Capacitor makes the circuit open?

[jim hardy]

Before continuing it is important to note that the entire flow of charge is through the battery and resistor- not through the region between the plates. In every sense of definition there is an open circuit between the plates of capacitor

Those are the words that made me confused. I thought if a circuit is open then how can charge be accumulated or flowed? Or 'flow of charge and accumulation of charge is different thing'-is the point I am not getting?

okay saimee, try this not-analogy

Charge flows along conductors we agree.
Whatever it is carrying that charge, usually electrons, moves slowly.
But the propagation of charge is quick - when a charge enters one end of the wire an indentical one comes out other end with almost no delay. But it's riding on a different electron .
Point of this paragraph - the wire neither stores nor absorbs energy, just passes it through. And charge is not energy.

Next paragraph
Cut 1mm out of the middle of wire and insert two plates parallel (like a capacitor symbol) of area A.
Now you have a capacitor of value εA/1mm .
Push one unit of charge into the wire. Where does it go? It moves infinitesimally into the wire. That increases the charge contained in the volume of the wire and its capacitor plate, which increases the electric field at its surface.
Point of this paragraph - the sea of electrons in a conductor can be made very slightly more dense. Electric field results.

Next paragraph
The second capacitor plate is now in presence of electric field from first plate. Charge in that plate is repelled by field from like charge in first plate. So some of it moves off the plate and down the wire, emerging at far end just as if it had flowed through the space between the plates (which it did not)
Point of this paragraph: That's how charge gives the illusion of moving through a capacitor.

Next paragraph
So now we have two plates , one slightly overpopuated with charge and the other slightly underpopulated. There should exist between them a physical attractive force .
our mind leaps immediately to $F=\frac{Q_1Q_2}{r^2}$
but that's the trouble with analogies, we sometimes jump ahead.
Force between two charged parallel plate is constant with distance not inverse square. That's because the charge on each plate is a sheet not a point. The electric field between the sheets depends only on the charge density and nature of dielectric between them.
Look right here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html#c2

E field, $E = \frac{\sigma}{\epsilon_0}$. No distance term.

And $F = QE$
Which means the mechanical force between parallel plates of a capacitor aren't like a spring, there's no increase of force with distance.
Point of this paragraph: you'll hear of spring analogies to capacitor. They are just that, analogies describing mathematical not mechanical similarities.

Next paragraph:
Unlike in the wire, Energy is stored in the electric field between the plates. In a good dielectric polar molecules realign with the E field and the work it took to make them do that is mostly recoverable. In that sense the dielectric resembles a spring. And that's why the equations for a capacitor's electical behavior are so similar to those describiing a spring's mechanical behavior.
Point of this paragraph: our "mental pictures" and analogies must be constantly refined as we learn more. And that's a lifelong process. I learned a lot writing this .

I was taught in high school by a remarkable man whose analogies have stood the test of time. I realize now how fortunate i was. He taught us boys about that mechanical behavior of capacitor plates without calculus. See if these links help. I stumbled across them while trying to write something similar to first one.

http://www.haverford.edu/physics-astro/course_materials/phys102b/phys102_S99/notes/capnote2.html

Then this one for e field between sheet charges (it's constant - no distance term !)
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html#c2

and some others for good measure

See https://www.physicsforums.com/showthread.php?t=99544

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html#c2

http://www.schoolphysics.co.uk/age1...t/Force_between_two_charged_plates/index.html
and http://www.regentsprep.org/Regents/physics/phys03/aparplate/
Would anyone who sees them please point out any mistakes in this ? I need to get better in this area.. corrections welcome.

respectfuly,

old jim himself

 

[sophiecentaur]

Since when did I tell anyone to misuse a tool? I admit I use analogies all the time but I am also the first to admit that they only go so far. If you come to my garage to borrow a 9/16 inch wrench to tighten the head bolts on your lawn mower and twist the bolt off, don't blame me. I really don't think there is any more to say about it.

The analogy you give is like a tool without the Complete instruction guide. The 'proper' model or tool comes complete with instructions in the form of all the other resources in books etc. You cannot always be sure that you have included all the necessary caveats when you introduce a home made analogy.
At the start of this thread I pointed out the shortcomings of a post that involved a very long story as a substitute for about two lines of the real thing. How much sub-editing had been done on it to avoid misconceptions?

