Why is as important as What. When If I know a collection of facts but do not
understand why they are so, I have no principles that I can apply in order to
apprehend other facts. So when struggling to understand something I may
appear to be arguing, as in stating that the other person is wrong. While I
have been known to do that some times, usually what I'm doing is trying to
get my mind around the facts.

That I never took college level physics may explain my difficulties, then
again maybe not since I don't know to what extent the issue of 'where a
charge is stored in a capacitor, might be answered by college level physics,
at least as much of it as an EE might take. As a technician I have often had
to just accept some premise, not having the knowledge to question them or to
verify them for that matter. And being pragmatic I see no problem with that
as long is it doesn't interfere with what I may be trying to do. For
example, who care which convention is used, current of electron flow as long
as we can follow the function of a circuit. Likewise knowing whether a
capacitors charge is stored, the plates or the dielectric won't help me
understand why the capacitor seems to allow current to flow right through it.
In fact I can operate as current does flow though the capacitor and when
working with them in actual circuits we assume a capacitor to be an AC short
circuit, albeit a frequency sensitive one.

All of this leads me to quote from Nigel Cook, DC-AC Second Edition, 1993.
In chapter 11, under the summary on capacitance he actually states that the
'charges on the plates produce an electric field...' Sound like he's saying
that the charge is stored on the plates. The last sentence in the paragraph
states...
'The energy in a capacitor is actually stored in the electric field within
the dielectric.' Sounds like it's stored in the dielectric. Now some
interpretation which I would be allowed if I were a student reading this
book, but which could be mistaken. Nothing new, I've been wrong before and I
plan to be wrong again before I go to that great capacitor in the sky to be
fully charged for eternity. :-)
I think both views may be wrong. The energy is stored in the electrostatic
field, not on the plates, not in the dielectric. That certainly would explain
why vacuum capacitors can work. It also agrees with my notion that a
conductor can't really store a charge. Oh, and this also agrees with the
idea that energy is stored in the magnetic field in an inductor, which always
seemed strange to me.

Am I right, is one of the other view right? I really don't give a poop. But I
have an explanation that will satisfy me until someone comes up with a better
one. And I feel that I understand capacitors better now. Maybe I'll take one
out to dinner some time.

Jim

开云体育

Yes Jim, You've got it.

Charge(voltage) and Electric field are to a capacitor what
Current and Magnitic field are to an inductor.

Doug Hale




Jim Purcell wrote:

Some snippage and comments below:

Ah, so it's the field that stores all the energy? And fields, like any EM radiation, don't need a medium.

When the field collapses the energy is transmitted to the plates, which goes through the wires...

Hmmm.

-Mike

From: "G Ramasubramani" <grama@...>
Reply-To: Electronics_101@...
To: <Electronics_101@...>
Subject: Re: [Electronics_101] The need to know!
Date: Mon, 29 Oct 2001 18:12:39 -0800

Jim,

Liked your mail. Charge is stored on the plates and the energy in the gap between the plates. This energy results from the field between the positive and negative charges on the plates. The dielectric helps in keeping the charge apart. The maximum amount of charge that can be kept apart depends on the dielectric. In case the amount exceeds this level, then the dielectric breaks down and the capacitor gets discharged real fast. The dielectric also helps in chanellising the field between the plates - this results in different capacitances for different dielectrics if the area of the plates and the distance between them remains the same.

Rama
----- Original Message -----
From: Jim Purcell
To: Electronics_101@...
Sent: Monday, October 29, 2001 4:51 PM
Subject: [Electronics_101] The need to know!


Why is as important as What. When If I know a collection of facts but do not
understand why they are so, I have no principles that I can apply in order to
apprehend other facts. So when struggling to understand something I may
appear to be arguing, as in stating that the other person is wrong. While I
have been known to do that some times, usually what I'm doing is trying to
get my mind around the facts.

That I never took college level physics may explain my difficulties, then
again maybe not since I don't know to what extent the issue of 'where a
charge is stored in a capacitor, might be answered by college level physics,
at least as much of it as an EE might take. As a technician I have often had
to just accept some premise, not having the knowledge to question them or to
verify them for that matter. And being pragmatic I see no problem with that
as long is it doesn't interfere with what I may be trying to do. For
example, who care which convention is used, current of electron flow as long
as we can follow the function of a circuit. Likewise knowing whether a
capacitors charge is stored, the plates or the dielectric won't help me
understand why the capacitor seems to allow current to flow right through it.
In fact I can operate as current does flow though the capacitor and when
working with them in actual circuits we assume a capacitor to be an AC short
circuit, albeit a frequency sensitive one.

