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Homework Help: Complex number division.

  1. Aug 26, 2009 #1
    When finding the current in a 4+j3 balanced load star connected circuit. with 400V line voltage at 50Hz.

    Do you find the current by dividing the phase voltage approx. 230V by 4+j3 ohms?

    and if so. how do you divide normal numbers with complex numbers?

    your help is appreciated.
  2. jcsd
  3. Aug 26, 2009 #2


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    Science Advisor

    You put the 230 volts over 4 + j3 then multiply by 4 - j3 over 4 - j3. (You can multiply anything by 1 and it stays the same.)

    So, the top line becomes 920 - j690

    The bottom line becomes 16 + 9 (because J = sqrt of -1 ). =25

    Dividing by 25, Current = (36.8 - J 27.6) amps


    Another way is to convert to polar notation.

    4 + j 3 is 5 ohms at an angle of +36.87 degrees { 5= SQRT(16 + 9) 36.87 = tan-1 of 3 on 4}

    so divide 230 by 5 and take the angle to the top line as a negative

    so the current is 46 amps at an angle of - 36.87 degrees.
    Last edited: Aug 26, 2009
  4. Aug 26, 2009 #3
    you mentioned in the first part of the question that the current is 36.8-J27.6 =

    is the 27.6 the angle?

    and what is the 46A magnitude?

  5. Aug 26, 2009 #4

    no, just flip the complex number to polar form
  6. Aug 26, 2009 #5


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    Dividing by 25, Current = 36.8 - J 27.6 = 46 amps magnitude.

    This was just a crosscheck on the second part.
    The magnitude is sqrt ( 36.8 * 36.8 + 27.6 * 27.6) =46 amps
    The angle of this is the inverse tan of -27.6 / 36.8 which is -36.87 degrees.

    I find it helps to picture this as a right angled triangle with sides of 36.8 and 27.6 with the right angle between these.
    The magnitude is the length of the 3rd side of the triangle. This works out to be 46. (you get this with Pythagoras's Theorem)
    The angle is the angle between the 46 and the 36.8 sides of the triangle.
  7. Aug 26, 2009 #6
    Convert everything to the phasor domain and carry on division as usual in the phasor domain.
  8. Aug 26, 2009 #7


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    Staff: Mentor

    Thread moved from EE to Homework Help.

    Please be careful, folks, responding to misplaced questions like this one. Ask youself if it might be a misplaced homework/coursework question, and if it could be, then most likely it will get moved out of the general technical forums, and the normal HH rules apply. So the OP should be doing all of this work, not you guys.
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