Capacitors, how do they work, or why?

[Argus25]

Originally Posted by Tyso_Bl

If a vacuum, what properties does a vacuum share with gases or solids that give rise to capacitance? Are we to take it that a vacuum or empty space isn't really empty?

Yes not empty, I think I explained this question (at least with the notions of space and the models we have) in my first post on this thread, but more detail might help.

The property that the vacuum (or space) has for an electric field, called permittivity, is always there as a property of our universe. All that happens is when you add physical material between the capacitor's plates, where say you started with a vacuum, you can add air, plastics, liquids, other solid insulators etc, it raises the permittivity between the plates by a factor of the dielectric constant (otherwise known as relative permittivity) of the material and thereby increases the capacitance. By definition a vacuum has a constant of 1 and say polystyrene around 2.5.

(asking why the permittivity property is there in the first instance and what it really is, is no different to asking why the universe exists in the first place and what it really is. The models we have for it help to use and exploit its properties, most of the time much better than they help us truly understand it. But the math and models are solid enough to send a man to the moon and return him safely which would not be possible if the models were generally defective)

Massive increases in capacitance can be had if an electrolyte is between the plates. The dielectric creates a new or modified electric medium by adding the dielectric material.

[Argus25]

Originally Posted by Tyso_Bl

From what I read it appears that "charge" is stored in the dielectric, not the plates.

I think this is correct to the extent that the charge distribution and polarization in the dielectric allows the capacitor as a whole to absorb more charge for a given terminal voltage, which raises the capacitance as C = Q/V or the charge to voltage ratio.

[Bazz4CQJ]

Originally Posted by Argus25

Yes not empty, I think I explained this question (at least with the notions of space and the models we have) in my first post on this thread, but more detail might help.

I'm sure I recall Jim Al Khalili saying in one of his programmes that the quantum physicists now have evidence that in a vacuum, various particles appear, together with their corresponding anti-particle...and then disappear, in a continous process. That said, I'm not sure such events influence permittivity.

B

[joebog1]

This has evolved into an excellent thread!!
Philosophy from many learned persons!!
I have reread all the posts and its surprising how much I have learned, AND remembered from uni. Stuff I forgot the moment I learned it.

Joe

[Argus25]

Originally Posted by Bazz4CQJ

I'm sure I recall Jim Al Khalili saying in one of his programmes that the quantum physicists now have evidence that in a vacuum, various particles appear, together with their corresponding anti-particle...and then disappear, in a continous process. That said, I'm not sure such events influence permittivity.

After learning the very basics of Quantum Mechanics and finding out that the cat was both simultaneously dead and alive, of which I couldn't be certain, I knew I would have great difficulty incorporating this branch of physics into my interests in radio & television.

The only "quantum mechanical" component in my workshop (whose function can't readily be explained well enough with the approximations and models of Newtonian physics) is the Tunnel diode, because electron tunnelling is a quantum mechanical effect. So I have to put up with a few of these "thought destabilizing diodes" in my Tek scopes.

[Mr Moose]

Hello,
If we consider a simple two plate capacitor when we apply a voltage to it this causes electrons (which are essentially free to move in a metal) to move from one plate to the other. This leaves one plate (the positive plate) with an excess of positively charged protons and so it acquires a positive charge. (The protons are fixed and unable to move). The other plate (the negative plate) has an excess of electrons and so acquires a negative charge.
These excess electrons are subject to three forces. Firstly there is the force from the applied voltage which is pushing the electrons onto the negative plate.
Secondly there is the attractive force from nearby positively charged protons on the positive plate which is also pulling the electrons onto the negative plate.
The third force is a repulsive force from other nearby excess electrons on the negative plate. This force is trying to push the electrons back off the plate. The more electrons that flow onto the plate the closer they will be together and so the greater will be the repulsive force.
Electrons will flow onto the negative plate until this negative force just balances the two positive forces when a state of equilibrium is reached and the capacitor is fully charged.

If you increase the area of the plate the the electrons would be further apart and so the repulsive force would decrease so more electrons will flow on to the plate till the distance between the electrons is restored and hence the repulsive force is restored which results in an increase in capacity.

This brings me to the edge effect. Electrons in the centre of the plate are surrounded by electrons and so are repelled in all directions. However electrons on the edge of the plate only have electrons repelling them on one side so are not subjected to the same force as the ones in the centre.
For a given shape, as the size decreases the ratio of edges to area increases so this effect becomes more pronounced as the size decreases.

If you move the plates closer together you increace the attractive force between the excess protons and the excess electrons and so draw more electrons onto the negative plate to achieve equilibrum which increases the capacitance.

Dielectrics are insulators so the electrons in them are not free to move, however they can move a little when subjected to an electric force, the ammount they can move depending how tightly they are bound, which varies from substance to substance which causes the variation in dielectric constant.

When the dieletric substance is placed between the plates the electrons in them are attracted to and so displaced towards the positive plate leaving a nett extra positive charge facing the negative plate and thus attracting more electrons onto the negative plate and increasing the capacitance.

Yours, Richard

[G8HQP Dave]

From what I read it appears that "charge" is stored in the dielectric, not the plates.

Charge is stored on the plates. However, in addition, charge may be moved in the dielectric - which is why a dielectric can modify capacitance.

How is the dielectric affected by the pumping round of electrons?

