A Pedantic Question

[nsaspook]

Just to toss an extra little hand grenade into the room...

... does this current which may or may not flow through a capacitor, obey Ohms Law?

JimB

Sure, Ohm's law in phasor form. https://en.wikibooks.org/wiki/Circuit_Theory/Phasor_Theorems
So now the question is 'why' do we now need a time based (phase relationship) calculation of current and what causes that?

Does a lamp filament in a incandescent bulb obey Ohms Law?

The grenade is a Dud.

 

[tvtech]

At least this Thread is in OT. Where it belongs. Where Newbies don't look. And get confused.

AC has always meant Alternating Current.
And DC has always meant Direct Current.

I am 50 Years old...and I get confused when Mr Smarty Pants (aka the Hopelessly Pedantic one) tries to impress...

Just imagine how a Kid feels...

Regards,
tvtech

 

[Ratchit]

Eric,

but I dont see any mention of voltage in my post,

Yes, you are correct. But how many times have I seen an appliance label with words to the effect "For 120 VAC only"? In context, I believe you did say in post #48 "but it works for me".

JimB,

Last time I looked capacitors were made of material.

Last time I looked, you asked about current.

Ratch

 

[tvtech]

Hello Ratch

We need you here in SA as a private weapon.......

You will make the people you teach so crazy that they will commit "you know what" in their masses. I have a place for you to stay. Wanna chance it??

And then, South Africa will be truly free.

Oh no. Almost Politics again

Regards,
tvtech

 

[Ratchit]

tvtech,

We need you here in SA as a private weapon.......

I am not weaponized, and I like it where I am.

You will make the people you teach so crazy that they will commit "you know what" in their masses. I have a place for you to stay. Wanna chance it??

No, not really. I don't want to be a part of a "final solution" you seem to have in mind.

And then, South Africa will be truly free.

And be happy for 1000 years?

Ratch

 

[tvtech]

LOL Ratch

You sometimes have a sense of humour too

Lighten up please. If only for me.

tvtech dislikes it when you complicate things....Rather start your own Thread when you feel the urge to discuss stuff.

There are a Number of people that are on the same level as you.....I am not one. I deal with practical issues. Not theories.

Regards,
tvtech

 

[steveB]

What about the professor referenced by nsaspook in post #37 of this thread, who specifically says that current does not exist through a cap?

Ratch,

Let's be fair and accurate here. Prof. Lewin was talking about displacement current in the beginning parts of the lecture. He is basically making the same mistake (although it may be a deliberate mistake in his case) I made in the other thread where I attributed all current associated with "going through" to displacement current. This is a natural approach/mistake to make when you are trying to describe displacement current. His comments about no charge flow current through the dielectric are relevant for vacuum dielectric caps (or air caps practically). Practical caps with material dielectrics have polarization current. Later in the lecture (time: 21:35 to 22:50) Prof. Lewin clarifies and corrects his previous mistake and mentions that displacement current does not involve real charge flow, but polarization current in the material is a real charge movement (I'll say movement rather than flow). "There is indeed a current" are his exact words at that point in the lecture. So it's not fair to pick and choose one statement to favor your opinion. I could quote the other part of the lecture and make the exact opposite point. In fact, both statements (although they contradict) are part of the full truth.

So, the idea of no charge movement (or flow) is attributable only to displacement current for vacuum, but not polarization current for material dielectrics. For any capacitor (even one with super high permittivity) there has to be some contribution from displacement current. So, even if >99% of the wire drift current can be associated with polarization current in the material, there is still missing charge flow that must be associated with displacement current which is not actual charges flowing (but is still called a current).

So we are seeing even an outstanding professor and scientist like Prof. Lewin will not speak perfect literal correct facts at all points in the teaching activity, especially when speaking (as opposed to writing where there is more time to think). It's simply too difficult to be perfect. Instead, teachers repeat points several times and show things from different points of view, and the sum total of what they say is correct (hopefully), when taken in full context. Sometimes the omissions and false statements are deliberate, with the teacher knowing that too much information will overwhelm. So they sometimes lie a little bit until the critical concepts are absorbed, and then they later clarify and include the pertinent details.

