AC flowing through a cap. What actually happens?

[BrownOut]

Notice the prof also said that there was no way to distinguish the displacement current from the conduction current because they are both the same.

Well I think that says it all.

 

[ericgibbs]

I believe I said that just because current can be measured on both sides of the capacitor was not proof that charge passes through the capacitor. If it did, the charge would not accumulate and it would not do what a capacitor does, store energy. Why didn't you discuss this earlier?

Ratch,
If you think that a capacitor dosn't store energy when an AC voltage is applied, just try disconnecting a capacitor from a mains source when the AC waveform is at its peak, then put your fingers across the terminals.!!

Or the other test you could do is to connect one end of a 2uF capacitor to the Line of the mains supply and touch the other end of the capacitor with your finger..

Of course I am NOT seriously suggesting that you do these as a practical test.

 

[Ratchit]

Mr RB,

Now I'm going to get pedantic while you are busy backpedalling.

Back-pedaling? Where have I back pedaled? I have been consistent in saying that current does not really pass through a capacitor.

I said; "CURRENT flows though a cpacitor" and
You said; "No it doesn't"

I deliberately used the most simple example of where you were completely wrong about capacitor current rather than nitpicking the specifics of what happens inside a capacitor becasue frankly that doesn't matter.

Correct, current does not really exist inside a capacitor. If you are not interested in how the changing E-field causes a B-field to form and act as if current is existing in a cap, fine. But remember the quotes I copied from the professor's lecture. Quotes like "Even though it is not current, it will act like current", "capacitor will act as if current is existing through it", "cap has no current in it at all". Did you view his lecture example with a cap and resistor? He more or less said the the current through the dielectric was a virtual or pretend current.

1. you have 3 mystery comonents in series,
2. your instrument is one ammeter at the end of the series which registers 1 amp, a single reading at one point in time
3. deduce which one of the 3 components is the cap because "there is no current flowing through it"

I don't know where we are going with this. I already said in a previous post that a ammeter will show deflection on both sides capacitor if the voltage across it is changing, and gave reasons for that happening. Why don't you say directly what your point is?

Ratch

 

[Ratchit]

ericgibbs,

If you think that a capacitor dosn't store energy when an AC voltage is applied, just try disconnecting a capacitor from a mains source when the AC waveform is at its peak, then put your fingers across the terminals.!!

Or the other test you could do is to connect one end of a 2uF capacitor to the Line of the mains supply and touch the other end of the capacitor with your finger..

Of course a capacitor stores electric energy, twice during each cycle. I never said it did not. And I know better than to ground myself when touching a "hot" line, even when a capacitor is between me and the line.

Ratch

 

[BrownOut]

He more or less said the the current through the dielectric was a virtual or pretend current.

I don't see any definition of virtual that means "pretend."

1: being such in essence or effect though not formally recognized or admitted

That means the effects are the same as current through a wire, but it's called something else, hence "displacement" current. Doesn't say it's pretend or not real. Fact is, it's very real.

 

[MrAl]

Hello again,


Im not sure anymore who is arguing what, but from one of that 'professors' videos he says this, and this is a direct quote, unchanged in any way:

"The changing E field will create a B field and according to the right hand rule, this will act like current, even though it is not current."

That tells me that he is saying that it's not a current but the effect makes it look like a current.

This is from "Displacement current example 2".


"The displacement current is equal to the actual current but the ammeter is not going to be able to tell the difference between the two".
"So the rest of the circuit has the regular current going through but the capacitor doesnt have any current going through it but because the E field is changing as a function of time it acts as if there is a displacement current equal to the actual current".

He is apparently verifying that what i posted about the capacitor a while back about the units being the same but not being a true current. Thus it is not comparable to the inductor at all except in other ways.

Maybe this would help here...

We go to the store and buy 3 apples but when we get to the register we realize we really wanted to buy 6 apples. We go back to the produce dept and pick up 3 more apples. We come back to the register and pay 50 cents for each apple because that's the price.
When we get home though we notice that the second set of apples was smaller but we still got charged 50 cents for them too.

So even though the second set of apples was smaller we still got charged the same amount because they were both "called" apples. That doesnt mean that they all were the same size or weighed the same.

So another question that comes up is this:
"If the current is a real current then why do we have to tack 'displacement' onto it, why dont we just call it 'current'?"

Another interesting point of view:
When we create a current though an insulator with a B field, the better the insulator the higher the voltage across that element, yet in the capacitor the better the insulator the lower the voltage (dv=i*dt/C).

