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Homework Help: Sinusoids represented by Phasors

  1. Jul 12, 2008 #1
    1. Find the sinusoids represented by these phasors:
    (a) I = -3 + j4 A


    well to convert first to polar they got the following

    they didnt show their steps:

    I = 5 <126.87

    then they said

    i(t) = 5 cos(wt + 126.87)

    What I dont get is the fact that how they got 126.87

    to get 5 i just take 3 and 4 sum their squares and take the square root.

    to find the angle i would do tan-1(y/x) = tan-1(4/-3)=-53.13

    ok since its tangent you may say u can add 180 and it is the same answer which is what they did -53.13 +180=126.87

    but my question is WHY. why do they add the 180 if its the same thing. Whats the use. I would just leave my final answer as

    i(t) = 5cos(wt - 53.13)

    why add 180?
  2. jcsd
  3. Jul 13, 2008 #2


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    Homework Helper

    It's easily seen if you sketch the Argand diagram for -3+j4. Remember that the angle you want is measured from the x-axis on the right to the complex number "vector". And that is given by 180 - 53.13.
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