 2 in parallel. You then have two resistors in parallel that give a resistance of 5 ohms and you have just calculated the value of one of them (R1 and R2 in parallel.) so you now can calculate R3. 2/ Calculate the current in R1. Calculate the current in R2. You can then see what the current in R3 must be. (As you have been given the total current.) From that you can calculate the value of R3.
    (I used the second method.) You now have all the information to calculate the power dissipated in each resistor.

    Les.
     
  7. bigtomoh

    bigtomoh New Member

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    r3 = 29.8 ohms

    thank you so much it was doing my head in
     
  8. bigtomoh

    bigtomoh New Member

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    your a champ
     
  9. Les Jones

    Les Jones Well-Known Member

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    Your value for R3 is not quite right. I suspect that you have not been working to enough decimal places. (This is a situation were fractions give a more accurate answer than decimals.)

    Les.
     
  10. bigtomoh

    bigtomoh New Member

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    in australia we have been told to round up
    200/10=20 200/15=13.3 r3=6.7=29.85 or 30 ohms ?
     
  11. bigtomoh

    bigtomoh New Member

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    thanks for your help have been studying all day my brain is a bit fried
     
  12. JoeJester

    JoeJester Active Member

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    If you were to round to zero decimal places, you would get:

    160305-resistors-in-parallel-round-zero.png

    If you were to round to three decimal places, you would get:

    160305-resistors-in-parallel-round-three.png

    In some circles, in another time, we rounded to two decimal places using scientific notation.

    160305-resistors-in-parallel-scientific-notation.png

    The calculated cells are marked in Grey. The yellow highlighted cell was calculated using the parallel resistance formula, taking the reciprocal of the sum of the conductance of the resistors.

    160305-resistors-in-parallel-calculated-cells.png
     
    Last edited: Mar 5, 2016

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