 

[sophiecentaur]

Am I being too heavy in introducing the subject of personal responsibility here?

 

[sheshank]

Unfortunately, what you write about dielectrics is not relevant to the basic function of a capacitor because two plates IN A VACUUM still behave as a Capacitor.

Also, as you introduced the idea of exponential decay, why carry on with an extra verbal description when the Maths has done it all for you?

Yes..Vaccum still behaves as a dielectric. I accept, and I did want to make it sweet and simple, so avoided such complexities. For your information, Capacitors are never made with 100% Vaccum. They still have some medium in between the plates (gas) which is pressurized to very low where Air can act as Ions and develop charges (get polarized). I hope you remember the Thomson's experiment of discovery of electrons : Gas conducts at very low pressures i.e.. almost vaccum.

Mathematics is just a tool not the soul. Mathematics just describes something quantitatively, not qualitatively. Qualitative analysis requires Imaginations and Imaginations are baseless without Idea. I just tried to create an Idea. I want everyone to grow everything on their own rather than depend on equations somebody else developed. Develop the same, or similar equation, on your self based on the ideas rather than somebody else's equations. That's, I feel, is the best way to learn rather than just looking at some equation as it can never be remembered with exact preciseness.

This is not in vengeance, this is just my view. That's all.

 

[sophiecentaur]

Frankly, I'm quite happy with Maxwell's work. If you can do what he did, all on your own then well done.

Free space has a Permittivity and has no dipoles. If a vacuum can conduct then I wonder how a thermionic diode is supposed to work. (Do you remember them?)

 

[sophiecentaur]

There are a few more details which you may need to sort out, such as the fact that the relative permittivity of a vacuum of 10^-7 Torr (εr is pretty much =1) so how much 'polarisation' of the gas molecules does this represent? (There may be some QM issues here, to consider, I think.)
Pictures in your mind are very useful but they need to agree with observation and well established theory.

 

[samieee]

Thanks to all folks for helping me understand the concept. For the last two days I was busy with it, studied from several sources. Now I have managed to solve my confusion.

 

[sheshank]

The conductivity depends on the applied voltage and the pressure... At some point the gas acts as a dielectric, similar to the ones applied in the conductor, and at some point it acts as a conductor and at some point it acts as a semi-conductor. There is a very narrow boundary between them.

Thermionic, or Vaccum tubes, involves in emission of electrons i.e..., diffusion not drifting. When there is diffusion, it doesn't require any media to travel. Do Earth required any media to travel? NO. Similarly, an electron doesn't require media to travel from cathode to anode of the diode. Its diffusion not drifting. The work is done by allowing the Cathode part to emit electrons which exhibits the exponential increase. Hence its used as a diode.

Its waste of time doing what Maxwell already did. I believe that there is an underlying philosophy, idea of every thing proposed till date. If we get to crack and understand that idea, then we can march forward rather than struggling to understand what it already said.

 

[sophiecentaur]

The conductivity depends on the applied voltage and the pressure... At some point the gas acts as a dielectric, similar to the ones applied in the conductor, and at some point it acts as a conductor and at some point it acts as a semi-conductor. There is a very narrow boundary between them.

Thermionic, or Vaccum tubes, involves in emission of electrons i.e..., diffusion not drifting. When there is diffusion, it doesn't require any media to travel. Do Earth required any media to travel? NO. Similarly, an electron doesn't require media to travel from cathode to anode of the diode. Its diffusion not drifting. The work is done by allowing the Cathode part to emit electrons which exhibits the exponential increase. Hence its used as a diode.

Its waste of time doing what Maxwell already did. I believe that there is an underlying philosophy, idea of every thing proposed till date. If we get to crack and understand that idea, then we can march forward rather than struggling to understand what it already said.

So a gas varies from a conductor, through being a semi-conductor to being a dielectric? Is this for all applied voltages? This is a novel idea.

