Transmission line phase

Discussion in 'Homework Help' started by jamus, Sep 13, 2013.

  1. jamus

    Thread Starter Member

    Feb 11, 2013
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    Simple question. In my electronics course, impedance was modeled as causing the current to lead or lag the voltage. Was this more of a "tool" or does it basically apply everywhere?
     
    Last edited: Sep 13, 2013
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Not sure what you mean by 'everywhere' but normally the current lags voltage in a purely inductive circuit.
    Max.
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I would say "everywhere" you have an impedance, you will have conditions a lead or lad condition.

    Your lab associated with this should illustrate and reinforce the "ELI" and "ICE" conditions.
     
  4. jamus

    Thread Starter Member

    Feb 11, 2013
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    What about in the case of a transmission line with a distributed inductance and shunt capacitance? The Vout would have the same characteristics as equivalent circuit elements?
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Since a transmission line looks like a low pass filter I would expect the phase to be a function of frequency.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    For the question to make any sense, you need to be clear on the conditions you are talking about. In general, for the course you are almost certainly in, this means:

    1) Sinusoidal steady state operation.

    2) Linear systems.

    In general the voltage across a device will lead the current through that device by an angle that equals the impedance angle of the device.

    Of course, keep in mind that the impedance angle could be zero, making the voltage and current in phase.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    If you are talking about transmission line theory in which you have distributed inductance and shunt capacitance (and possibly resistance and shunt conductance), then you need to take a step back and walk through the notion of "characteristic impedance" and what that means and how it is derived.
     
  8. vk6zgo

    Active Member

    Jul 21, 2012
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    Reactance is that property which causes the current to lead or lag the voltage by 90°

    Impedance is normally used for circuits which possess both Inductive & Capacitive Reactance (& in the real world,resistance).

    Depending on the relative values of the two types of Reactance,current may lead voltage or vice versa,but,except where one Reactance =0Ω,it will not be by 90°.

    Resonance
    For every LC network,there is a frequency where Inductive & Capacitive Reactance are the same value.

    At those frequencies,the voltage & current as applied to the input terminals of the network are in phase.
    Hence the impedance at Resonance appears resistive.
     
  9. jamus

    Thread Starter Member

    Feb 11, 2013
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    Alright, thanks for the help. I get it now.
     
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