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what is electricity? part 8: turning DC current into AC, mostly

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Canto: So before we go into detail about turning direct current into alternating current, I want to know, in detail, why AC is better for our grid system. I’m still not clear about that.

Jacinta: It’s cheaper to generate and involves less energy loss over medium-long distances, apparently. This is because the voltage can be varied by means of transformers, which we’ll get to at some stage. Varying the voltage means, I think, that you can transmit the energy at high voltages via power lines, and then bring the voltage down via transformers for household use. This results in lower energy loss, but to understand this requires some mathematics.

Canto: Oh dear. And I’ve just been reading that AC is, strictly speaking, not more efficient than DC, but of course the argument and the technical detail is way beyond me.

Jacinta: Well let’s avoid that one. Or…maybe not. AC isn’t in any way intrinsically superior to DC, it depends on circs – and that stands for circuits as well as circumstances haha. But to explain this requires going into root mean square (RMS) values, which we will get to, but for now let’s focus on converting DC into AC. Here’s a quote from ‘all about circuits’:

If a machine is constructed to rotate a magnetic field around a set of stationary wire coils with the turning of a shaft, AC voltage will be produced across the wire coils as that shaft is rotated, in accordance with Faraday’s Law of electromagnetic induction. This is the basic operating principle of an AC generator, also known as an alternator

The links explain more about magnetic fields and electromagnetic induction, which we’ll eventually get to. Now we’ve already talked about rotating magnets to create a polarised field…

Canto: And when the magnet is at a particular angle in its rotation, no current flows – if ‘flow’ is the right word?

Jacinta: Yes. This same website has a neat illustration, and think of the sine curves.

Canto: Can you explain the wire coils? They’re what’s shown in the illustration, right, with the magnet somehow connected to them? And the load is anything that resists the current, creating energy to power a device?

Jacinta: Yes, electric coils, or electromagnetic coils, as I understand them, are integral to most electronic devices, and according to the ‘industrial quick search’ website, they ‘provide inductance in an electrical circuit, an electrical characteristic that opposes the flow of current’.

Canto: OMG, can you explain that explanation?

Jacinta: I can but try. You would think that resistance opposes the flow of current – like, to resist is to oppose, right? Well, it gets complicated, because magnetism is involved. We quoted earlier something about Faraday’s Law of electromagnetic induction, which will require much analysis to understand. The Oxford definition of inductance is ‘the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing’, if that helps.

Canto: Not really.

Jacinta: So… I believe… I mean I’ve read, that any flow of electric current creates a magnetic field…

Canto: How so? And what exactly is a magnetic field?

Jacinta: Well, it’s like a field of values, and it gets very mathematical, but the shape of the field is circular around the wire. There’s a rule of thumb about this, quite literally. It’s a right-hand rule…

Canto: I’m left-handed.

Jacinta: It shouldn’t be difficult to remember this. You set your right thumb in the direction of the current, and that means your fingers will curl in the direction of the magnetic field. So that’s direction. Strength, or magnitude, reduces as you move out from the wire, according to a precisely defined formula, B (the magnetic field) = μI/2πr. You’ll notice that the denominator here defines the circumference of a circle.

Canto: Yes, I think I get that – because it’s a circular field.

Jacinta: I got this from Khan Academy. I is the current, and μ, or mu (a Greek letter) stands for the permeability of the material, or substance, or medium, the wire is passing through (like air, for example). It all has something to do with Ampere’s Law. When the wire is passing through air, or a vacuum, mu becomes, or is treated as, the permeability of free space (μ.0), which is called a constant. So you can calculate, say, with a current of 3 amps, and a point 2 metres from the wire that the current is passing through, the magnitude and direction of the magnetic field. So you would have, in this wire passing through space, μ.0.3/2π.2, or μ.0.3/4π, which you can work out with a better calculator than we have, one that has all or many of the constants built in.

Canto: So easy. Wasn’t this supposed to be about alternating current?

Jacinta: Okay forget all that. Or don’t, but getting back to alternating current and how we create it, and how we switch from AC to DC or vice versa…

Canto: Let’s start, arbitrarily, with converting AC to DC.