Steve

 

[tcmtech]

So how does all of this relate to the principle of power factor shifting in AC circuits then if no current is being conducted or whatever some want to describe it as?

That and what is my amp clamp reading when I clamp it over the conductor going to or from a capacitor that is being used in an AC circuit?

 

[Ratchit]

steveB,

Let's be fair and accurate here.

That works for me.

Prof. Lewin was talking about displacement current in the beginning parts of the lecture. He is basically making the same mistake (although it may be a deliberate mistake in his case) I made in the other thread where I attributed all current associated with "going through" to displacement current. This is a natural approach/mistake to make when you are trying to describe displacement current. His comments about no charge flow current through the dielectric are relevant for vacuum dielectric caps (or air caps practically). Practical caps with material dielectrics have polarization current. Later in the lecture (time: 21:35 to 22:50) Prof. Lewin clarifies and corrects his previous mistake and mentions that displacement current does not involve real charge flow, but polarization current in the material is a real charge movement (I'll say movement rather than flow). "There is indeed a current" are his exact words at that point in the lecture. So it's not fair to pick and choose one statement to favor your opinion. I could quote the other part of the lecture and make the exact opposite point. In fact, both statements (although they contradict) are part of the full truth.

OK, I see your point. I listened and observed the video at the time you pointed out very carefully. I don't think he made any misstatements or said anything incorrectly. Here is why. He describes current existing in between the plates containing a dielectric, and while doing so, moves both his hands alternately closer together and farther apart as if to to describe an oscillation between the plates. He did not say the current exists across the plates, and did not sweep his hand in one direction to indicate that current existed through the capacitor. That makes sense because he was talking about polarization current which only exists in a material dielectric, and is bound to a dielectric. Afterwards he iterates that current does not exist through a capacitor. Strangely, he did not say "polarization current".

So, the idea of no charge movement (or flow) is attributable only to displacement current for vacuum, but not polarization current for material dielectrics. For any capacitor (even one with super high permittivity) there has to be some contribution from displacement current. So, even if >99% of the wire drift current can be associated with polarization current in the material, there is still missing charge flow that must be associated with displacement current which is not actual charges flowing (but is still called a current).

Nothing in this video has shown me that the displacement term does not come from the energy transferred through whatever dielectric is present, then separates the charges on the plates, and gives the appearance of current through the cap. I have seen no evidence that a electron hops from one plate into the dielectric while another electron jumps from the opposite side of the dielectric to the opposite plate.

So we are seeing even an outstanding professor and scientist like Prof. Lewin will not speak perfect literal correct facts at all points in the teaching activity, especially when speaking (as opposed to writing where there is more time to think). It's simply too difficult to be perfect. Instead, teachers repeat points several times and show things from different points of view, and the sum total of what they say is correct (hopefully), when taken in full context. Sometimes the omissions and false statements are deliberate, with the teacher knowing that too much information will overwhelm. So they sometimes lie a little bit until the critical concepts are absorbed, and then they later clarify and include the pertinent details.

Hmm, I think he could make some of his points a little clearer, and include some more detail, but I don't think he was trying to fool us.

Ratch

 

[Ratchit]

tcmtech,

So how does all of this relate to the principle of power factor shifting in AC circuits then if no current is being conducted or whatever some want to describe it as?

Current is existing in the branch containing the capacitor, but the current is not going through the capacitor for reasons which have been explained in this thread.

That and what is my amp clamp reading when I clamp it over the conductor going to or from a capacitor that is being used in an AC circuit?

It is reading the current value of the branch containing the capacitor.

Ratch

 

[steveB]

I don't think he was trying to fool us.