That is when electrons have been released from the Cathode there is no conduction from the Anode when reverse biased - because the near-vacuum doesn't conduct on its own, once the thermionic electrons have reached an electrode. (You know that, surely?)
And, if the gas molecules in a vacuum capacitor (Pressure 10^-7Torr) contribute to the capacity significantly, how come the capacity is not very much more than the value with just 'free space' inside it? Look at some values of ε_r for some gases - then reduce the density to near-zero (i.e. for a low vacuum). Where do your "dipoles" come into the model, under those conditions. Some dodgy Science at work in what you say about this, I think.

Is it a "waste of time" to get down to it and understand what Maxwell tell us? Have you actually sussed it all out and do you actually have a better model that accounts for all the phenomena that Maxwell does? I'm very impressed if you have; the Maths is non-trivial and can't be ignored.

And have you "cracked it" ?

 

[sheshank]

So a gas varies from a conductor, through being a semi-conductor to being a dielectric? Is this for all applied voltages? This is a novel idea.

That is when electrons have been released from the Cathode there is no conduction from the Anode when reverse biased - because the near-vacuum doesn't conduct on its own, once the thermionic electrons have reached an electrode. (You know that, surely?)
And, if the gas molecules in a vacuum capacitor (Pressure 10^-7Torr) contribute to the capacity significantly, how come the capacity is not very much more than the value with just 'free space' inside it? Look at some values of ε_r for some gases - then reduce the density to near-zero (i.e. for a low vacuum). Where do your "dipoles" come into the model, under those conditions. Some dodgy Science at work in what you say about this, I think.

Is it a "waste of time" to get down to it and understand what Maxwell tell us? Have you actually sussed it all out and do you actually have a better model that accounts for all the phenomena that Maxwell does? I'm very impressed if you have; the Maths is non-trivial and can't be ignored.

And have you "cracked it" ?

My intention behind saying "waste of time" is something of the kind of " Why always try to hang on it? Why don't we try some of them ourselves." It doesn't mean to replace any of them or of any such sort, but just to try them on our own. Maths, as I always feel, is secondary not the primary one. Primary one's the idea giving rise to imagination, put down on paper in the form of diagrams and described precisely using a universally recognized language called 'mathematics'. You just can't catch anything and everything with mathematics. The best example : vital error which maths assumes i.e.. everything is ideal and it turns out way far from practical where nothing is ideal ; there will always be error which can never tend to Zero. Mathematical models are just models and have and ought to be reconstructed.

Well, coming to the emission of electrons from cathode it so happens and I stated. I think you have mistaken a little. I suggest you recheck your basics because I already did it before I am posting this reply and I am sure what I typed was right. Electrons can never be emitted if they aren't in excess. Voltage only pulls up electrons - with a uniform speed- which are excess at the other side, not the electrons of the material itself. If it pulls, then the whole thing blasts up- because the charges need to get accelerated not drifted which means change in energy-and the wires burn up and light's emitted etc.. I guess you know the capacitor failures and bad odor they emit. (di electric failures where the di electric turns)

Not all the gases act as dielectric. It depends on the composition.

Read this -an extract from a website

In 1896, Thomson visited America to give a course of four lectures, which summarised his current researches, at Princeton. These lectures were subsequently published as Discharge of Electricity through Gases (1897). On his return from America, he achieved the most brilliant work of his life - an original study of cathode rays culminating in the discovery of the electron, which was announced during the course of his evening lecture to the Royal Institution on Friday, April 30, 1897. His book, Conduction of Electricity through Gases, published in 1903 was described by Lord Rayleigh as a review of "Thomson's great days at the Cavendish Laboratory". A later edition, written in collaboration with his son, George, appeared in two volumes (1928 and 1933).

Happily, I was able to read both the volumes and I am confident as long as they hold their truth value, I hold mine. At the same pressure, Hydrogen conducts in the discharge tube while normal air acts as a dielectric and oxygen acts as a semi-conductor with a function of temperature and applied voltage. If its not for pressure then why aren't we filling the incandescent bulb with some other gas and why only Noble gases? Got the clue?

And there can't be capacitance with just "free-space" in it. We, humans, can never attain 100% vacuum. I bet, you'll be clear with all your doubts (confusions) once you read the books mentioned above. Just revise them once and you'll confusion will be solved.