Jacinta: Okay, so this involves the use of diodes. So, a diode conducts electricity in one direction only…. but, having had my head spun by the notion of diodes, and almost everything else electrical, I think we should start again, from the very beginning, and learn all about electrical circuits, in baby steps.

Canto: Maybe we should do it historically again, it’s more fun. People are generally more interesting than electrons.

Jacinta: Well, maybe we should do a bit of both. It’s true that we’re neither of us too good at the maths of all this but it’s pretty essential.

Canto: Okay, let’s return to the eighteenth century…

References

https://www.allaboutcircuits.com/textbook/direct-current/chpt-15/magnetic-fields-and-inductance/

Alternating Current vs Direct Current – Rms Voltage, Peak Current & Average Power of AC Circuits (video – the organic chemistry tutor)

 

Written by stewart henderson

January 16, 2022 at 6:19 pm

what is electricity? part 5: volts, amps, currents, resistance and final acceptance, almost

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A vintage Edison carbon filament light bulb – I stole this from Amazon.com, which every decent person should do

Jacinta: So the struggle continues, but I do feel we’re making progress, after having perused our previous posts. We’re perhaps being too hard on ourselves.

Canto: Yes, we’re geniuses actually, asking all the smart questions, not taking anything for granted. Anyway, we posted an image with part 4 of this series, which might help us to understand volts, watts and so forth. It tells us that volts are ‘a force that makes electricity move’, and that voltage measures ‘the potential difference between two points in a circuit’. I don’t fully understand this. It also tells us that watts are the product of voltage and current, P = VI, which we’ve already stated. I’m worried that we’ll be able to make calculations without really understanding the forces involved.

Jacinta: I suspect that our lack of hands-on experimental experience is hindering us. Even brilliant.org won’t really give us that.

Canto: This ‘potential difference’ concept is hard to grasp. Here’s another, apparently very different definition:

Voltage is the pressure from an electrical circuit’s power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light.

This takes us back to the safe ground of comparing electricity with water. But how does ‘pressure’ equate with ‘potential difference’?  Fluke.com goes on to introduce another headache. Voltage can come in two forms, alternating current (ac) and direct current (dc). We’re definitely not ready for that complication.

Jacinta: But further on this site gives an explanation of potential difference, again using the water analogy. Like water in a tank, voltage is more powerful (has more potential energy) the more water is stored, the bigger the tank etc. When the valve to the tank is opened, that’s like switching on the current, but there will always be resistance (ohms) in the dimensions of the valve or the pipe (the conductivity of the wire). And of course we’re talking of the ‘flow’ of electrons, but I seem to recall it’s more like the electrons are bumping against each other rapidly, a sort of knock-on effect. I may be wrong about that though.

Canto: Voltage is a measure of the potential capacity to do work – to push electrons into activity, whatever the detail of that activity is. I think that’s right. I don’t understand why it’s called potential difference, though, rather than potential energy, say.

Jacinta: Okay, I’ve just asked the internet that question. On Quora, someone with a PhD in theoretical physics says that it’s not actually potential energy, though somewhat related. There’s an equation, U = qV, in which U is potential energy, q is charge, and V is potential, or voltage.

Canto: Right, so voltage is very close to potential energy, because it’s the next letter in the alphabet.

Jacinta: Haha, your knowledge has always been too alphabetical. But apparently it has something to do with fields, and scalar and vector properties. Let’s not go there.

Canto: We might not be able to avoid it. Another Quora answer gives voltage the symbol E, apparently due to Ohm’s Law, I think because in Ohm’s day voltage was described as electromotive force.

E (or V) = IR (I being the current, R the resistance).

Jacinta: We haven’t really talked about electrons thus far, because we’ve been treating the subject historically and we’ve not got past the 18th century, but let’s jump to a modern understanding for a while. We now know that metals and other materials that can pass electrons from atom to atom easily are called conductors. Or rather, we now know that the reason metals are good at conducting electric currents is because of their atomic structure, where valence electrons, the electrons in the outer or valence shell, are loosely bound and can move or bounce from atom to atom within the atomic lattice. I think. Materials like glass and rubber are insulators for the opposite reason – tightly bound electrons.

