an autodidact meets a dilettante…

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Posts Tagged ‘Volta

what is electricity? part 7 – alternating current explained, maybe

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Canto: So, alternating current is electrical current that alternates, or wobbles, or zig-zags, or cycles back and forth, at fifty or sixty cycles per second, aka hertz, but how and why?

Jacinta: Well, as Sabine would say, that’s what we’re going to talk about today. As always, when we look online for explanations, they tend to assume the reader or viewer has background knowledge by the bucketful. Here’s a typical example:

Many sources of electricity, most notably electromechanical generators, produce AC current with voltages that alternate in polarity, reversing between positive and negative over time. An alternator can also be used to purposely generate AC current.

It goes on to explain what an alternator is, but not very effectively for types like us.

Canto: We really need our own ‘For Dummies’ library.

Jacinta: The alternating current that’s used in our electrical grids has a neat sine wave form, undulating at precise intervals above and below a time line.

I’ll try to find out how we bring about alternating current, but first some points about its usefulness. As I think we mentioned before, AC is useful for transporting electrical energy, because it produces lower current at higher voltages (I DON’T REALLY UNDERSTAND THIS), so creating less resistance in the power lines, and so less energy lost as heat.

Canto: Some simple definitions, via Wikipedia et al, which we really need to keep reinforcing. Voltage is electric potential, or pressure, or tension. It’s usually analogised as water in a tank, or a boulder at the top of a mountain, ready to unleash its ‘tension’ by rolling downhill, and meeting resistance along the way, which makes things happen.

Jacinta: Did you know that there’s also three-phase AC power? OMG. But we talked in an earlier post about electrons only moving slightly, bumping the next electron along and so on. But, duh, I didn’t think that one through – that bumping action would be continuous, like people in a queue. You’d bump the person before and be bumped by the person behind, so the movement would be continuous, more or less, they’d all move from the positive to the negative. It’s what they call a chain reaction.

Canto: Interesting, but back to these analogies, I understood that a water tank has the potential to pour out water, and that a boulder has a potential to release kinetic energy down a mountain, but what is this potential energy that a battery has? It’s something called voltage, but that’s what I don’t understand. It’s the storage of a certain amount of electricity, like so much water. But I can visualise stored water. I can’t visualise stored electricity, or electric potential, or whatever.

Jacinta: Well, one day, understanding will dawn. Meanwhile, AC power, that’s when you get electrons to oscillate backwards and forwards, for example via a spinning magnet, which alternately repels and attracts electrons. It’s the movement of the electrons rather than their direction that creates the current.

Canto: Changing polarity. That’s what a spinning magnet will do (and maybe that’s what is meant by an alternator, or something like). And it will do it in an undulating rather than abrupt way. Very fast undulating – 50 cycles a second.

Jacinta: So I think we need to look at transformers, which are able to change the ac voltage, but not dc. Don’t ask why, at least not yet.

Canto: I’m looking at a vid which says that with AC the voltage varies, creating a sinusoidal function, as in the graphic above. But this explains nothing to me. Voltage is electric potential, but what really is that? I don’t want fucking analogies, I want the reality of it. How do you store this ‘electric potential’ in a battery, or whatever? And what really gets me about this and other videos are the comments – ‘great explanation’, ‘what a great teacher you are’, I’ve learned more from this than from months of study’ etc etc etc. And I’m thinking – am I a complete moron or what?

Jacinta: I feel your frustration, but we’ve promised to focus on AC, so just hold on to that question, which can be formulated as – How can a battery (or any other device) store electric potential for later use?

Canto: Which I suppose is something the same as – what is a battery (or an electric potential storage device)? How can you make one?

Jacinta: Anyway, a battery is used for DC energy, flowing from its positive to its negative terminal. That’s why, if you have batteries in series, like in the tube of my computer keyboard, they have to be in the right order, positive connected to negative terminals.

Canto: And if you have, say, three 1.5v batteries in series, that means you have 4.5v of ‘electric potential’?

