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Towards James Clerk Maxwell 2 – Francis Hauksbee’s experiments

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an electrostatic generator – one of Hauksbee’s many ingenious experimental devices

Canto: So we’ve witnessed electricity since we’ve had the wit to witness, in lightning. And through our attempts to understand and harness those scary bursts of energy we’ve transformed our world.

Jacinta: We’ve written about lightning before, but the info we presented there was accumulated over centuries. Now we’re going to travel back to the early years of the Royal Society in England, the early 1700s, a mere 300 years ago, to reflect on the first experiments with electricity – remembering that there was no electric power and light in those days, that gods were in the air and much was mysterious.

Canto: Electricity from the start was much sexier, and scarier, than magnetism – lightning very very frightning was the most obvious physical manifestation, and its power was easily recognised. It could kill at a stroke, while magnetism seemed all about metals getting stuck together, and needles pointing north. Interesting, but hardly earth-shattering.

Jacinta: Lightning was all about gigantic sparks shattering the sky, and the ancients, who spent so much of their time in darkness, must have seen other, less impressive and dangerous sparks, the sparks of static electricity, and wondered.

Canto: In the recent BBC documentary The story of electricity, narrator Jim Al-Khalili begins by describing Francis Hauksbee‘s experiments with static electricity and electroluminescence in the early 1700s, which dazzled visitors to the Royal Society. These were the first properly documented experiments with the mysterious force, and a collection of his papers describing these experiments was widely read by the 18th century cognoscenti – including Joe Priestley and Ben Franklin. He employed the newly-invented air pump (simply a pump for pushing out air, as in a common bike pump), popularised in England by Robert Hooke some decades before. Hauksbee made his own improvements, enabling the pump to create a vacuum.

Jacinta: Yes Hauksbee was a more interesting figure than The story of electricity presents. He didn’t ‘lose interest’ but worked on his experiments and reflected on them until his final illness in 1713 – and I’m thinking that illness, since he was only in his late forties – may have been due to mercury poisoning. Hauksbee was ‘lower class’ so few details of his life are documented. However, in these experiments he wasn’t thinking so much of electricity as of ‘attractive forces’. Also as an experimenter who must always have seen himself as an underling (in his book he mentions his ‘want of a learned education’), he doubtless felt obliged to follow the guidance of his Royal Society ‘master’, Newton, which took him into different fields of research….

Canto: The term ‘electricity’ was possibly not in common use then? You’re right, though, about Hauksbee, who rose from obscurity to become a member of the Royal Society, probably under the auspices of Newton. In late 1705, as a result of some spectacular effects displayed to the Society he became intrigued by ‘mercurial phosphorus’. The fact that mercury, in a vacuum, glowed when shaken, had already been noted by Jean Picard, a 17th century French astronomer, and the Swiss mathematician Johann Bernoulli.

Jacinta: And this has to do with electricity?

Canto: We shall see. Hauksbee wanted to work out the conditions under which this mercurial light was produced. He found that the more air in the container, the weaker the light. Also the light’s intensity depended on the movement of the mercury. He concluded that the friction of the mercury against the glass was the major cause. But was it only mercury that had this property, and was it only glass that brought it out? He experimented with other materials, finding a means of rubbing them together in a section of his air pump, Amber rubbed with wool produced a light, brightened in the absence of air. By contrast metal on flint only produced sparks when air was present. Remember, oxygen wasn’t known about at the time. In late 1705 Hauksbee presented one of his most spectacular experiments for the Society. Ingenious instrument-maker that he was, he created a glass globe, from which air could be pumped in and out, on a rotating spindle. The spinning globe came into contact with woollen cloth, and the contact created a weird purple light inside the evacuated globe, which reduced as air was let in. It was a fantastic mystery.

Jacinta: I’m hoping you can solve it.

Canto: Great expectations indeed. He experimented further, and found that when he pressed his own hands against a spinning evacuated globe, the same bright purple glow was produced, which again faded when air was let in to the globe.

