# 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

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
54
0
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:

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
18,087
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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
54
0
Alright, thanks for the help. I get it now.