Canto: So wires of good conducting material, such as copper, are insulated with rubber, to contain and direct the current. If these conducting materials don’t have a current connected to them, the valence electrons will move about randomly. Attaching an electric current to these materials pushes the electrons in a particular direction. Which raises the question – how does this happen? Where exactly does this force come from? 

Jacinta: It apparently comes from the voltage – but that sounds like magic. Of course, the source is a battery or some kind of electrical grid which is connected to households – a  sort of power storehouse. The source is a force.

Canto: Nice. But how does an electrical current move these electrons? For example, we know how water in a stream flows from the mountains to the sea. That’s the force of gravity. And I know how that works, sort of.

Jacinta: What is gravity? Will that be our next 50-part series?

Canto: In yet another intro to electricity I get the analogy of voltage and water pressure, which sort of explains how the force works, like water released from a tank, but it doesn’t explain what the force is. That’s the question – what actually is electricity.

Jacinta: But surely it actually is electrons flowing in a circuit, in a particular direction. Or in lightning. And here’s another definition – of a volt. It’s a joule per coulomb. A joule, in this definition, is a unit of energy or work, and a coulomb is a ‘group of flowing electrons’.

Canto: Fluids again. Anyway, the direction of the current seems to be described as positive to negative (though I’m wondering if the ac/dc distinction comes into play here), as in a small circuit connecting to those terminals in a battery. But why does a current ‘flow’ in this direction, assuming it does? Or are these just arbitrary designations, made up by Franklin?

Jacinta: And here comes another problem thrown up by one of these ‘explanations for dummies’. It distinguishes between a closed circuit, which enables ‘flow’, and an open circuit which prevents the electrons from flowing. I’ve never heard these terms before. Sounds counter-intuitive, but no explanation is given.

Canto: The meaning seems to be that you have to close the circuit to make the flow happen, between one battery terminal and the other for example. And that circuit might include light bulbs, heaters etc. Switching the bulb off means opening the circuit and stopping the flow, at least to that particular bulb. If it’s really a flow.

Jacinta: Well it does seem to be, according to this explainer. The claim is that electron flow is measured in coulombs or amps, because one coulomb equals one amp, though why they confuse us with two measures for the same thing is as yet a mystery. Apparently we can measure the flow of electrons as easily as we can measure the flow of water in a pipe. Which is surely bullshit. The explainer goes on to tell us that this electron flow is called current, and the unit of measure is an amp, or a coulomb.

Canto: Aren’t we going to find out about Monsieur Ampère and Monsieur Coulomb?

Jacinta: Not for a while. Our explainer tells us that one amp or coulomb equals the flow of 6242 plus fifteen more zeroes of electrons over a single point in the circuit in one second.

Canto: Hmmm. I’d hate to be the one counting that, especially within the time limit. Not so easy-peasy. But this is called a unit of electric charge, or electron charge, or elementary charge, and Britannica tells us this about it:

In 2018 the General Conference on Weights and Measures (CGPM) agreed that on May 20, 2019, the ampere would henceforth be defined such that the elementary charge would be equal to 1.602176634 × 10−19 coulomb. Earlier the ampere was defined as the constant current which, if maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length.

Jacinta: Clarity at last! I need a drink. Anyway, our previous explainer seems to distinguish the group of electrons (a coulomb) from their passing one point in a second (an amp). I think. But let’s move on to something else to be confused about. Electrical currents don’t have to pass through wires of course but they do in our everyday electric circuits. And all these wires have a certain level of resistance.

Canto: Ohm, I think I know where you’re going with this.

Jacinta: The longer the wire, the more the resistance. The thicker the wire, the less the resistance. So in everyday circuits we have to find a compromise. And resistance is also temperature-dependent. Our circuits often incorporate resistors to deliberately restrict electron flow, which seems to be essential for lighting. Resistance within materials occurs when electrons collide with atoms, apparently.

Canto: And ‘conduction’ involves dodging atoms?