 

[BrownOut]

I think you can cherry-pick anything you want to hear from the lecture. But as Amphere's law shows, magnetic fields arise from current. So, although displacement current has some disimilarities to conduction current, it also has some identical features. I'm comfortable living with that duality, especially when I see the results every day. And it's not the only duality I've come across in my study of physics.

 

[Ratchit]

MrAl,

That tells me that he is saying that it's not a current but the effect makes it look like a current.

Right on Al. He is saying that the changing E-field causes a B-field of the same direction and magnitude to form as would happen if a current existed in the capacitor. But there is no real current in the capacitor, just an E-field and B-field. Although the calculated values of the conduction and apparent values are the same. One is real and the other is not. One current exists and the other does not exist. One is virtual and the other is real.

Ratch

 

[BrownOut]

If I turn on my radio, I hear the results of displacement current.
If I trun on a light, I see the results of displacement current.
Until someone proves I can get real results from an unreal quantity, then I must conclude displacement current is real.

 

[MrAl]

If I turn on my radio, I hear the results of displacement current.
If I trun on a light, I see the results of displacement current.
Until someone proves I can get real results from an unreal quantity, then I must conclude displacement current is real.

Well, if i throw a baseball at a painted steel stop sign and it hits it on the right side so it twists a little, then generate a magnetic field to make the stop sign rotate, both things caused the sign to rotate (a little) but they are not the same cause. For that matter, if i throw a basketball at the same sign, same thing happens, yet a basketball is not the same as a baseball.

Usually when we want to prove something like this we try to isolate the dimensions and try to eliminate the ones that we dont want to play with so we can see what happens with the ones we do. We find out more information about something by splitting it into more than one basic thing, and that tells us something more about it.

Another way of looking at it would be something like this....
There are two types of people in the world, those that believe that the universe is infinitely simple and those that believe that the universe is infinitely complex.
Infinitely simple here would mean that we will eventually know everything about the universe, while infinitely complex here would mean that whatever we do we end up finding out something but also end up finding more questions to answer so we never know everything.

The people who believe that the world is infinitely simple observe a limited amount of information like E=I*R and want to say that everything in the universe is almost that simple. Newton did this with his law of gravitation, but then someone else we all know came along and realized that the world was just not that simple, even though he did want to push a completely ordered system and not allow randomness to enter into the complexity.

I happen to be one of those people that believe that the universe is infinitely complex, where we will never know everything, and that means that everything is not as simple as it looks. Ampere's Law was modified, why did it need to be modified. Under this view, the situation became more complex than originally thought (as i believe many things are to begin with).

Maybe it doesnt matter externally what is happening with this new 'current', but it's interesting to note where it comes from and why they dont want to call it just another plain old current.

Who is that professor anyway? The quotes that i quoted from that lecture are straight from the lecture, so can we really 'cherry pick' to show that he actually is saying that the displacement current is the same as any other current? If you can show this then i guess we would have to dismiss that lecture (note we dont actually do this though of course).

 

[BrownOut]

Well, if i throw a baseball at a painted steel stop sign and it hits it on the right side so it twists a little, then generate a magnetic field to make the stop sign rotate, both things caused the sign to rotate (a little) but they are not the same cause. For that matter, if i throw a basketball at the same sign, same thing happens, yet a basketball is not the same as a baseball.

I know what you're trying to say, but you anology falls flat, as you're talking about apples and oranges. We aren't talking about different well known forces that move objects, we're talking about creating a magnetic field, and only a current can do that. Come back and see me when you can throw a baseball and create a magnetic field. Placing static charge on the leather doesn't count

Another way of looking at it would be something like this....

Or, one might say that someone who believs in the complex world believes current can happen without charge, and is not discouraged by the duality of certain phenomina.

so can we really 'cherry pick' to show that he actually is saying that the displacement current is the same as any other current? If you can show this then i guess we would have to dismiss that lecture.

Yes, we definitely can ( cherry pick to show dispalcement current is real ), but now you're saying you have to dismiss the lecture when it opposes your preconceived notion, while you are willing to embrace it when it supported the same. Sorry, I don't think like you.

 

[MrAl]

I know what you're trying to say, but you anology falls flat, as you're talking about apples and oranges. We aren't talking about different well known forces that move objects, we're talking about creating a magnetic field, and only a current can do that. Come back and see me when you can throw a baseball and create a magnetic field. Placing static charge on the leather doesn't count




Or, one might say that someone who believs in the complex world believes current can happen without charge, and is not discouraged by the duality of certain phenomina.



