Capacitor Charging- Where are all the electrons? 11

[WhMcC]

Greetings,

I am hoping that someone may have some insight here:

When a capacitor is charged, the electrons that are held in the electric field within the device must be physically distributed in some manner- how and where? Do the additional electrons fill normally vacant positions in the valence orbits of the material that comprises the plates of the capacitor? Or, is there some other mechanism by which the electrons are located and held in the field?

Thanks for your thoughts on this one!

 

[jlhmaint]

Take a look at this link it helps explaine it.

http://www.amasci.com/emotor/cap1.html

 

[IRstuff]

I would be a little wary of believing everything on that set of pages. His explanation of capacitors are essentially OK, but he seems to have issues with physics education in general.

TTFN

 

[jlhmaint]

Ok, i just figure the laymans info on capacitors might help whmcc, i dont take everything to heart off the internet but it is a decent concept on what whcc is looking for

 

[xnuke]

I have a degree in physics and I think it's an excellent explanation, and much better than those I've seen in most physics and EE textbooks. If a person with an appropriate level of background education can't understand the explanation of a concept, it hasn't been explained well enough.

 

[WhMcC]

Thanks to all for the responses thus far. I am still struggling with this to some degree and I apologize in advance for being thick-headed in this matter. While I believe myself to be reasonably educated this eludes me yet and perhaps this is more of a physics-centric question at this level... At any rate, here's what I think I know:

- Capacitors store energy.
- Energy is stored in the electrostatic field created by a charge imbalance between the plates of the capacitor.
- The charge imbalance is a result of more electrons in one plate versus fewer electrons in the other.

My ongoing confusion is still this: in the plate with more electrons where are they held? In the water sphere analogy it is simple to see that when charged one side has a greater volume than the other even though the overall volume water in the sphere is unchanged and further that energy is held in the displaced rubber plate. It is simple also to say that the volume of electrons has increased on one side of the capacitor but it seems unlikely to say that the dielectric has been bent in a physical manner similar to the rubber plate in the water sphere analogy. If (thinking aloud now) electrons have mass and no space can be occupied by more than one electron at any given instant the electrons on a plate in excess of those normally associated with the material constructing the plate must be somewhere in or around the plate? Again, where? In the normally vacant orbitals of the valence shell of the atoms of material comprising the plate? Randomly across the surface area of the plate as dictated by maximum charge separation based on the repulsion of like charges given the strength of the electrostatic field? Or elsewhere?

Thanks again for any follow-up thoughts!

 

[buzzp]

An educated guess is they will be on the surface area of the plate, evenly dispersed. When there is too many on the plate (surface of the plate), the dieletric breaks down causing the electrons to goto the other side until an equillibrium is reached. Just making an educated guess.

 

[IRstuff]

If you wish to get to that level, the electrons are essentially in overall electron cloud in the metal, but closer to the surface that's closest to the other plate.

In metals, the electrons are free to move about, which is where the electrical conductivity comes from. When the metal is charged with additional electrons, they are simply shared amongst the atoms that are closest to the other plate. Anywhere else, there would be an internal electric field that would cause a current flow.

The internal distribution is driven externally by the dielectric field attracting electrons to the inner surface of the plate, but limited by the Coulombic repulsion between electrons in the cloud.

TTFN

 

[cbarn24050]

Hi, the plates of a capcitor do not hold extra electrons, nor do they hold any charge.All the enegry is in the electrostatic field.

 

[IRstuff]

You may wish to review your physics again.

There is no electric field without charges.

TTFN

 

[WhMcC]

Thanks all and extra thanks to IRstuff for the very detailed response!

It is apparent now that this question leans more towards the underlying physics rather than the electronics applications and I appreciate all the responses as they have helped shine light in many of the dim corners of the topic.

I have a next question for anyone who might respond to a true physics follow-up based on this first query discussion. It would not be appropriate for this forum at that level so I would invite individuals to contact me at whksmcc@aol.com if interested in helping tackle the next question.