Jacinta: Uhhh, let’s focus on AC. So, in Australia we typically have 230v household sources of AC electricity, oscillating, or changing polarity, at a frequency of 50 cycles/second, or 50 hertz. Imagine if you have a battery that’s spinning around so that the polarity is, well, spinning around too.

Canto: So if we have a 230v AC source in every home, is that like a gigantic spinning battery? I’d like to see that. Is that what an alternator is?

Jacinta: Well, if you look up ‘What’s an alternator’, you’ll generally find stuff about motor vehicles, but it’s definitely all about alternating current. And if you think polarity, you should think magnetism. So an alternator is essentially a magnet connected to an electric circuit, that changes polarity, usually by spinning, which creates a smooth alternation – back to the sine wave. We’re talking here about one-phase AC.

Canto: Yeah, we don’t presumably have alternators in our homes because it’s already AC in the wires, so it’s all AC?

Jacinta: Don’t confuse me. Running an electric current through a wire – usually copper – creates a magnetic field, and you can strengthen this magnetic field by coiling the wire. I’m not sure why, but this is essential electromagnetism, which we might understand one day. Anyway, this coil of wire is now an electromagnet, with its own polarity. Increasing the current induces a stronger magnetic field. If we run a magnet through the coil, we’ll create a stronger electric current, in DC form. Stop the magnet, and you stop that current. Reverse the magnet and you reverse the current. Push and pull the magnet in and out, and you create an AC current.

Canto: So that’s how sex can be electrifying – if it’s done fast enough?

Jacinta: Hmmm. The speed of the magnet’s movement does create a stronger current, as does the strength of the magnet.

Canto: Ahh, so it’s both the meat and the motion? Anyway, how to transform DC into AC – I’ve heard of a new device, or whatever – an inverter.

Jacinta: Ok, backing up, you’ve no doubt heard of the big battle between Edison and Tesla regarding AC and DC, back at the end of the 19th century. Well, Edison proved himself a bit of an arsehole during this battle, though the hero-worship of Tesla has since become a bit extreme. Since then, it’s been AC for big electrical networks worldwide, but DC is still used for car batteries and other smaller scale power. And, yes, an inverter is the device used to convert DC to AC.

Canto: Let me say that I do understand how AC works to create energy. It doesn’t matter if the movement is in one direction, or two, or a thousand. It’s the movement itself that creates the energy, which creates heat to boil your kettle or light your lamp.

Jacinta: Good, now there are rectifiers, which are a collection of diodes, which can convert AC to DC, but that’s for another post. An inverter comes in more than one type. Some use electromagnetic switches, reversing the flow abruptly, even brutally, with a pattern very different from our sine wave. More like castle crenellations. But electronic inverters use components such as capacitors and inductors – yes, they’ll be explained eventually – to smooth out the transitions. Transformers can also be used to change DC input voltage into a quite different AC voltage output, though of course, according to the law of conservation of energy, (first law of thermodynamics) you can’t get more power out of the system than you put in.

Canto: Changing the subject yet again, I was getting aerated about batteries, and I should’ve thought about them a bit more – I know that they get their electric potential from chemistry. I’ve been reading about Volta’s battery, made from zinc, silver and cloth or paper soaked in salty water. But that, and later improvements, and the mechanisms involved, are also for later posts.

Jacinta: Yes, a battery has an anode and a cathode and an electrolyte material separating them. A fun topic to explore more thoroughly. But we’re onto inverters. We need them to convert DC voltage providers, such as batteries and solar panels, into AC power for households. So batteries work to cause a current to flow, in say, a copper wire, and this creates a circuit between the cathode and the anode, heating up lamps and kettles along the way. But inverting the current, to create the sine wave pattern, or multiple such patterns, requires a magnet, coils and such. It’s complicated, so our next post will be horrible.

a pure sine wave inverter, apparently

References

What is Alternating Current (AC)? – Basic AC Theory – AC vs. DC (video)

Electric current (Khan Academy)

https://www.britannica.com/science/conservation-of-energy

https://www.explainthatstuff.com/how-inverters-work.html

 

Written by stewart henderson

January 11, 2022 at 5:35 pm

towards James Clerk Maxwell: 1 – a bit about magnetism

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the terrell, or model globe, with which Gilbert conducted experiments

Canto: So what do you know about magnetism?

