Physical explanation of leading capacitive current (270 degrees!) 5

[bvc]

Dear all,
Kindly correct my observations if wrong :
1) In any circuit, the current is produced by means of application of a voltage. Thus voltage is the cause and current the effect.
2) The phasors representing the various currents and voltage show the phase relationship between the components having the same frequency. Thus a phasor which is shown leading w.r.t another can be termed as leading the second phasor in time domain also. Thus, I can say that the first quantity say for example , a voltage is applied before the second quantity e.g current. In case of a resistance, I can say the current appears as soon as the voltage is applied. In case of an inductive circuit, the inductive current appears after a time delay of application of voltage as the inductor does not allow the current through it to change suddenly. In case of a capacitor, the current takes more time to reach steady state as it is an insulator so naturally, it must take more time for the current to flow through it after application of the voltage across it. So in this regard, is it fine to say that the current in case of a capacitor lags the voltage by 270 degree ( that is more time than inductor or resistor) ? I cannot fit a leading current with the time lag explanation as I don't understand how a current can flow through a capacitor without application of voltage ?

Waiting for the comments.

Thanks.

 

[electricpete]

none of the current flows INSIDE the wires

I haven't heard that one. Can you explain what you mean?

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(2B)+(2B)' ?

 

[bvc]

Well, it is nice to know the various view points by industry experts. Myself, I am only 9 years into Electrical engineering work. Some two weeks back , I was going through the "Performance and design of AC machines" by MG Say, for certain information on transformers and I was forced to go back to the no load phasor diagram. Was slightly shocked to know that my understanding of the transformer phasors was wrong. I never read this book ( or for that matter any text book of such standards) during my engineering school days. So some of the links were missing in my understanding of things . Never my intentions to challenge well established facts but only to understand them and that too in the best possible way , not by blindly memorising any formula or phasor diagram! My mentor used to say "never hesitate to beg for knowledge ".

Anyway thanks to everyone out here for their patience and time. I have got my clarifications.

Cheers !!

 

[Skogsgurra]

I think that I should apologize for being too outspoken. You have the right attitude!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bvc]

Thanks ! Even the 270 degree stuff was not one of my own making but rather it was conveyed during a training session by an old Engineer ( well past his 60's) some 3 years back.
I used to be a lazy bone during my college days -. With the passing of time, I am sure I will become a better Engineer.

 

[mikekilroy]

electricpete,

I do not want to get into trouble with discussing a topic of how current flows OUTSIDE wires and get banned here.....but just a teaser....

In defense of our friend bvc, schools, including engineering universities, teach the 'bump' theory - that electrons travel at the speed of light down the inside of the wire to the load by bumping into each other..... they do this as it is easiest to convey the CONCEPT of current flow - just as they teach current flows THRU a capacitor (insulator). but we also know that electron flow inside the wire is at a SLOW rate of only a few inches per second! So how does this instantaneous energy get from the source to the load?

Energy does not travel in the wire at all but travels outside the wire in the surrounding TWO fields 90 degrees to each other: magnetic field and electric field and this happens near instantaneously. The only energy inside the wire is due to losses from these fields cutting across it..., there are some really good discussions of this elsewhere if one wants pursue it.

 

[electricpete]

mike - I get your gist. If we look for where the power is transmitted electromagnetically (associated with the Poynting vector S=ExH), it is not within a perfect conductor because the fields are zero there. Or for that matter even in a real conductor the radial fields are approximately zero. So the power is transmitted electromagnetically in the associated fields which may lie between conductors or between a conductor and a shield. It is not such a surprise that the E and H fields extend outside the conductor. What you said originally was a little different... that the current flows outside the conductor. But maybe I'm splitting hairs. Good discussion.



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(2B)+(2B)' ?

 

[electricpete]

I complicated the whole thing by mentioning perfect conductor, where current flows on the skin. Let me remove that distractor. I should've stuck with real conductor. Radial fields are zero inside the real conductor, so the power transmitted (longitudinally) by a set of real conductors lives in the radial fields outside the conductors.

=====================================
(2B)+(2B)' ?

 

[electricpete]

All of which goes to prove, you can make anything more complicated than it needs to be.

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(2B)+(2B)' ?

 

[DRWeig]

Occam's razor, Pete.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[veritas]

Skogsgurra - thank you and thank you also for all your posts. I've read many of them and they've been very helpful. Keep it up.

 

[Bronzeado]

Hi folks,

Really, I enjoyed this discussion. Back to fundamentals!

I think the confusion from the ideias as because some of you thinks physically and others mathematically.
Regarding to bvc starting question, if the angle between the voltage and current applied on a capacitor
is 90 in advance or 270 behind, it is happens only on steady state condition, with the phasor theory been applied (maths).

However, as Skogsgurra said, as soon the voltage is applied on a capacitor, a current in it is generated (physics).

About current flowing through a capacitor, physically it does not, but mathematically it may depending on the model applied to solve an engineering problem. The same thinking may be applied to current outside the lines.

At the end of the day, all of you are right!

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[bvc]

The same electrical phenomenon can be explained based on Circuit Theory or Field Theory ( may be some more theory I am not aware of) giving rise to different explanations.

 

[ScottyUK]

All of which goes to prove, you can make anything more complicated than it needs to be.

I seem to recall that in school, up until the age of maybe 14 or 15 years, teachers insisted that it was impossible to take a square root of a negative number. Then one day in high school 'i' appeared and we realised that we had been lied to for years, only for 'i' to become 'j' in electrical principles class a few years later... and then having gotten to grips with the 'j' operator we are confronted by the 'h' operator in a bid to confuse us even further, on the off chance that Messrs Laplace, Dirac, et. al had not already already succeeded in that goal...

 

[PHovnanian]

we are confronted by the 'h' operator in a bid to confuse us even further,

I understand the root of the 'i' vs 'j' schism.

But 'h'? Why? Is someone still trying to use Fortran?

 

[ScottyUK]

You're more than familiar with the 'j' operator that is used to denote a 90° phase shift relative to a reference vector. The 'h' operator does the same except it denotes a 120° shift. Mathematically it is the complex cube root of 1. I'm sure you can see its immediate usefulness in 3-phase analysis by avoiding the awkward -1/2 ± j1/[√]3 term which occurs when the 'j' operator is used to describe a 3-phase system.

Fortran? Now that is something I haven't had nightmares about for a long time.

 

[electricpete]

I think it's interesting someone chose the words "real" and "imaginary".
Imaginary almost sounds like "fictional"
A complex subject.

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(2B)+(2B)' ?

 

[xnuke]

Scotty

Most of our textbooks on this side of the pond seem to use "a" instead of "h".

electricpete, nice pun.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sibeen]

Most of our textbooks on this side of the pond seem to use "a" instead of "h".

That applies in the antipodes as well. When Scotty mention the 'h' operator I had no idea what he was on about. When he described it I went, hold on , that's 'a' or alpha.

 

[ScottyUK]

I wonder if it is a British thing? I have seen the 'a' operator in texts and never really questioned it; just assumed that it was similar to 'i' versus 'j' between the mathematics world and the engineering world.

 

[PHovnanian]

I wonder if it is a British thing?

I think its British, or European. Like 'u' for voltage instead of 'v'.

I've heard 'a' used on this side of the pond. Although that might have something to do with my proximity to Canada, eh?