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what is electricity? part 3: capacitors, dielectrics and confusion

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an electrophorus, apparently

Canto: I’ve found a useful website on the history of the capacitor, which tells us that the term condenser was an early term for a capacitor, presumably because it accumulates charge, condensing it – like condensed milk?

Jacinta: Condensation in chemistry, or whatever, means transformation from a gas, or vapour, to a liquid. Remember they were thinking of an electrical fluid in the early days.

Canto: Well this excellent website on the early days tells me that the effect they were creating by rubbing  a glass globe is now called the triboelectric effect. And by the way it was Franklin who worked out that it was the glass that was creating the effect – nout to do with water, it seems.

Jacinta: Yes, it’s an everyday effect – you can get it just through combing your hair, or rubbing a plastic pen on your sleeve and then picking up bits of paper. I did it at school! I was very sciencey in them days.

Canto: Interestingly, there are lots of nice comments on this website, pointing out that the term for capacitor in a number of European languages is kondensator, or variants thereof. But we get yet another story here on early Leyden jars, which I’ll need to unpick:

It was realized also at Leyden University that it worked only if the glass container was held in your hand and not if it was supported by an insulating material. Today we realize that the alcohol or water in contact with the glass was acting as one plate of the capacitor and the hand was acting as the other while the glass was the dielectric. The high voltage source was the friction machine and the hand and body provided a ground.

Jacinta: So sometimes water was used as a ‘plate’ instead of the tin foil on the inner surface, and the hand was acting as the other plate. So, different versions of Leyden jars. And the dielectric? Yet another unexplained term.

Canto: Yeah, they just never simplify things enough for fuckwits like us. A dielectric is apparently an insulator. Or, as Wikipedia expands it, it’s ‘an electrical insulator that can be polarised by an applied electric field’. Now, I thought that an insulator was the opposite of a conductor, that it tends to be a bad conductor, something that’s difficult for a charge to pass through. Or is that a resistor? Anyway, I can see how dielectric, meaning two, has to do with polarisation, positive and negative, but it still remains vague. I just thought an insulator kind of protects people from getting electric shocks.

Jacinta: So, going back to Crump, here’s a quote:

Franklin succeeded in giving Leyden jars both positive and negative charges, and showed that the force itself was stored in the glass of the jar with the charge being proportional to its surface area.

I DO NOT UNDERSTAND THIS. I WANT TO UNDERSTAND. Does he mean positive and negative charges at the same time? Is that what a dielectric is? And when he says the force was stored in the glass, and the charge bore a mathematical relation to the surface area of the glass, does he mean a different thing by force and charge? And if the charge is proportional to the surface area of the glass, does that mean that if the surface area of the glass was equal to, say, the surface area of a glassy planet Earth, you’d get a more than respectable charge? And if our universe has a surface area?

Canto: The universe isn’t made of glass, I learned that from Dava Sobel’s The glass universe. Or not.

Jacinta: Okay, let me look up some common definitions before we go on.

dielectric is a material that transmits electricity without conducting. That’s to say, an insulator (BUT I DON’T UNDERSTAND WHAT THIS MEANS). Examples of dielectric materials include glass, ceramics, metallic oxides, plastics and dry air.

An insulator, electrically speaking, is a material in which electricity can’t flow freely. In such materials, electrons are tightly bound – though it’s all relative. They’re said to be resistive. So presumably there’s a connection between resistors and insulators. Most insulators are non-metals.

conductor is a material that allows a flow of electrrical charge, aka a current. Metals, such as copper wire, are commonly used as conductors.

Electric charge – and I think this is really the biggie – is a state or property of matter when a certain force from an electromagnetic field is applied to it. Or when it is within an electromagnetic field. But we won’t try to define an electromagnetic field until part 30 or so. An electric charge can be positive (carried by protons) or negative (electrons). This is not, of course, a full definition.

Triboelectricity is a charge of electricity gained by friction. The triboelectric effect can be varied and unpredictable, depending on the precise structure of the materials being rubbed together.

A capacitor, originally called a condenser, a term first coined by Volta, is… well, we posted a piece over four years ago called ‘What are capacitors?’ – but we’ve never thought about them since…

Canto: Yes, I’ve skimmed through that piece and I barely understand it. Let’s just say for now that a capacitor is a device for temporarily storing electricity, but that it differs from a battery somehow.