Yes, we definitely can ( cherry pick to show dispalcement current is real ), but now you're saying you have to dismiss the lecture when it opposes your preconceived notion, while you are willing to embrace it when it supported the same. Sorry, I don't think like you.

Maybe i didnt make the point clear about dismissing the lecture. I meant that if someone says that the displacement current isnt real and then says it is real isnt that a contradiction unless they are still within the process of discovery?
Still, i need a direct quote if you are going to argue that point, if you dont mind i'd like to see where he says this. Which lecture was it in? I'll watch it.

Im not talking about fruit (this time ) im talking about two things looking like the same thing when really they are two different things. We can say Electromagnetic Theory as one thing, or we can say Magnetic Theory alone, but that doesnt mean that Electrostatics are the same 'exactly' as Magnetics. It's a very simple concept.

I will admit though that i might be wrong about the B field in a poor conductor because the voltage does not only depend on the insulator properties, it depends on the properties of the circuit too. This deserves more thought though that's all i'll say for now about that.

 

[BrownOut]

Maybe i didnt make the point clear about dismissing the lecture. I meant that if someone says that the displacement current isnt real and then says it is real isnt that a contradiction unless they are still within the process of discovery?
Still, i need a direct quote if you are going to argue that point, if you dont mind i'd like to see where he says this. Which lecture was it in? I'll watch it. I don't think he ever said displacement current isn't real. That was someone's liberal interpretation of what he said.

Im not talking about fruit (this time ) im talking about two things looking like the same thing when really they are two different things. We can say Electromagnetic Theory as one thing, or we can say Magnetic Theory alone, but that doesnt mean that Electrostatics are the same 'exactly' as Magnetics. It's a very simple concept. There is continuity between electrostatics, magnetic and electromagnetic theory. They cannot be discussed as though they are 3 distinct things. I know you're trying to say "something" else can produce a magnetic field, but I disagree. Magnetic fields are literally defined by current. Until someone finds that elusive "something else", it can only be current.

I will admit though that i might be wrong about the B field in a poor conductor because the voltage does not only depend on the insulator properties, it depends on the properties of the circuit too. This deserves more thought though that's all i'll say for now about that.

The quote I was referring to was in pt. 2 of the discussion on displacement current, "We can see now that in addition to regular current, we have displacement current flowing through the capacitor..." I really see no contradiction in the lectures, because he simply made a distinction between "regular" ( moving charge ) current and displacement current. But you should ask yourself, why would he spend so much time treating a subject that doesn't exist? Clearly he believes it is real and important.

I've learned two things studying physics. 1) you can't observe every physical phenomenon. You can't see electrons; all you can do is observe some of the effects and postulate about their existence and mechanics. If you postulates agree with experimental results, and field applications, then they become the physics of that particular phenomenon. 2) You can't be inflexible in your thinking. From time to time physical observations seem to be ambiguous. Is light a wave or a particle? Sometimes you just need to accept the ambiguity and move on.

I still have to go back over your previous posts. Maybe this weekend.

 

[MrAl]

**broken link removed** Originally Posted by MrAl **broken link removed**
Maybe i didnt make the point clear about dismissing the lecture. I meant that if someone says that the displacement current isnt real and then says it is real isnt that a contradiction unless they are still within the process of discovery?
Still, i need a direct quote if you are going to argue that point, if you dont mind i'd like to see where he says this. Which lecture was it in? I'll watch it. I don't think he ever said displacment current isn't real. That was someone's liberal interpretation of what he said.

Im not talking about fruit (this time ) im talking about two things looking like the same thing when really they are two different things. We can say Electromagnetic Theory as one thing, or we can say Magnetic Theory alone, but that doesnt mean that Electrostatics are the same 'exactly' as Magnetics. It's a very simple concept. There is continunity between electrostatics, magnetics and electro magnetic theroy. They cannot be discussed as though they are 3 distinct things. I know you're trying to say "something" else can produce a magnetic field, but I disagree. Magnetic fields are literally defined by current. Until someone finds that elusive "someting else", it can only be current.



Hello again,

Ok i'll keep this short so you dont have to read a book just to read what my point is about...


This is a DIRECT quote, unmodified in any way from the lecture. This is not a rewording or reinterpretation...

"The changing E field will create a B field and according to the right hand rule, this will act like current, even though it is not current."

This is from the lecture: "Displacement current example 2".

What does "even though it is not current" mean to you?

 

[BrownOut]

**broken link removed** Originally Posted by MrAl **broken link removed**



Hello again,

Ok i'll keep this short so you dont have to read a book just to read what my point is about...