If there is a dielectric, then charge in it can be moved - but typically only by a very small distance. A lot of charge moving a very small distance can have a big effect.

If a vacuum, what properties does a vacuum share with gases or solids that give rise to capacitance?

Permittivity.

Thinking about the connection between electric and magnetic fields, if they are static and not changing in intensity do they exist independently of each other?

Static fields are not coupled to each other, which is why we can study electrostatics without worrying about magnetism and vice versa.

What about "edge effects" in the calculation of capacitance, why are they usually ignored, and is there any connection with how capacitance arises?

They are ignored because they greatly complicate calculations, yet are often sufficiently small that they make little practical difference and they provide no special insight. You might have to consider edge effects if you are designing a traditional variable capacitor.

[grampy2]

If I remember correctly from school cert physics as it was in 1947,I saw a Leyden jar which is only a capacitor charged and then dismantled into two conductors and a glass dielectric,on reassembly it would appear to be charged still. Why?.

[Neutrino]

Watch the link in post 26.

[PJL]

The traditional explanation is take 2 plates with a potential difference and put them in a vacuum and there will be an accumulation of electons on the -ve plate and a depletion of electrons on the +ve plate allowing energy to be stored. The amount of energy that can be stored is dependent on the area of the plates and the distance between the plates.

Now place an insulating material with a dielectric constant > 1 (polarisable material) between the plates and the electrons within the molecules of the material will align with the electric field allowing energy to be stored in the material itself rather than just the plates.

The dielectric constant is not the main characteristic that makes a good capacitor. A good capacitor material must be an excellent insulator by not readily allowing the release of electrons in the presence of a strong electric field.

[JacKam_]

You do not need two plates to make a capacitor.

[grampy2]

A problem that's been bugging me for sixty years is solved,thanks.

[G8HQP Dave]

You need two plates to make a capacitor. You can have self-capacitance with just one conductor - it uses the rest of the universe as the other plate, although dominated by nearby parts of the universe such as the table the apparatus is standing on.

[JacKam_]

Correct, Dave.

I do not fully agree with the "Wiki" definition of capacitor. One terminal can store electric charge and allows to pump a charge into it or draw from it depending on voltage at the terminal. The bigger is the surface at the other end of the terminal the bigger capacitance.

Anyway, from electronics point ov wiev the Wiki definition describes human made device, usually for a purpose and is correct for 99.9 % of people. Therefore I am not going to argue as I am outside <-3,+3> standard deviation range, so not normal. For me a capacitor is anything that stores electic charge and creates electric field. A battery is not a capacitor, but its terminals - yes.

As an experinent anybody can blow a baloon, wrap it in aluminium foil and touch it to an output of an RF generator. It WILL act as a huge load !

[Tyso_Bl]

Thank you to everyone that has posted, its taken a while for me to digest what has been said and to do other bits of reading to clarify things.

The explanations by Argus25 have been most helpful to me, but that doesn't mean that all the other posts haven't helped, they all did.

So it really comes down to a property of the universe which we understand as permittivity, which we understand well enough to deal with even if there isn't a "proper" explanation of how it arises.

That will have to do for me, there are projects exploring the nature of the universe and I suppose if an ultimate answer is to be found as to why the physical constants are what they are it will really be a big news story.

Back to melting solder and replacing capacitors which have has their permittivity bypassed.

T

[G8HQP Dave]

To explain something you have to describe how it works in terms of simpler systems/effects. If the thing you are talking about is fundamental then there can be no simpler explanation. Electrical charge appears to be fundamental; if so there can be no explanation.

We can explain how dielectrics work in raising apparent permittivity, because what happens in bulk material is not itself fundamental but a consequence of fundamental things.

[Radio Wrangler]

In the past, scientists have generally thought that they'd got down to fundamentals and there seemed to nothing left to discover. For a while, that is,, then along came relativity and quantum mechanics and a whole slew of new particles to prove a new level below the current level of excavations.

Of course, just because they got it wrong in the past is no proof or even indication they won't ever reach a truly fundamental level.

THey don't seem to be finding new 'particles' at quite the rate they once did. Does this mean that they're running out of ones to find, or that the next level is at dramatically higher energies, or that they're now digging in the wrong direction?

If the true fundamentals are reached, proving it will be an interesting task

David

[G8HQP Dave]

I suspect that we have now found many of the fundamental particles and forces. For example, there is no evidence of any structure inside an electron. Only time will tell.

New particles seem to be either at much higher energies, or only interact very weakly with what we can now see. In that sense I feel we may need to stop digging 'down' and instead dig 'sideways'. To put it another way, look at forces (bosons) rather than matter particles (fermions).

[G0HZU_JMR]

I'd generally recommend people to try to see how they get on with old classic edition of Nelkon and Parker (Advanced Level Physics) as a basic no frills physics reference. It covers capacitance and capacitors quite well for example. At school I recall that we had old dog eared copies of the classic third edition (red/white cover?) but we had to give these back. I bought the 6th edition some years later and still refer to it quite often. I'd recommend the sixth edition if you can find it cheaply. It should be dirt cheap these days. It's the version with the egg and the laser on the front cover. I've not read any of the later editions so I don't know how it changed over the years.

It's presentation style is going to look very basic and dated to younger readers but for my generation this book was a classic

[G8HQP Dave]

We used Nelkon and Parker, but I have no idea which version (early 1970s).