Jacinta: Okay, what Hauksbee was exploring in these experiments are what we now call triboelectric effects. I remember playing around with this in schooldays by rubbing a plastic pen along the sleeve of my jersey and watching the fibres stand on end as the pen passed, and hearing the prickling sound of static electricity. The pen was then capable of lifting scraps of paper from the desk, for a time. But I didn’t see any purple lights and I’m not sure how the presence or absence of air relates to it all.

Canto: Yes, triboelectricity is about the exchange of electric charge between different materials – the build-up and discharge of electrical energy. It seems that some materials have a more or less positive charge and some have a more or less negative or opposite charge (because positive and negative are really arbitrary terms, the key point is their relation to each other), and we know that like charges repel and opposite charges attract.

Jacinta: You’ve brought up the word ‘charge’ here, and I’m wondering if that’s just an arbitrary word too – like degree of positive charge just means degree of being repulsed by its opposite, negative charge. In other words, different materials are attracted to or repulsed by each other to varying degrees under various conditions, and that degree or ‘amount’ of attraction or repulsion is referred to as ‘charge’. So ‘charge’ is a relational term…

Canto: Ummm. Maybe. In any case, if you take these different materials down to the atomic level, and I’m not sure how you take plastic and wool down to that level – I mean I know plastic is a petrochemical product, but wool, which I’ve just looked up, has a very complex chemistry – but the fact that the plastic pen, after some rubbing, pulls the fibres of your woollen sleeve towards it is because there’s an attractive force operating between opposite charges. In fact there’s a movement of electrons between the materials, from the wool to the plastic. This electron transfer leaves those woollen fibres with a net positive charge, which is attracted to the now negatively charged plastic due to the electron flow. I think.

Jacinta: Mmm. None of this was understood in the early eighteenth century, obviously. But before we go back there, we’ll stay with this concept of charge, which is nowadays calculated as a fundamental or base unit, based on the electron or its opposite, charge-wise, the proton. These elementary particles have the same but opposite charge, though they’re very different in mass (something which seems suspect to me). Anyway, taking things on trust, a unit of charge is ‘defined’ in standard macro terms as a coulomb, named for the 18th century French physicist Charles-Augustin de Coulomb. One coulomb equals approximately 6.24 x 1018 protons (or electrons). We’ll come back to this later, no doubt. Returning to Hauksbee’s experiments, he soon realised that it was the glass, not the mercury inside it, that was the agent of electrical effects. His experiments with glass globes were written down in great detail, a boon to later researchers.

Canto: Interestingly, I’ve discovered that, more or less exactly at the same time, one Pierre Polinière was conducting and presenting experiments on electroluminescence in Paris:

A closer examination of these experiments reveals not only that Polinière had personally presented them before the French Academy of Sciences, but that Polinière and Hauksbee, starting from a common interest in the ‘mercurial phosphor’, had conducted similar investigations and had in fact simultaneously announced their independent discoveries of the luminescence of evacuated glass containers.

Pierre Polinière, Francis Hauksbee and electroluminescence: a case of simultaneous discovery.
David Corson, 1968.

Jacinta: So we might finish by trying to explain our current understanding of electroluminescence (EL) and its applications. It’s a sort of combo of electricity and light, as you can imagine, or electrons and photons on the level of particles. For example, semiconductors emit light when subjected to a strong electric field or current….

Canto: Is that the basis of LED lighting?

Jacinta: Absolutely. Electrons in the semiconductor material recombine with electron holes, emitting energy in the form of photons. So it has taken us three centuries to really effectively harness the electroluminescent effects demonstrated by Hauksbee in the early days of the Royal Society.

Canto: What are electron holes? I’m thinking not ‘holes in electrons’ but holes left by electrons as they’re displaced in an electric current?

Jacinta: Yes, or almost. It’s like the lack of an electron where you might expect an electron to be. These holes where you might expect an electrically charged particle (an electron) to be, act like positively charged particles – a positron, say – and move through a lattice like an electron does. We could get into very complicated electronics here, if we had the wherewithal, but these holes are examples of quasiparticles, which mostly exist within solids. Fluid movement within solids (not apparently a contradiction in terms) is extremely complicated. Who would’ve thunk it? This movement of electrons and protons within solids is ‘regulated’ by Coulomb’s Law. Remember him? We’ll be getting to that law very soon, as it’s essential to the field of electromagnetism. And that’s our topic don’t forget.

technomagic – the tellingbone

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weirdly wired – the first telephone

The telephone remains the acme of electrical marvels. No other thing does so much with so little energy. No other thing is more enswathed in the unknown.