Jacinta: Well not a lot but I’m hoping to learn a lot. Some metals – but perhaps it’s only iron – appear to be attracted by other metals – or other bits of iron – so that they’re pulled together and are hard to pull apart, depending on the strength of the magnetism, which is apparently some kind of force. And I believe it’s related to electricity.

Canto: We shall learn more together. All this enquiry stems from a perhaps vague interest in James Clerk Maxwell, who famously connected electricity and magnetism in an equation, or a series of equations, or laws, with a great deal of mathematical sophistication, which I don’t have. Maxwell is hardly a household name in the way that Newton and Einstein are, but he’s undoubtedly revered among mathematical physicists. My own interest is twofold – I’d like to understand more about physics and maths in general, and – I’m Scottish, sort of. That is, I was born there and grew up among Scottish customs, though I’ve lived in Australia since I was five, and I always like to say that I haven’t a nationalist cell in my body. I’ve never waved a flag or sung any of those naff national anthems, and I have dual British/Australian citizenship only as a matter of convenience – and I suppose the more nations I could become a citizen of, the more convenient it would be. And yet. I’ve always felt ‘something extra’ in noting the Scottish contribution to the sciences and the life of the mind. James Hutton, Charles Lyell, James Watt, Adam Ferguson, David Hume and Adam Smith are names I’ve learned with a glimmer of unwonted or irrational pride over the years, though my knowledge of their achievements is in some cases very limited. And that limitation is perhaps most extreme in the case of Maxwell.

Jacinta: So we’ll get back to him later. There are good, easily available videos on all matters scientific these days, so I’ve looked at a few on magnetism, and have learned a few things. Magnetism apparently occurs when the atoms in a block of material are all aligned in the same direction, because atoms themselves are like tiny magnets, they’re polarised with a north and south pole, which I think has something to do with ionisation, maybe. Most materials have their atoms aligned in an infinity of orientations, with a net effect of no magnetism. Don’t quote me on that. The Earth itself is a gigantic magnet with a north and south pole. If it wasn’t, then the solar wind, which is a plasma of charged particles, would strip away the ozone that protects us from UV radiation. Because that field is sucked in at the poles, we see that plasma in the northern and southern latitudes, e.g. the northern lights. We now know that magnetism is essential to our existence – light itself is just a form of electromagnetic radiation (I think). But what we first learned about this stuff was pretty meagre. There were these rocks called lodestones, actually iron ore (magnetite), which attracted iron objects – swords and other tools of the iron age. What was this invisible force? It was named magnetism, after the region of Magnesia in what’s now modern Greece, where presumably lots of these lodestones were to be found. Early discoveries about magnetism showed that it could be useful in navigation…

Canto: But that wasn’t too early – there’s something of a gap between the discussions in Aristotle and Hippocrates and the 12th century realisation that a magnetic needle could be used for navigation. At least in Europe. The Chinese were well ahead in that regard. But I should stop here and say that if we’re going to arrive at Maxwell, it’s going to be a long, though undoubtedly fascinating road, with a few detours, and sometimes we might move ahead and turn back, and we’ll meet many brilliant characters along the way. And, who knows, we may never even arrive at Maxwell, and of course we shouldn’t assume that Maxwell is at the summit of all this.

Jacinta: So the first extant treatise on magnets was the Epistola de Magnete, by Petrus Peregrinus, aka Pete the Pilgrim, in 1269. It was described as a letter but it contained 13 chapters of weighty reading. The first 10 chapters apparently describe the laws of magnetism, a clear indication that such laws were already known. He describes magnetic induction, how magnetism can be induced in a piece of iron, such as a needle, by a lodestone. He writes about polarity, being the first to use the term ‘pole’ in this way – in writing at least. He noted that like poles repel and unlike poles attract, and he wrote of a south pole and a north pole. That’s to say, one end of a needle points north when given its head – for example when suspended in water. He also describes the ‘dry’ pivoted compass, which was clearly well in use by that time.