Jacinta: Well, it’s just electrons, not a game of Red Rover. Incandescent lights work by incorporating resistors, because the collisions release energy, which heats up the resistant tungsten wire in the bulb, producing light.

Canto: Ahh, that’s a real light bulb moment for me. And that’s not even a joke, though it also is.

Jacinta: Sounds like a great note to finish on. We’ll camp here for the night. For the road is long and winding, and I fear has no end….

References

https://www.fluke.com/en-au/learn/blog/electrical/what-is-voltage

https://www.quora.com/Why-is-voltage-sometimes-called-potential-Its-not-potential-energy-so-what-is-it-the-potential-of-The-electric-field

How electricity works – working principle (video – the engineering mindset)

https://www.britannica.com/science/ampere

https://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-whats-a-resistor.html

Written by stewart henderson

December 23, 2021 at 10:58 pm

what is electricity? part 4: history, hysteria and a shameful sense of stupidity

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to be explored next time

Canto: So we’re still trying to explore various ‘electricity for dummies’ sites to comprehend the basics, but they all seem to be riddled with assumptions of knowledge we just don’t have, so we’ll keep on trying, as we must.

Jacinta: Yes, we’re still on basic electrostatics, but perhaps we should move on, and see if things somehow fall into place. Individuals noted that you could accumulate this energy, called charge, I think, in materials which didn’t actually conduct this charge, because they were insulators, in which electrons were trapped and couldn’t flow (though they knew nothing about electrons, they presumably thought the ‘fluid’ was kind of stuck, but was polarised. I presume, though, that they didn’t use the term ‘polarised’ either.

Canto: So when did they stop thinking of electricity as a fluid?

Jacinta: Well, a French guy called du Fay postulated that there were two fluids which somehow interacted to cause ‘electricity’. I’m writing this, but it doesn’t make any sense to me. Anyway this was back in 1733, and Franklin was still working under this view when he did his experiments in the 1740s, but he proposed an improvement – that there was only one fluid, which could somehow exist in excess or in its opposite – insufficiency, I suppose. And he called one ‘state’ positive and the other negative.

Canto: Just looking at the Wikipedia article on the fluid theory, which reminds me that in the 17th and early 18th century the idea of ‘ether’, this explain-all fluid or ‘stuff’ that permeated the atmosphere somehow, was predominant among the cognoscenti – or not-so-cognoscenti as it turned out.

Jacinta: Yes, and to answer your question, there’s no date for when they stopped thinking about ether or electrical fluid, the combined work of the likes of Coulomb, Ørsted and Ampère, and the gradual melding of theories of magnetism and electricity in the eighteenth and nineteenth centuries led to its fading away.

Canto: So to summarise where we’re at now, Franklin played around with Leyden jars, arranging them in sets to increase the stored static charge, and he called this a battery but it was really a capacitor.

Jacinta: Yes, and he set up a system of eleven panes of glass covered on each side by thin lead plates, a kind of ‘electrostatic’ battery, which accumulates and quickly discharges electric – what?

Canto: Electrical static? Certainly it wasn’t capable of creating electrical flow, which is what a battery does.

Jacinta: Flow implies a fluid doesn’t it?

Canto: Oh shit. Anyway, there were a lot of people experimenting with and reflecting on this powerful effect, or stuff, which was known to kill people if they weren’t careful. And they were starting to connect it with magnetism. For example, Franz Aepinus, a German intellectual who worked in Russia under Catherine the Great, published a treatise in 1759 with translates as An Attempt at a Theory of Electricity and Magnetism, which not only combined these forces for the first time but was the first attempt to treat the phenomena in mathematical terms. Henry Cavendish apparently worked on very similar lines in England in the 1770s, but his work wasn’t discovered until Maxwell published it a century later.

Jacinta: Yes, but what were these connections, and what was the mathematics?

Canto: Fuck knows. Who d’you think I am, Einshtein? I suppose we’re working towards Maxwell’s breakthrough work on electromagnetism, but whether we manage to get our heads around the mathematics of it all, that’s a question.

Jacinta: To which I know the answer.

Canto: So let’s look at Galvani, Volta and Coulomb. Galvani’s work with twitching dead frogs pioneered the field of bioelectricity – singing the body electric.