All of this leads me to quote from Nigel Cook, DC-AC Second Edition, 1993.
In chapter 11, under the summary on capacitance he actually states that the
'charges on the plates produce an electric field...' Sound like he's saying
that the charge is stored on the plates. The last sentence in the paragraph
states...
'The energy in a capacitor is actually stored in the electric field within
the dielectric.' Sounds like it's stored in the dielectric. Now some
interpretation which I would be allowed if I were a student reading this
book, but which could be mistaken. Nothing new, I've been wrong before and I
plan to be wrong again before I go to that great capacitor in the sky to be
fully charged for eternity. :-)
I think both views may be wrong. The energy is stored in the electrostatic
field, not on the plates, not in the dielectric. That certainly would explain
why vacuum capacitors can work. It also agrees with my notion that a
conductor can't really store a charge. Oh, and this also agrees with the
idea that energy is stored in the magnetic field in an inductor, which always
seemed strange to me.

Am I right, is one of the other view right? I really don't give a poop. But I
have an explanation that will satisfy me until someone comes up with a better
one. And I feel that I understand capacitors better now. Maybe I'll take one
out to dinner some time.

Jim


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Rama,

?Charge is stored on the plates and the energy in the gap between the plates.

No that was not my point, correct or not I was saying that the energy is stored in the e.s. field not on the plates or in the dielectric. Just energy stored in an inductor is stored in the mag. field.

This energy results from the field between the positive and negative charges on the plates.

Yes, the parameters of the field are determined by the size of the plates, their distnce apart, i.e. the thickness of the dielectric and the dielectric constant.

The dielectric helps in keeping the charge apart.

I think it would be misleading to say that, in one sense it's true but that's not it's purpose.?

The maximum amount of charge that can be kept apart depends on the dielectric.

Now that language is becomming more grotesque and losing meaning, I don't about keeping charge apart. .

In case the amount exceeds this level, then the dielectric breaks down

Your mixing dielecric constant with dielectric strength.

and the capacitor gets discharged real fast.

Are you talking leakage due to exceeding dielecric strength?

The dielectric also helps in???? chanellising

What does that word meain? If you had plates that had less area than the dielectric, I think the dielectric that was 'sticking out' would be as it it weren't there. Like lunch meat that sticks out of the sandwich. :-)

the field between the plates - this results in different capacitances for different dielectrics if the area of the plates and the distance between them remains the same.

Yes, I think that the distance between the plates is more important than dielectric thickness. If the plates were farther apart than the dielectric thickness you'd have a mixture of dielectrics, the normal dielectric plus some air (or vacuum.)

Jim

Neal,

'The energy in a capacitor is actually stored in the electric field within
the dielectric.' Sounds like it's stored in the dielectric. Now some
interpretation which I would be allowed if I were a student reading this

The problem is wording. The field goes _through_ the dialectric. It is not
_in_ the dialectric.

Sound like a split hair to me. How does the field get through the dielectric
without
being in it. If the dielectric is swapped after the original charging voltage is
removed,
will the field strength change? Of course the process messes things up, how do
you
swap dielectrics without upsetting things?

Jim

开云体育

G,

Yes, the parameters of the field are determined by the size of the plates, their distnce apart, i.e. the thickness of the dielectric and the dielectric constant.

It is guided not only by the thickness of the dielectric and dielectric constant, also by the distance between the plates. In school we have had to determine theoretically the capacitance when the thickness of? the dielectric constant was less than the distance between the plates.

??????????? As far as I can see, unless there is a space between the plates and the dielectric, the
?????????? distance between the plates and the dielectric thickness are the same. I don't see how
?????????? you can relate dielectric constant with the plate distance. They are apples and oranges.
?????????? One has units, the other is only a number.

Well. You have 2 plates. Positive charge on one, negative on another. How high can this charge build up before? the dielectric breaks down? That is determined by the dielectric.

??????????? Yes dielectric strength, not dielectric constant. And until the dielectric breaks down
??????????? it has little to do with the energy storage, etc.

In case the amount exceeds this level, then the dielectric breaks down

Your mixing dielecric constant with dielectric strength.

No. I never mentioned dielectric contant in the context of breakdown. But, if I remember vaguely, the dielectric strength in some way depends on the dielectric strength.

??????????? Oops, you repeated yourself. I assume that one of those above was supposed
??????????? to be dielectric constant.

??????????? The two parameters have little to do with each other.

and the capacitor gets discharged real fast.

Are you talking leakage due to exceeding dielecric strength?

Yeah. Maybe discharge is not the correct term, but what the heck - I am not writing a book :-)

???????????? It's a lot more serious than discharge. If breakdown is reached the cap. is probably destroyed.

