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Calculate phasor of voltage in circuit

  1. Nov 15, 2015 #1
    1. The problem statement, all variables and given/known data
    Calculate phasor of the output voltage (Vout). The circuit is powered by a sine wave voltage source (Vs) with amplitude=10V and frequency=1000Hz.

    2. Relevant equations
    Voltage= Current*Impedance (V=IZ)
    Z(R) = R
    Z(C) = 1/jwC
    i(t) = Acos(wt - phi)
    V(out)=[Z(C) / (R+Z(C))]*Vs
    3. The attempt at a solution
    Total capacitance = 6.8nF+15nF=21.8nF
    Z(R)=1000 Ohms
    Z(C)=(-j)(1/(1000*21.8nF) = -45870j
    Vout = [Z(C) / (R+Z(C))]*Vs = 0.98*Vs

    Phasor question.png
     
  2. jcsd
  3. Nov 15, 2015 #2

    gneill

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    Staff: Mentor

    The capacitors are not in parallel; R1 is between their top connectors.

    Hint: You should note that C1 is connected directly across the voltage source. Does C1 affect the rest of the circuit in any way?
     
  4. Nov 15, 2015 #3
    ah ok thanks. I always struggle to differentiate between series and parallel. However, is the final equation I used correct?
     
  5. Nov 15, 2015 #4

    gneill

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    Staff: Mentor

    Yes, if you use the right value for the capacitance when calculating it.
     
  6. Nov 17, 2015 #5
    I'm actually unsure what Vs is?
     
  7. Nov 17, 2015 #6

    gneill

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    Staff: Mentor

    Vs(t) is described as a 10 V amplitude sinewave with a frequency of 1000 Hz. What's the phasor of a sinewave?
     
  8. Nov 17, 2015 #7
    v(t)= A sin(ω t + φ)?
     
  9. Nov 17, 2015 #8

    gneill

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    Staff: Mentor

    No, that's the original time domain form. What do your class notes or text say?
     
  10. Nov 17, 2015 #9
    we didn't really cover this in much detail. Is it Ae^jφ
     
  11. Nov 17, 2015 #10

    gneill

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    Staff: Mentor

    So in this case there's no phase shift specified for the sinewave, making φ equal to zero. What does that leave?
     
  12. Nov 17, 2015 #11
    just A, which is 10v? so Vs is simply 10v?
     
  13. Nov 17, 2015 #12

    gneill

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    Staff: Mentor

    Yes. The phasor representation for a sinewave without a phase shift is simply the amplitude of the sinewave.
     
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