Herbert Casson ‘The history of the telephone”, 1910. Quoted in “The Information”, J Gleick

I recently had a conversation with someone of my generation about the technology of our childhoods, and how magical they seemed to us. So let me start with the motor car, or auto-mobile. Our first family car was a Hillman Minx, which was bought in maybe 1964 or so, not too long after we arrived in Australia. The model probably dated from the early or mid-fifties – we certainly weren’t wealthy enough to buy a brand new car. But that didn’t make it any less magical. How was it that you could turn a key and bring an engine to life, and with a bit of footwork and handiwork get the beast to move backward and forward and get its engine to putter or roar? I hadn’t the foggiest.

Next in the mid-sixties came the television box, fired by electrickery. Somehow, due to wires and signals, we could see a more or less fuzzy image of grey figures from faraway, giving us news of Britain and the World Cup, and shows from the USA like Hopalong Cassidy and the Cisco Kid, all made from faraway – even one day from the moon – for our entertainment and enlightenment. Wires and signals, I mean, WTF?

Next we became the first people in the street to have our own tellingbone (or that’s what we proudly told ourselves, actually we had no idea). So people would ring us from the other side of town and then talk to us as if they were standing right next to us!! It was crazy-making, yet people seemed generally to remain as sane as they had been. I would lie in bed trying to work it out. So someone would dial a number, and more or less instantaneously a ringing sound would come out of the phone miles and miles away, and a person there would pick up this bone-shaped piece of plastic with holes in it, and they would talk into one end and listen through the other end, and they could hear this person on the ‘end of the line’ miles away far better than they could hear someone else talking in the next room, all thanks, we were informed, to those wires and signals again.

So, forward to adulthood. One of the most informative books I’ve read in recent years is titled, appropriately enough, The Information, by James Gleick. It’s a history of information processing and communication from tribal drumming to the latest algorithms, and inter alia it tells the story of how the telephone became one of the most rapidly universalised forms of information transfer in human history in the period 1870-1900, approximately. And of course it didn’t come into existence out of nowhere. It replaced the telegraph, the first electrical telecommunications system, itself only a few decades old. Previous to this there were many experiments and developments in the field by the likes of Alessandro Volta, Johann Schweigger and Pavel Schilling. Studying electricity and its potential was the hottest of scientific activities throughout the 19th century, especially the first half.

The telegraph, though, was a transmission-reception system run by experts, making it very unlike the telephone. Gleick puts it thus:

The telegraph demanded literacy; the telephone embraced orality. A message sent by telegraph had first to be written, encoded and tapped out by a trained intermediary. To employ the telephone, one just talked. A child could use it.

Nevertheless the system of poles and wires, the harnessing of electricity, and the concepts of signal and noise (both abstract and exasperatingly practical) had all been dealt with to varying degrees of success well before the telephone came along.

So now let’s get into the basic mechanics. When we talk into a phone we produce patterned sound waves, a form of mechanical energy. Behind the phone’s mouthpiece is a diaphragm of thin metal. It vibrates at various speeds according to the patterned waves striking it. The diaphragm is attached to a microphone, which in the early phones consisted simply of carbon grains in a container attached to an electric current, which were compressed to varying degrees in response to the waves vibrating the diaphragm, modulating the current. That current flows through copper wires to a box outside your home which connects with other wires and cables in a huge telecommunications system.

Of course the miracle to us, or to me, is how a sound wave signal, moving presumably more or less at the speed of sound, and distinctive for every human (not to mention dogs, birds etc), can be converted to an electrical signal, moving presumably at some substantial fraction of the speed of light, then at the end of its journey be converted back to a mechanical signal with such perfect fidelity that you can hear the unmistakeable tones of your grandmother at the other end of the line in real time. The use of terms such as analogue and digitising don’t quite work for me, especially when combined with the word ‘simply’, which is often used. In any case, the process is commonplace enough, and has been used in radio, in recorded music and so forth.

