Phasors

Discussion in 'Homework Help' started by regexp, Nov 28, 2010.

  1. regexp

    Thread Starter New Member

    Nov 20, 2010
    24
    0
    Hi,

    I have this simple circuit
    [​IMG]

    i Just need to find the phasor current I. But i must be doing something wrong,
    because my answer differs a bit from the answer in the book.

    1. Replace the capacitance with it's complex impedance
    -\frac{1}{333\cdot10^{-5}} = -J300.3

    2. find Z_eq = \frac{1}{1/115 + 1/-300.3} = 107.39

    3. 1585/107.39 = 14.76, the correct answer is supposed to be 15.11.

    What am i doing wrong?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You forgot to include the j in the Zeq equation.
     
  3. regexp

    Thread Starter New Member

    Nov 20, 2010
    24
    0
    How do you mean? The angle?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It should be:

    Zeq=1/[1/115-1/j300.3]=1/[1/115+j/300.3]

    which equals:

    Zeq=1/(0.0093<20.95)=107.53<-20.95
     
  5. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    On the picture the voltage source will be 1585 * 1.4142 (root 2) i.e 2241.19 Volt and the polar angle is 12 deg.

    Good luck
     
  6. TweedleDee

    New Member

    Nov 9, 2010
    5
    0
    Hmm, I'm getting the same answer as the OP.
     
  7. TweedleDee

    New Member

    Nov 9, 2010
    5
    0

    But you still use 1585 rms for the calculation right?
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Accurately,

    Zeq=107.39<-20.95 Ω

    and if the RMS value for voltage is used,

    I=14.759<32.95 A
     
  9. regexp

    Thread Starter New Member

    Nov 20, 2010
    24
    0
    [​IMG]

    What am i doing wrong?
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    These answers are wrong!
     
  11. regexp

    Thread Starter New Member

    Nov 20, 2010
    24
    0
    Ok, thanks for clearing that up.
    I f#&%? hate my book.
     
  12. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Well, do not always trust me too. :p
     
  13. regexp

    Thread Starter New Member

    Nov 20, 2010
    24
    0
    Hi,

    I have one last question.

    If i have parallel RLC phasors, how do i get the angle of the resulting phasor.

    For example, i would like to find the equivalent of these two parallel phasors  37.94\angle71.5 and
     133.33\angle-90

    I know how to get |Z| , just not how to get the angle.

    \frac{1}{\frac{1}{37.94} + \frac{1}{133.33}} =  29.53
     
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