Canto: What he didn’t know was why a needle points north – actually magnetic north, which isn’t the same as the north pole – but close enough for most navigational purposes. He didn’t know that the Earth was a magnet.

Jacinta: On compass needles, there’s a neat essay online on how compasses are made. I’m not sure about how GPS is making compasses obsolete these days, but it’s a bit of a shame if it’s true…

Canto: So the next name, apart from the others, to associate with work on magnets was William Gilbert, who published De Magnete in 1600. This gathered together previous knowledge on the subject along with his own experimental work. One of the important things he noted, taken from the 1581 work The Newe Attractive, by Robert Norman, was magnetic inclination or dip, probably first noted by the Bavarian engineer and mathematician Georg Hartmann in the mid sixteenth century. This dip from the horizontal, either upward (steepest at the south pole) or downward (north pole) is a result of the Earth’s magnetic field, which doesn’t run parallel to the surface. Inspired by Norman’s work, Gilbert conducted experiments with a model Earth he made, concluding that the Earth was a magnet, and that its core, or centre, was made of iron…

Jacinta: Just how did he he work that out? Did he think that a bar magnet passed through the centre of the Earth from north to south pole?

Canto: I don’t think so, it’s probably more like he thought of Earth as a gigantic spherical lodestone with iron at its centre. It’s understandable that he would infer iron to be inside the Earth to make it magnetic, but he was the first to give a geocentric cause for the behaviour of compass needles – others had thought the attractive force was celestial. Interestingly, Gilbert was also a Copernican, in that he thought it absurd that the stars, which he believed to be vastly distant, revolved around the Earth. So he argued that the Earth turned, a view that got Galileo into so much trouble a few decades later.

Jacinta: Useful to be a Protestant in those times. Thank Dog for Henry VIII.

Canto: He also took an interest in what was later called electricity, though he didn’t consider it connected to magnetism. He built a versorium, the first electroscope, used to detect static electric charge. It was simply a metallic needle pivoted on a pedestal, like a compass needle but not magnetised. The needle would move towards a statically charged object, such as rubbed amber. In fact, Gilbert’s experiments strove to prove that static electricity was distinct from magnetism, which was an important development in early modern science.

Jacinta: I suppose we’re going to learn exactly what ‘static’ electricity is and how it fits in the over-all picture?

Canto: We shall try, though I shudder to think about what we’re embarking on here.

Jacinta: And I shudder to think about what cannot possibly be avoided – mathematics.

Canto: Well, yes, as we enter the 17th century, we’ll be encountering some great mathematical developments – with figures like Descartes, Pascal, Fermat, Liebniz and Newton all adding their weighty contributions to Galileo’s claim that nature is a book written in the language of mathematics.

Jacinta: Shit, I’m having a hard enough time trying to understand this stuff in English.

Canto: Hopefully it’ll be a great and rewarding adventure, and on the way we’ll learn about Coulomb’s inverse-square law, which is central to electrostatics. Meanwhile, it seems not much was added to our understanding of magnetism for a couple of hundred years, until Hans Ørsted’s more or less accidental discovery in 1819 that an electric current could create a magnetic field, by noting that a compass needle moved when placed near an electrified wire. Alessandro Volta had invented the voltaic pile, or battery, twenty years earlier, leading to a pile of electrical experiments in subsequent years.

Jacinta: But we’ll have to go back to the eighteenth century or beyond to trace developments in electricity before Ørsted’s finding brought the two fields together. And maybe we’ll look at the mathematics of
Charles-Augustin de Coulomb and others in the process. Let’s face it, we can’t progress towards Maxwell without doing so.

Canto: Tragic but true.

Written by stewart henderson

March 31, 2019 at 1:37 pm