It all bears some relation to the work of the greatest physical theorist of the 19th century, James Clerk Maxwell, who recognised and provided precise relationships between electrical impulses, magnetism and light, bringing the new and future technologies together, to be amplitude-modified by engineers who needed to understand the technicalities of input, output, feedback, multiplexing, and signal preservation. But as the possibilities of the new technology expanded, so did technological expertise, and switchboards and networks became increasingly complex. They eventually required a numbering system to keep track of users and connections, and telephone directories were born, only to grow in size and number, costing acres of forestry, until in the 21st century they didn’t. I won’t go into the development of mobile and smartphones here, those little black boxes of mystery which I might one day try to peer inside, but I think I’ve had enough armchair demystifying of the technomagical for one day.

Yet something I didn’t think of as a child was that the telephone was no more technomagical than just speaking and listening to the person beside you. To speak, to make words and sentences out of sounds, first requires a sound-maker (a voice-box, to employ a criminally simplistic term), then a complex set of sound-shapers (the tongue, the soft and hard palates, the teeth and lips) into those words and sentences. Once they leave the speaker’s lips they make waves in the air – complex and variable waves which carry to the hearer’s tympanum, stimulating nerves to send electrical impulses to the auditory cortex. This thinking to speaking to listening to comprehending process is so mundane to us as to breed indifference, but no AI process comes close to matching it.

References

The information, James Gleick, 2011

https://electronics.howstuffworks.com/telephone1.htm

https://www.antiquetelephonehistory.com/telworks.php

https://www.thoughtco.com/how-a-telephone-works-1992551

Written by stewart henderson

March 1, 2019 at 4:31 pm

On electrickery, part 1 – the discovery of electrons

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Canto: This could be the first of a thousand-odd parts, because speaking for myself it will take me several lifetimes to get my head around this stuff, which is as basic as can be. Matter and charge and why is it so and all that.

Jacinta: so let’s start at random and go in any direction we like.

Canto: Great plan. Do you know what a cathode ray is?

Jacinta: No. I’ve heard of cathodes and anodes, which are positive and negative terminals of batteries and such, but I can’t recall which is which.

Canto: Don’t panic, Positive is Anode, Negative ICathode. Though I’ve read somewhere that the reverse can sometimes be true. The essential thing is they’re polar opposites.

Jacinta: Good, so a cathode ray is some kind of negative ray? Of electrons?

Canto: A cathode ray is defined as a beam of electrons emitted from the cathode of a high-vacuum tube.

Jacinta: That’s a pretty shitty definition, why would a tube, vacuum or otherwise, have a cathode in it? And what kind of tube? Rubber, plastic, cardboard?

Canto: Well let’s not get too picky. I’m talking about a cathode ray tube. It’s a sealed tube, obviously, made of glass, and evacuated as far as possible. Sciencey types have been playing around with vacuums since the mid seventeenth century – basically since the vacuum pump was invented in 1654, and electrical experiments in the nineteenth century, with vacuum tubes fitted with cathodes and anodes, led to the discovery of the electron by J J Thomson in 1897.

Jacinta: So what do you mean by a beam of electrons and how is it emitted, and can you give more detail on the cathode, and is there an anode involved? Are there such things as anode rays?

Canto: I’ll get there. Early experiments found that electrostatic sparks travelled further through a near vacuum than through normal air, which raised the question of whether you could get a ‘charge’, or a current, to travel between two relatively distant points in an airless tube. That’s to say, between a cathode and an anode, or two electrodes of opposite polarity. The cathode is of a conducting material such as copper, and yes there’s an anode at the other end – I’m talking about the early forms, because in modern times it starts to get very complicated. Faraday in the 1830s noted a light arc could be created between the two electrodes, and later Heinrich Geissler, who invented a better vacuum, was able to get the whole tube to glow – an early form of ‘neon light’. They used an induction coil, an early form of transformer, to create high voltages. They’re still used in ignition systems today, as part of the infernal combustion engine

Jacinta: So do you want to explain what a transformer is in more detail? I’ve certainly heard of them. They ‘create high voltages’ you say. Qu’est-ce que ça veux dire?