I think that the distance between the plates is more important than dielectric thickness.

Look at the formula, that will tell you.

If the plates were farther apart than the dielectric thickness you'd have a mixture of dielectrics, the normal dielectric plus some air (or vacuum.)?I am not sure of more important or less important - both play a role.

?????????? I can't remember either to be truthful. My point was that if the plate distance was different from the dielectric thickness then there must be something between the plates and the dielectric, so you have a mixture of dielectrics, i.e. the original and air or whatever.

Jim

开云体育

G,

?

Call me Rama.

Yes, the parameters of the field are determined by the size of the plates, their distnce apart, i.e. the thickness of the dielectric and the dielectric constant.

It is guided not only by the thickness of the dielectric and dielectric constant, also by the distance between the plates. In school we have had to determine theoretically the capacitance when the thickness of? the dielectric constant was less than the distance between the plates.

??????????? As far as I can see, unless there is a space between the plates and the dielectric, the
?????????? distance between the plates and the dielectric thickness are the same. I don't see how
?????????? you can relate dielectric constant with the plate distance. They are apples and oranges.
?????????? One has units, the other is only a number.

?

As you mention, yes - I am referring to space between the plates and dielectric. I thought that was pretty obvious. It can also be some other dielectric. I don't see how? these are apples and oranges. I did not mean to type 'constant' after dielectric. Slip of the keyboard. And before you say you have never seen such capacitors, neither have I. It is just theory.

Well. You have 2 plates. Positive charge on one, negative on another. How high can this charge build up before? the dielectric breaks down? That is determined by the dielectric.

??????????? Yes dielectric strength, not dielectric constant. And until the dielectric breaks down
??????????? it has little to do with the energy storage, etc.?

?

I repeat. I don't recall me ever saying dielectric constant was instrumental in breakdown. And , no, I can emphatically say that I never mentioned energy storage causing breakdown. Wonder where you got that idea from.

In case the amount exceeds this level, then the dielectric breaks down

Your mixing dielecric constant with dielectric strength.

No. I never mentioned dielectric contant in the context of breakdown. But, if I remember vaguely, the dielectric strength in some way depends on the dielectric strength.

??????????? Oops, you repeated yourself. I assume that one of those above was supposed
??????????? to be dielectric constant.

??????????? The two parameters have little to do with each other.

?

Yeah. I checked up. Thats true.

and the capacitor gets discharged real fast.

Are you talking leakage due to exceeding dielecric strength?

Yeah. Maybe discharge is not the correct term, but what the heck - I am not writing a book :-)

???????????? It's a lot more serious than discharge. If breakdown is reached the cap. is probably destroyed.

?

Whatever. As long as you get the gist of what I am saying.

I think that the distance between the plates is more important than dielectric thickness.

Look at the formula, that will tell you.

If the plates were farther apart than the dielectric thickness you'd have a mixture of dielectrics, the normal dielectric plus some air (or vacuum.)?I am not sure of more important or less important - both play a role.

?????????? I can't remember either to be truthful. My point was that if the plate distance was different from the dielectric thickness then there must be something between the plates and the dielectric, so you have a mixture of dielectrics, i.e. the original and air or whatever.

?

Yes. Thats true.

?

?

Rama

Rama,

Call me Rama.

Call me anything but late for dinner.
?
?As you mention, yes - I am referring to space between the plates and dielectric. I thought that was pretty obvious.

YOu seemed to be making a distinction between plate distance and dielectric thickness when you
said they cold differ. If they differ then you must have air as an additional dielectric.
?

Are you talking leakage due to exceeding dielecric strength?

Yeah. Maybe discharge is not the correct term, but what the heck - I am not writing a book :-)

??????????????????? I'm sure that the cap will discharge but worse, it might be
??????????????????? Permanently damaged in any but air capacitors. Air is generally
??????????????????? a self healing dielectric.

If the plates were farther apart than the dielectric thickness you'd have a mixture of dielectrics, the normal dielectric plus some air (or vacuum.) I am not sure of more important or less important - both play a role.

????????????????? Easy to get that from the formula for capacitance as a function of
????????????????? physical design parameters.? In this formula the plate distance is the
????????????????? only term on the bottom and dielectric constant is on top with
????????????????? a constant and plate area. So distance has greater effect on
????????????????? capacitance. And dielectric thickness is generally assumed to be
????????????????? the same as plate distance.
?

Jim
?
?

开云体育

Rama,

?? It is tough responding to this mail since you seem to have misunderstood nearly everything I wrote.

I have a way of doing that. Partly because I am a literalist. I take what someone says/writes at face value unless 'he couldn't mean that'.

Jim

开云体育