I also don't think he was trying to fool us. I think he is trying to educate us. I think he does his job better than most.

what is my amp clamp reading when I clamp it over the conductor going to or from a capacitor that is being used in an AC circuit?

You will always measure the current that you clamp around, no matter where you place the clamp. You can even place in around the cap because displacement currents, drift currents, polarization current or any other current you can think of creates the same integrated magnetic field around the loop. The clamp meter is based on the modified Amperes' Law that Prof. Lewin discusses in the lecture 18 referenced above.

 

[nsaspook]

Ratch,

So, the idea of no charge movement (or flow) is attributable only to displacement current for vacuum, but not polarization current for material dielectrics. For any capacitor (even one with super high permittivity) there has to be some contribution from displacement current. So, even if >99% of the wire drift current can be associated with polarization current in the material, there is still missing charge flow that must be associated with displacement current which is not actual charges flowing (but is still called a current).
Steve

Well said.

The polarization current in dielectrics is often overlooked but can be used in some very interesting ways. https://www.electro-tech-online.com/custompdfs/2013/08/singleton_040629.pdf

 

[tcmtech]

Personally I am just going to continue on living with my perceived assumptions and the math that goes along with it.

 

[shortbus=]

You guys are all wrong about this stuff! Even us dummies know it a depends on the smoke. Let the smoke out and no more working.

 

[nsaspook]

You guys are all wrong about this stuff! Even us dummies know it a depends on the smoke. Let the smoke out and no more working.

I work at a magic smoke factory, we use only the best unicorn tears for our products.

 

[MrAl]

Hi,

I agree that the Professor did not lie but was simply speaking in a transitory way like we all do. For example, if i say out loud: "I'd like to discuss the current that flows through the capacitor" what i might really mean is the current that POSSIBLY flows through the capacitor not that i want people to believe that it is really flowing through, and later i would clarify that as i just did now so the right information should get conveyed.

And as to the dielectric, this is exactly why whenever i talk about this i try to immediately clarify that we are talking about a vacuum capacitor and that is usually assumed to act similar to an air capacitor. When we bring in a dielectric we immediately complicate matters that dont need to be complicated but need instead to be simplified if possible.
But temporarily speaking, if he did move his hands back and forth then he was most likely indicating that the movement of the charge was back and forth.

What no one else has talked about yet and i am a little surprised, is the thoughts on the movement of charge in the capacitor plates themselves not the dielectric or vacuum or air between the plates. We absolutely should be able to determine more about the capacitor by understanding the movement of charge in the plates themselves. We know that the charge (at least on one plate) moves out onto the plate in all directions, but we also know this would not happen unless there was another plate somewhere to provide the opposite charge which to me looks like a motivator for the charge on the plate. But in any case, it moves out along the plate and that means at some point it must be traveling ni a direction that is perpendicular to the current in the wire itself. This is interesting because the other plate then has the same action except the charge must be moving in the other direction. So we'll see action similar to a transmission line, and this point of view has actually been taken up at one point in the past. This kind of transmission line would be very different than the ones we normally think about and work with, but would have a circular orientation as well as the parallel orientation. We could probably work up some equations to describe this action and then compare it to results found by the lab that did the internal field measurements of a small cap held in a vacuum at near absolute zero. Or at least maybe gain some more insight into this incredibly interesting phenomenon.

 

[Mr RB]

Hi MrRB,
Well you are arguing with me about something telling me that 'your' single view is better than the 'dual' view where we 'know' that there are two different approaches.

That's really well phrased; "Single view or dual view". The single view is better. If this is purely intellectual banter for the sake of it (ie it does not help in electronics - which is true) then sure go for the "dual view".

On the other hand if this occurs in a beginner's thread about using his cap (which is almost always true ) then the "single view" is better.

Ask yourself; is it better for an overwhelmed beginner to go away remembering;
A. "AC current flows through a cap, DC current does not".
or;
B. "Electrons don't go from one side of the cap to the other."