Canto: Do you want me to explain an induction coil, a transformer, or both?

Jacinta: Well, since we’re talking about the 19th century, explain an induction coil.

Canto: Search for it on google images. It consists of a magnetic iron core, round which are wound two coils of insulated copper, a primary and secondary winding. The primary is of coarse wire, wound round a few times. The secondary is of much finer wire, wound many many more times. Now as I’ve said, it’s basically a transformer, and I don’t know what a transformer is, but I’m hoping to find out soon. Its purpose is to ‘produce high-voltage pulses from a low-voltage direct current (DC) supply’, according to Wikipedia.

Jacinta: All of this’ll come clear in the end, right?

Canto: I’m hoping so. When a current – presumably from that low-volage DC supply – is passed through the primary, a magnetic field is created.

Jacinta: Ahh, electromagnetism…

Canto: And since the secondary shares the core, the magnetic field is also shared. Here’s how Wikipedia describes it, and I think we’ll need to do further reading or video-watching to get it clear in our heads:

The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of volts. This voltage is often sufficient to cause an electric spark, to jump across an air gap (G) separating the secondary’s output terminals. For this reason, induction coils were called spark coils.

Jacinta: Okay, so much for an induction coil, to which we shall no doubt return, as well as to inductors and electromagnetic radiation. Let’s go back to the cathode ray tube and the discovery of the electron.

Canto: No, I need to continue this, as I’m hoping it’ll help us when we come to explaining transformers. Maybe. A key component of the induction coil was/is the interruptor. To have the coil functioning continuously, you have to repeatedly connect and disconnect the DC current. So a magnetically activated device called an interruptor or a break is mounted beside the iron core. It has an armature mechanism which is attracted by the increasing magnetic field created by the DC current. It moves towards the core, disconnecting the current, the magnetic field collapses, creating a spark, and the armature springs back to its original position. The current is reconnected and the process is repeated, cycling through many times per second.

A Crookes tube showing green fluorescence. The shadow of the metal cross on the glass showed that electrons travelled in straight lines

Jacinta: Right so now I’ll take us back to the cathode ray tube, starting with the Crookes tube, developed around 1870. When we’re talking about cathode rays, they’re just electron beams. But they certainly didn’t know that in the 1870s. The Crookes tube, simply a partially evacuated glass tube with cathode and anode at either end, was what Rontgen used to discover X-rays.

Canto: What are X-rays?

Jacinta: Electromagnetic radiation within a specific range of wavelengths. So the Crookes tube was an instrument for exploring the properties of these cathode rays. They applied a high DC voltage to the tube, via an induction coil, which ionised the small amount of air left in the tube – that’s to say it accelerated the motions of the small number of ions and free electrons, creating greater ionisation.

x-rays and the electromagnetic spectrum, taken from an article on the Chandra X-ray observatory

Canto: A rapid multiplication effect called a Townsend discharge.

Jacinta: An effect which can be analysed mathematically. The first ionisation event produces an ion pair, accelerating the positive ion towards the cathode and the freed electron toward the anode. Given a sufficiently strong electric field, the electron will have enough energy to free another electron in the next collision. The  two freed electrons will in turn free electrons, and so on, with the collisions and freed electrons growing exponentially, though the growth has a limit, called the Raether limit. But all of that was worked out much later. In the days of Crookes tubes, atoms were the smallest particles known, though they really only hypothesised, particularly through the work of the chemist John Dalton in the early nineteenth century. And of course they were thought to be indivisible, as the name implies.

Canto: We had no way of ‘seeing’ atoms in those days, and cathode rays themselves were invisible. What experimenters saw was a fluorescence, because many of the highly energised electrons, though aiming for the anode, would fly past, strike the back of the glass tube, where they excited orbital electrons to glow at higher energies. Experimenters were able to enhance this fluorescence through, for example, painting the inside walls of the tube with zinc sulphide.

