Voltage/Current phase diff and more

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Discussion Overview

The discussion revolves around the physical interpretation of voltage and current phase differences, particularly in the context of impedance, reactance, and the behavior of capacitors and inductors in AC circuits. Participants seek clarification on these concepts and their implications in electrical engineering.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants describe that a phase difference between voltage and current indicates that maximum voltage and maximum current do not occur simultaneously.
  • One participant explains the behavior of a pure capacitor, noting that current peaks 90 degrees before voltage peaks due to the charge dynamics in the capacitor.
  • Another participant contrasts this with inductors, stating that the voltage peak occurs 90 degrees before the current peak.
  • There is mention of the significance of real and imaginary components of impedance, though the physical implications remain unclear to some participants.
  • One participant highlights that power dissipated by inductors and capacitors is considered imaginary, indicating that these components absorb rather than dissipate power, leading to zero net energy consumption over complete cycles.
  • Another participant expresses difficulty in understanding the behavior of inductors compared to capacitors, suggesting that explanations often favor the capacitive case.
  • Euler's formula is referenced as a useful tool for understanding the real and imaginary aspects of impedance.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the phase relationships and the implications of reactance. While some explanations are appreciated, there remains uncertainty and a lack of consensus on the physical significance of the real and imaginary components of impedance and the behavior of inductors compared to capacitors.

Contextual Notes

Some participants note that discussions on inductors are often less detailed than those on capacitors, which may lead to gaps in understanding. The conversation also touches on the complexities of power dynamics in AC circuits, particularly regarding energy absorption and release.

martix
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Voltage/Current phase difference and more...

Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).
 
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martix said:
Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).
A phase difference between voltage and current simply means that points of maximum voltage do not occur at the same time as maximum current.

For a pure capacitor the greatest current occurs when the applied voltage begins to increase from 0 as there is not yet any charge on the capacitor. As the applied voltage increases the charge on the capacitor increases due to the current flow. At the same time, the capacitive reactance (ie. the voltage caused by the charge build up in the capacitor) increases and opposes the applied voltage, thereby limiting current. When the applied voltage is maximum, the applied voltage is the same as the opposing voltage from the build up of charge on the capacitor, so the current is zero. The applied voltage starts to decrease and the charge on the capacitor starts flowing out of the capacitor. The discharge current is maximum when the applied voltage is 0.

So you can see from that qualitative description of a capacitor that current is out of step with voltage. If you plot the sine curve for current and voltage, the current peak will occur 90 degrees (or 1/4 of a cycle) before the voltage peak.

The opposite occurs for an inductor. For an inductor, the voltage peak occurs 90 degrees before current peaks.

AM
 
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martix said:
Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).

And here's a thread from the EE forum where we discussed this also:

https://www.physicsforums.com/showthread.php?t=174615

.
 


berkeman said:
And here's a thread from the EE forum where we discussed this also:

https://www.physicsforums.com/showthread.php?t=174615

.
This is a useful thread. But with respect to this comment:

what said:
Also note that power dissipated by an inductor or a capacitor is imaginary. That means instead of dissipating power as heat like a resistor, capacitor or an inductor is absorbing power (because of lead or lag).
The terminology might be misleading. In a circuit with a pure inductor or capacitor (R=0) to which an alternating voltage is applied, power is not dissipated. The energy consumed over any number of complete cycles is zero. That is:

[tex]\int_{0}^{nT} \vec{V}\cdot\vec{I}dt = 0[/tex]

This, of course, ignores loss due to electromagnetic waves, which becomes significant at high frequencies.

A capacitor or inductor does not just absorb energy, it also releases energy. It is the periodic absorption and release of energy that results in no net dissipation of energy.

AM
 
A great explanation from Andrew Mason. Its amazing how the right formulation can put things into perspective. Had a little trouble figuring what happens to an inductor though. As someone mentioned in the other thread - "Most expositions explain the capacitive case but gloss over the inductive one."
As for the real/imag part - I found out about Euler's formula in the mean time. Amazing thing really. :)
 
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