...
... but you cant tell me that i am not allowed to question or think about that second view, and surely you cant tell me that i cant talk about it can you?
...

Please don't be offended, I have a lot of respect for you and my post was only very partially directed at you. My post expresses annoyance not with you or your opinion but it was annoyance at the fact this cap topic always occurs in beginner threads or general electronics cap thread (and it did this time too, before this thread was split).

I'm perfectly happy with the concept that individual electrons might not cross the dielectric. Sounds logical.

BUT, whether or not they do cross the dielectric has zero value to any of the general electronics cap use questions we get here on the forum. The thing that matters is that the AC current goes through the cap circuit regardless of the cap internals, ie in one cap leg and out the other.

When is it of use to know/remember that individual electrons don't get from one plate to the other? Does this need to be in most people minds when using caps in electronics?

... It's not a topic for a beginner and i never said it was, but we are not beginners are we, so we can talk about it. ...

Now we're on a much closer wavelength. I'm happy. This argument did occur again in a beginner or general cap thread, and derailed things.

... Do your students ask how current can exist through a cap when the dielectric is an insulator? What about the professor referenced by nsaspook in post #37 of this thread, who specifically says that current does not exist through a cap?

1. Never ever.
2. He is a professor of what exactly?

Is he a professor of practical electronics teaching basic electronics students? Because that is much closer to this forum, and the beginner/hobbyist/student cap threads that you constantly derail with pedantic nickpicking of useless theory about dielectrics.

I'm not going to argue further with you because you are one of those people who is good at grabbing one particular tech point and hammering it home with vigorous argument, but you never seem to be up to the task of seeing the big picture and flexible enough to focus on the important things, taken in context.

 

[Mikebits]

Well, you can fill up one side of the capacitor with blue electrons and the other with red electrons. Then apply an AC current. If you have trouble colouring the electrons, you could try the equivalent experiment using dyed water with a rubber diaphragm in place of the dielectric.

That sounds a lot like osmosis.

 

[MrAl]

Hello again MrRB,

START QUOTE
Ask yourself; is it better for an overwhelmed beginner to go away remembering;
A. "AC current flows through a cap, DC current does not".
or;
B. "Electrons don't go from one side of the cap to the other."
END QUOTE

I asked myself that yesterday but i still have not received an answer

Really though i think it would depend on the individual i was talking to and just how basic we were talking. So i think i'd want to go on a case by case basis. If it was for circuit analysis however i would go with the thinking that the current flows through the cap because it is easier to think about it that way, period. If we want to think about it more accurately we would have to say that the current flows in the circuit external to the cap but not in the cap itself except as it moves inside the plates. That takes more wind and wont change the equations so i'd go with View #1 for that.

I guess it comes up when an individual is curious enough to ask how charge (or electrons) can get through the dielectric. It's then that we have to take the time to point out that does not happen. We cant just say, "forget about it" so we have to come up with some kind of explanation, similar to how the professor did in the video in this thread (same video i posted in the other thread too). He is (or was) a physics professor at MIT.

This is one of those truly amazing things about nature. The two slit experiment is also an amazing thing about nature where the electron can travel as a wave until it is observed and then it turns into a particle. It's like nature is just trying to be tricky just to keep us guessing.

Either of these discussions fits into the general topic of physics.

 

[shortbus=]

I know I'm no where on the level or ever will be, of the rest of you. But in post #77 by RB, this quote by Ratchit really hit me and I had to investigate - "Originally Posted by Ratchit
... Do your students ask how current can exist through a cap when the dielectric is an insulator?"

I then looked up 'dielectric' and found this - "While the term "insulator" implies low electrical conduction, "dielectric" is typically used to describe materials with a high polarizability. The latter is expressed by a number called the relative permittivity (also known in older texts as dielectric constant). The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor's surface charge.[1]" From https://en.wikipedia.org/wiki/Dielectric

Doesn't this explain how a cap "seems" to transfer AC?