Jacinta: So the point is, though electrical experiments had been carried out since the days of Benjamin Franklin in the mid-eighteenth century, and before, nobody knew how an electric current was transmitted. Without going into much detail, some thought they were carried by particles (like radiant atoms), others thought they were waves. J J Thomson, an outstanding theoretical and mathematical physicist, who had already done significant work on the particulate nature of matter, turned his attention to cathode rays and found that their velocity indicated a much lighter ‘element’ than the lightest element known, hydrogen. He also found that their velocity was uniform with respect to the current applied to them, regardless of the (atomic) nature of the gas being ionised. His experiments suggested that these ‘corpuscles’, as they were initially called, were 1000 times lighter than  hydrogen atoms. His work was clearly very important in the development of atomic theory – which in large measure he initiated – and he developed his own ‘plum pudding’ theory of atomic structure.

Canto: So that was all very interesting – next time we’ll have a look at electricity from another angle, shall we?

 

Written by stewart henderson

October 1, 2017 at 8:14 pm

what are capacitors?

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the shapes and sizes of capacitors – a screenshot taken from the youtube vid – What are Capacitors? – Electronics Basics 11

Jacinta: We’re embarking on the clearly impossible task of learning about every aspect of clean (and sometimes dirty because nothing’s 100% clean or efficient) technology – batteries, photovoltaics, turbines, kilo/megawatt-hours, glass electrolytes, powerwalls, inverters, regen, generators, airfoils, planetary gear sets, step-up transformers, nacelles AND capacitors.

Canto; Enough to last us a lifetime at our slow pace. So what, in the name of green fundamentalism, is a capacitor?

Jacinta: Well I’ve checked this out with Madam Youtube…

Canto: Professor Google’s co-dependent…

Jacinta: And in one sense it’s simple, or at least it sounds simple. Capacitors store electric charge, and the capacitance of a capacitor relates to how much charge it can hold.

Canto: So how does it do that, and what’s the purpose of storing electric charge?

Jacinta: Okay now you’re complicating matters, but basic to all capacitors are two separated pieces of conducting material, usually metal. Connected to a battery, they store charge…

Canto: Which is a kind of potential energy, right?

Jacinta: Umm, I think so. So take your battery with its positive and negative terminals. Attach one of the bits of conducting material (metal) to the positive terminal and you’ll get a flow of negatively-charged electrons to that terminal, because of ye olde law of attraction. This somehow means that electrons are repelled from the negative terminal  (which we’ve hooked up to the other bit of metal in the capacitor). So because the first strip of metal has lost electrons it has become positively charged, and the other bit of metal, having gained electrons, has an equal and opposite charge. So each piece of metal has the same magnitude of charge, measured in coulombs. This is regardless of the size and shape of the different metal bits.

Canto: But this process reaches a limit, though, yes? A kind of saturation point…

Jacinta: Well there comes a point where, yes, the accumulated charge just sits there. This is because there comes a kind of point of equilibrium between the positive battery terminal and the now positively charged strip of metal. The electrons are now caught between the attractive positive terminal and the positive strip.

Canto: Torn between two lovers, I know that foolish feeling.

Jacinta: So now if you remove the battery, so breaking the circuit, that accumulated charge will continue to sit there, because there’s nowhere to go.

Canto: And of course that accumulated or stored charge, or capacitance, is different for different capacitors.

Jacinta: And here’s where it gets really complicated, like you know, maths and formulae and equations. C = Q/V, capacitance equals the charge stored by the capacitor over the voltage across the capacitor. That charge (Q), in coulombs, is measured on one side of the capacitor, because the charges actually cancel each other out if you measure both sides, making a net charge of zero. So far, so uncomplicated, but try and get the following. When a capacitor stores charge it will create a voltage, which is essentially a difference in electric potential between the two metal strips. Now apparently (and you’ll have to take my word for this) electric potential is high near positive charges and low near negative charges. So if you bring these two differently charged strips into close proximity, that’s when you get a difference in electric potential – a voltage. If you allow a battery to fully charge up a capacitor, then the voltage across it (between the two strips) will be the same as the voltage in the battery. The capacitance, Q/V, coulombs per volt, is measured in farads, after Micky Faraday, the 19th century electrical wizz. I’m quoting this more or less verbatim from the Khan Academy video on capacitors, and I’m almost finished, but here comes the toughest bit, maths! Say you have a capacitor with a capacitance of 3 farads, and it’s connected to a nine volt battery, the charge stored will be 27 coulombs (3 = 27/9). 3 farads equals 27 coulombs of charge divided by nine volts, or 27 coulombs of charge is 3 farads times 9 volts. Or, if a 2 farad capacitor stores a charge of 6 coulombs, then the voltage across the capacitor will be 3 volts.