Otherwise, I want to pass along a link to a great site I found very recently while searching for information. The site covers a lot of ground with regard to electricity, magnetism, and physics that one might find useful and does so in a very user-friendly and informative manner:

http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html

 

[IRstuff]

I think you should go ahead and post away, there are plenty of smart and experienced people in this forum. Some problem solutions require the synergy of a large forum to get the final answer.

TTFN

 

[jlhmaint]

ya post the question we will help if we can.

 

[cbarn24050]

Hi Irstuff, were you refering to me about about reviewing my physics? Consider this as you say no charges means no field, and the converse is true. in a charged capacitor there is no current flow in the plates, therefore no pd in a plate, therefore no electric field in a plate, and therefore no charge in a plate. You might also like to try and find a printed reference to your "electron cloud" theory.

 

[IRstuff]

Sorry, can't take credit for the standard model for conduction in metals; see Chap 6 "Free Electron Fermi Gas" in "Introduction to Solid State Physics" by C. Kittel, J. Wiley & Sons, (c) 1976.

See also Chapter 4.6 "Electrical Conductitivty of Metals" in "Electricity and Magnetism" by Purcell, McGraw-Hill, (c) 1965.

See also, Chapter 15-B, "The Free-Electron Theory of Metals" in "Electronic Structure and the Properties of Solids" by Harrison, W.H. Freeman, (c) 1980.


As to the capacitor itself, see any physics text such as "Fundamentals of Physics" Halliday and Resnick, J. Wiley & Sons, (c) 1974, Chap 26, "Capacitors and Dielectrics"

If there is no excess charge, how does the displacement current arise from discharging a capacitor and how does the integral of the displacement current conveniently equals Q=V*C?

TTFN

 

[jmarko]

Hi, everybody!
I think that key of how capacitor works is in material between metal plates : in dielectric .
There is where really takes place a distribution of charge carriers (electrons or holes-positive charged atoms or molecules).
And really, in solid dielectric, some atoms (or molecules) loses electrons, and some atoms (or molecules) gains electrons, and the charge is then evenly distributed in solid material between conductive plates : geometrical middle is a virtual border between positive and negative charge .
Special case is gas or vacuum or liquid dielectric capacitor. Here are all excess electrons (and holes - negative electrons) distributed on surface of capacitor conductive plates : in space between plates there are no charge carriers (true only when transition effects are bygone - DC voltage only).
So, partially is everbody right.
Best regards, Jmarko

 

[IRstuff]

I'm not sure why there is the need for having two separate mechanisms for vacuum and other dielectrics. Surely, since the equations of behavior are the same, there should be no reason to expect two different physical phenomena.

TTFN

 

[jmarko]

Hi, IRstuff!
You are right, equations for capacitance are the same regardless the dielectric. But original question was: where are the electrons; and physical picture is not the same. In solid dielectric, charge carriers are distributed evenly in the material between plates - atoms and molecules are hold still, while in gas dielectric all charge carriers are atracted to plates.

And also, there is no surplus of electrons -regardless charged or discharged- capacitor (in whole capacitor sum of negative and positive charge carriers is always zero - DC conditions);in charged capacitor electrons are only distributed from one half of material or plate to another with help of external voltage (or electric field).
Therefore, maybe better word is polarised instead of charged, which is probably confusing.
Best regards, Jmarko

 

[WhMcC]

Hello again,

Great dialog folks- thanks!

JMarko, am I to understand that it is the atoms/molecules of the dielectric that lose or gain electrons based on being located on one side or the other of the geometric middle of the dielectric material as a whole? The conductive plates neither gain nor lose electrons in the process but rather provide a conduction path for the migration of the electrons to and from the dielectric?

Just to review, if an atom gains an electron it then occupies a space within the orbital of the valence shell of the atom, with the total numer of electrons to be gained being limited by the number of vacancies in that valence orbital.

Best regards,

WhMcC.

 

[cbarn24050]

Hi Irstuff, it seems you didn't read mt post slowly enough. The energy is in the field betwwen the plates not on the plates, you cannot charge up a metal plate to any significant amount because the number of electrons in a give mass of a given metal is constant.