This is a DIRECT quote, unmodified in any way from the lecture. This is not a rewording or reinterpretation...

"The changing E field will create a B field and according to the right hand rule, this will act like current, even though it is not current."

This is from the lecture: "Displacement current example 2".

What does "even though it is not current" mean to you?

Later in his lecture, he says, "We can see now that in addition to regular current, we have displacement current flowing through the capacitor..." So, there is "regular" current and "displacement" current. Taken out of context, and taken far too literally, his comment about not "being current" might suggest something else. But considering context and remembering he was speaking extemporaneously, and thus needs to be interpreted with intelligence and not dogma, he was most likely referring to "regular" current or moving charge. I believe he clears that up later in the lecture.

 

[MrAl]

Later in his lecture, he says, "We can see now that in addition to regular current, we have displacement current flowing through the capacitor..." So, there is "regular" current and "displacement" current. Taken out of context, and taken far too literally, his comment about not "being current" might suggest something else. But considering context and remembering he was speaking extemporaneously, and thus needs to be interpreted with intelligence and not dogma, he was most likely referring to "regular" current or moving charge. I believe he clears that up later in the lecture.

Hello again,

Well then perhaps we need a better experiment because everything i heard in that lecture says that the displacement current is not the same thing as real current.

We need a better experiment, one that no one can weasel out of

Using an AC voltage source and a resistive wire, we apply the voltage. With some frequency f, the *current* goes through *zero* every so often and the B field 'almost' follows it. However, at the point where the current goes thought zero there is still a tiny B field around the wire. This is because of the small displacement current.
So here we have the *current* at zero yet we still have what is called the *displacement current*.
Obviously displacement current has to be something else here because we cant say that the current is zero AND the current is not zero.

I think the only thing to remember is that the E field can create a B field without requiring any current.

 

[BrownOut]

Well then perhaps we need a better experiment because everything i heard in that lecture says that the displacement current is not the same thing as real current.

He didn't say that.

So here we have the *current* at zero yet we still have what is called the *displacement current*.
Obviously displacement current has to be something else here because we cant say that the current is zero AND the current is not zero.

Conduction current is zero and displacement current is non-zero. What's the problem? BTW, the magnetic field will be zero when the conduction current induction cancels the displacement current induction, but this doesn't require either to be zero.

 

[MrAl]

More direct quotes from the lecture, without modification:

"When you change the electric flux you get something that acts like current".
"It has some purpose".
"The capacitor will act like it has a current flowing through it"

"something that acts" like current. You dont have to specify it if it is the same as current, or you'd be saying "current that acts like current".
The capacitor will "act" like it has a current flowing through it. You dont have to say "act like" if it actually has current flowing through it.

If you dont accept this then argue with someone else because his lecture is clearly indicating that something else is happening here and it's not exactly the same as current. You cant have zero current and say it's not zero both in the same sentence. If you say that displacement current is the same as current then you cant later distinguish between the two. The current can not be zero if ANY current is flowing.

 

[BrownOut]

Then he later elaborates on what that "something" is:

"We can see now that in addition to regular current, we have displacement current flowing through the capacitor..."

First he says it's "something" and later he defines what "something" means.

You can say conduction current is zero and displacement current is non-zero in the same sentence. You still haven't said what's wrong with that. So, as the professor clearly states, there is "regular" ( conduction ) current and there is displacement current. You have to consider the whole lecture, and how he elaborates and clears it up in the end. I don't see any way of making it more clear.

 

[MrAl]

Hello again,


Well, then where does the charge come from for this current? If the B field is going to move any charge, it's got to get it from somewhere and the dielectric can not supply any electrons. 'Normal' current is the movement of charge, and the displacement current is NOT that.

You're saying that the displacement current is the same as current, and then you say that you can have BOTH displacement current AND regular current in the same wire. The problem is that you can not sum the two and get zero, if they are both REAL current. ie: for I regular and Id displacement current:
I=0
but you're saying that
I+Id=0
yet your saying that
I=0 AND Id>0.
If Id>0 then
I+Id>0 when I=0.
In other words, if you measure 0 amps with the ammeter then you can not say that there is a displacement current that exists yet is not going to be measured, and still say that the displacement current is the same as regular current.
You're saying that the displacement current is real current but then when we measure 0 amps you're saying that although it exists we cant measure it. If we cant measure it like real current then it's not real current.

Just to note, "I" above is really i(t) for the sine wave sin(w*t) excitation and note that i(t)=0 when the current sine goes through zero.


I should really move on to other more profitable things anyway....I think Monk is on