 

[sheshank]

There are a few more details which you may need to sort out, such as the fact that the relative permittivity of a vacuum of 10^-7 Torr (εr is pretty much =1) so how much 'polarisation' of the gas molecules does this represent? (There may be some QM issues here, to consider, I think.)
Pictures in your mind are very useful but they need to agree with observation and well established theory.

Here is where all my friends, my lecturers, my colleagues fail. They fail to understand the basic underlying concept. First of all the permittivity is not naturally set one. We take it as one fundamental concepts. It varies with everything around you. Moreover, Permittivity is only compared not measured. Comparisions can never be depended upon unless you have a complete control of it. As you, yourself mentioned, its relative permittivity so recheck it again. Its relative, not absolute or anything of that sort.

Pictures are clue to the understanding. Theories come and go. They should fall apart one or the other day, when a better theory comes. Not every theory stands still. Not even Newton's theories stood still. Leave about theories. Pictures need to agree with observations I agree, but there are many other factors which affect the observations one or the other way and can't be eliminated. The only choice we have is to picturize it. Picturization may give rise to the description of it, precisely and Quantitatively. That'll never agree with observation again. Its like dog chasing its own tail.

Gases never polarize. Polarization happens only in the molecules in which they themselves can't drift and only their electrons can drift (like in all solids). When the molecules are ready to move, like in all liquids and gases, they move for the conduction. And here, in case of capacitor they accumulate at the surfaces of the plates.

 

[jim hardy]

And there can't be capacitance with just "free-space" in it.

uh-oh. I hope that's not so else my belief in aether has to go away.


or were you speaking of tha capacitor plates having to be in it ?

 

[sophiecentaur]

@sheshank

I conclude, from what you have written, that you have not 'The Maths" to cope with Maxwell or you would appreciate how elegant it all is when stated in mathematical form.
I also conclude that you have not heard of Thermionic Emission or the details of how electrons were first discovered.
As Jim says, you are an adherent to the idea of Aether -a very outdated and pretty well discredited idea. You have not heard of the permittivity of free space? (To which all 'relative permittivities' are related and which is a basic quantity involved with the speed of light).

I think a little more learning and a little less ego could help you a lot with your Science.

Lecturers and Colleagues all fail. Well, that is always the problem for the true Genius. I wish you well with your (non-mathematical) Physics studies and look forward to finding your name in the Nobel Prizewinners list.

 

[sheshank]

@sheshank

I conclude, from what you have written, that you have not 'The Maths" to cope with Maxwell or you would appreciate how elegant it all is when stated in mathematical form.
I also conclude that you have not heard of Thermionic Emission or the details of how electrons were first discovered.
As Jim says, you are an adherent to the idea of Aether -a very outdated and pretty well discredited idea. You have not heard of the permittivity of free space? (To which all 'relative permittivities' are related and which is a basic quantity involved with the speed of light).

I think a little more learning and a little less ego could help you a lot with your Science.

Lecturers and Colleagues all fail. Well, that is always the problem for the true Genius. I wish you well with your (non-mathematical) Physics studies and look forward to finding your name in the Nobel Prizewinners list.

What you concluded is right. I am not a pure or atleast partial mathematics student. I don't like everything around me to be complex. I want them to be simple and easy. Mathematics assumes everything to be ideal, and expects the same with the period of time which is not ideal. So, It stays away from reality. That's what troubles me and makes me hard to accept Math in everything.

I definitely read and watched the visuals and did almost everything learning in the discovery of Atom and its structures. Perhaps, with your stress, I think I'll have to read them again. Permittivity is not perfectly defined just like a charge or mass or time. Its just relative and not absolute. We assume it be to be some quantity and compare everything with it. I doubt the whole scientific progression regarding this.

I should apologize If I meant to express anything of the sort of Ego here. I didn't really mean it. Many people around me say that I express ego and thanks for that. I don't know how or where I express it, but its repeated again and again. I am still learning and didn't stop it. Well, finding my name in Noble Prize winners ! That's really a very big way. I think its too much for me taking my age of 19 into consideration. I have to learn Mathematics, but even Mathematics is not providing exact answer for my ideas. Anyway, thanks for your concern.