Jacinta: Okay, one more term used in Hackaday’s ‘History of the capacitor’ that needs explaining is hygroscopic. It says that soda glass, whatever that is, is hygroscopic. Franklin used soda glass in his experiments, apparently.

Canto: Google only tells me something about soda-lime glass, which I’m hoping is the same thing. It’s the most prevalent type of glass, composed of 70% silicon dioxide, or silica, 15% soda (sodium dioxide) and 9% lime (calcium oxide). The other 6% is made up of ‘other’. Hygroscopic materials attract water molecules from the surrounding environment, either by absorption or adsorption, but Wikipedia, which gives a large list of hygroscopic materials, makes no mention of glass or silicon as hygroscopic, though it does mention sodium salts.

Jacinta: So let’s move on with the history of these electrical discoveries, and maybe we’ll solve the problem of our own ignorance along the way. I note that potted histories of the battery, such as the one I’m about to quote from, don’t bother to distinguish between a battery and a capacitor:

Ben Franklin built an electric battery using glass window panes and thin lead plates. Using his “electric battery,” a term he coined himself, he showed how electricity could be stored in the glass and passed through it. Shouldn’t we call it the great-grand-dad of electric batteries?

So let’s not worry about it, though I suspect Yank jingoism is at play here. Let’s move on to Alessandro Volta.

Canto: And the continuous current battery. Volta’s first contribution to electricity was to improve on the electrophorus…

Jacinta: And here’s a great definition of the electrophorus, a device actually named by Volta:

An electrophorus or electrophore is a simple manual capacitive electrostatic generator used to produce electrostatic charge via the process of electrostatic induction.

Canto: Clear as mud. An electrophorus apparently consists of a dielectric plate…

Jacinta: Yeah, something that transmits electricity without conducting it.

Canto: Okay, let’s clear that up – perhaps. Dielectric materials don’t have free electrons for conducting electricity – they’re insulators. Electrons are, of course, electrically charged particles, and in dielectrics they’re tightly bonded to their nuclei. What does happen when an electric field or current is applied is that they become polarised. This raises the question of what polarisation actually is, and what it is about an electric field/current that causes this polarisation.

Jacinta: Not to mention whether there is a difference between an electric field and an electric current.

Canto: Okay, more work to be done. There are different types of polarisation. The polarisation of light, for example, is a complex story which we’ll have to deal with in another 50,000 part series. But here’s a general description from Britannica:

polarization, property of certain electromagnetic radiations in which the direction and magnitude of the vibrating electric field are related in a specified way.

So, just off the top of my head, an electric current seems to imply direction, whereas electric field not so much. On electric polarisation, ScienceDirect, which takes material from scientific papers, has this:

Electric polarization refers to the separation of center of positive charge and the center of negative charge in a material. The separation can be caused by a sufficiently high-electric field.

I think this means that dielectrics can be separated in terms of overall positive and negative charge in their individual atomic make-up, so that they can become magnetised, sort of? Because I think of magnetism in terms of polarity. They can become polarised, like magnets, while not being able to conduct an electric charge. Maybe.

Canto: We seem to have come a long way from capacitors.

Jacinta: We got lost on electrophoruses. An electrophorus consists of a dielectric plate..

Canto: Okay, here’s another definition, from Oxford Reference:

An early form of electrostatic generator. It consists of a flat dielectric plate and a metal plate with an insulated handle. The dielectric plate is charged by friction and the metal plate is placed on it and momentarily earthed, which leaves the metal plate with an induced charge of opposite polarity to that of the dielectric plate. The process can be repeated until all of the original charge has leaked away.

Jacinta: So this gives me a visible image, of sorts. The flat dielectric plate – and I assume a plate is something flat and thin – is polarised by friction, and a metal plate, that’s to say a conductor, is brought into contact with it and then momentarily earthed (I DON”T UNDERSTAND THIS), which leaves an induced charge of opposite polarity on this other plate )I DON”T UNDERSTAND THIS EITHER), and with repetition the original charge is leaked away (DITTO).

Canto: It seems every explanation needs further explanation, and we’re constantly changing electricity’s tail. And we’ve only just begun 🎵.

References

History of the Capacitor – The Pioneering Years

what are capacitors?

https://en.wikipedia.org/wiki/Triboelectric_effect

https://en.wikipedia.org/wiki/Hygroscopy

https://www.britannica.com/technology/soda-lime-glass

https://www.britannica.com/science/polarization-physics

https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095746578

Written by stewart henderson

December 12, 2021 at 12:34 pm