Canto: Actually, that’s not so difficult to follow, the maths is the easiest part for me… it’s more the concepts that get me, the very fact that matter has these electrical properties…

Jacinta: Okay here’s the last point made, more or less verbatim, on the Khan Academy video, something worth pondering:

You might think that as more charge gets stored on a capacitor, the capacitance must go up, but the value of the capacitance stays the same because as the charge increases, the voltage across that capacitor increases, which causes the ratio to stay the same. The only way to change the capacitance of a capacitor is to alter the physical characteristics of that capacitor (like making the pieces of metal bigger, or changing the distance between them).

Canto: Okay so to give an example, a capacitor might be connected to an 8 volt battery, but its capacitance is, say, 3 farads. It will be fully charged at 24 coulombs over 8 volts. The charge increases with the voltage, which has a maximum of 8. The ratio remains the same. Yet somehow I still don’t get it. So I’m going to have a look at another video to see if it helps. It uses the example of two metal plates. They start out as electrically neutral. You can’t force extra negativity, in the form of electrons, into one of these plates, because like charges repel, and they’ll be forced out again. But, according to the video, if you place another plate near the first, ‘as electrons accumulate in the first metal plate, they will repel the electrons in the second metal plate’, to which I want to respond, ‘but electrons aren’t accumulating, they’re being repelled’. But let’s just go with the electron flow. So the second metal plate becomes depleted of electrons and is positively charged. This means that it will attract the negatively charged first metal plate. According to the video, this makes it possible for the first plate to have more negative than positive particles, which I think has something to do with the fact that the electrons can’t jump from the first plate to the second, to create an equilibrium.

Jacinta: They’re kind of attracted by absence. That’s what they must mean by electric potential. It’s very romantic, really. But what you’ve failed to notice, is that a force is being continually applied, to counteract the repulsion of electrons from the first plate. If the force no longer applies then, yes, you won’t get that net negative charge in the first plate, and the consequent equal and opposite charge in the second. My question, though, is how can the capacitance increase by bringing the plates closer together? I can see how it can be changed by the size of the conducting material – more electrons, more electric potential. I suppose reducing the distance will increase the repulsive force…

Canto: Yes, let’s assume so. Any, a capacitor, which stores far less charge than a similarly-dimensioned battery can be used, I think, to briefly maintain power to, say, a LED bulb when it is disconnected from the battery. The capacitor, connected to the bulb will discharge its energy ‘across’ the bulb until it achieves equilibrium, which happens quite quickly, and the bulb will fade out. If the capacitor is connected to a number of batteries to achieve a higher voltage, the fully charged capacitor will take longer to discharge, keeping the light on for longer. If the metal plates are larger, the capacitor will take longer to charge up, and longer to discharge across the LED bulb. Finally, our second video (from a series of physics videos made by Eugene Khutoryansky) shows that you can place a piece of ‘special material’ between the two plates. This material contains molecules that change their orientation according to the charges on the plates. They exert a force which attracts more electrons to the negative plate, and repel them from the positive plate, which has the same effect as increasing the area of the plates – more charge for the same applied voltage.

Jacinta: An increase in capacitance.

Canto: Yes, and as you’ve surmised, bringing the two plates closer together increases the capacitance by attracting more electrons to the negatively charged plate and repelling them from the positively charged one – again, more charge for the same voltage.

Jacinta: So you can increase capacitance with a combo of the three – increased size, closer proximity, and that ‘special material’. Now, one advantage of capacitors over batteries is that they can charge up and discharge very quickly. Another is that they can endure many charge-discharge cycles. However they’re much less energy dense than batteries, and can only store a fraction of the energy of a same-sized battery. So the two energy sources have different uses.

Canto: Mmmm, and we’ll devote the next post to the uses to which capacitors can be put in electronics, and EVs and such.

 

Written by stewart henderson

August 28, 2017 at 6:27 pm