Jacinta: Brainwaves and shit. Neurotransmitters – we were electrical long before we knew it. Interestingly, Galvani’s wife Lucia was heavily involved in his experimental and scientific work. She was the daughter of one of Galvani’s teachers and was clearly a bright spark, but of course wasn’t fully credited until much later, and wouldn’t have been formally educated in those Talibanish days. She died of asthma in her mid-forties. I wish I’d met her.

Canto: So what exactly did they do?

Jacinta: Well they discovered, essentially, that the energy in muscular activity was electrical. We now recognise it as ionic flow. Fluids again. They also recognised that this energy was carried by the nerves. It was Alessandro Volta, a friend and sometime rival of the Galvanis, who coined the term galvanism in their honour – or rather in Luigi’s honour. Nowadays they’re considered pioneers in electrophysiology, the study of the electrical properties of living cells and tissues.

Canto: So now to Volta. He began to wonder about Galvani’s findings, suspecting that the metals used in Galvani’s experiments played a much more significant role in the activity. The Galvanis’ work had created the idea that electricity was a ‘living’ thing, and this of course has some truth to it, as living things have harnessed this force in many ways throughout their evolution, but Volta was also on the right track with his skepticism.

Jacinta: Volta was for decades a professor of experimental physics – which sounds so modern – at the University of Pavia. But he was also an experimenter in chemistry – all this in his early days when he did all his practical work in physics and chemistry. He was the first person to isolate and describe methane. But here’s a paragraph from Wikipedia we need to dwell on.

Volta also studied what we now call electrical capacitance, developing separate means to study both electrical potential (V) and charge (Q), and discovering that for a given object, they are proportional. This is called Volta’s Law of Capacitance, and for this work the unit of electrical potential has been named the volt.

Canto: Oh dear. I think we may need to do the Brilliant course on everyday electricity, or whatever it’s called. But, to begin – everyday light bulbs are designated as being 30 amps, 60 amps and so forth, and our domestic circuits apparently run on 240 volts. That latter is the electric potential and the amps are a measure of electrical output? Am I anywhere close?

Jacinta: I can’t pretend to know about that, but I was watching a video on neuroanatomy this morning…

Canto: As you do

Jacinta: And the lecturer informed us that the brain runs on only 20 watts. She was trying to impress her class with how energy-efficient the human brain is, but all I got from it was yet another electrical measure I need to get my head around.

Canto: Don’t forget ohms.

Jacinta: So let’s try to get these basics clear. Light bulbs are measured in watts, not amps, sorry. The HowStuffWorks website tells us that electricity is measured in voltage, current and resistance. Their symbols are V, I and R. They’re measured in volts, amps and ohms. So far, so very little. They use a neat analogy, especially as I’ve just done brilliant.org’s section on the science of toilets. Think of voltage as water pressure, current as flow rate, and resistance as the pipe system through which the water (and effluent etc) flows. Now, Ohm’s Law gives us a mathematical relationship between these three – I = V/R. That’s to say, the current is the voltage divided by the resistance.

Canto: So comparing this to water and plumbing, a hose is attached to a tank of water, near the bottom. The more water in the tank, the more pressure, the more water comes out of the hose, but the rate of flow depends on the dimensions of the hose, which provides resistance. Change the diameter of the hose and the outlet connected to the hose and you increase or reduce the resistance, which will have an inverse effect on the flow.

Jacinta: Now, to watts. This is, apparently, a measure of electrical power (P). It’s calculated by multiplying the voltage and the current (P = VI). Think of this again in watery terms. If you increase the water pressure (the ‘voltage’) while maintaining the ‘resistance’ aspects, you’ll produce more power. Or if you maintain the same pressure but reduce the resistance, you’ll also produce more power.

Canto: Right, so now we’re adding a bit of maths. Exhilarating. So using Ohm’s Law we can do some calculations. I’ll try to remember that watts are a measure of the energy a device uses. So, using the equation I = P/V we can calculate the current required for a certain power of light bulb with a particular voltage – but using the analogy of voltage as water pressure doesn’t really help me here. I’m not getting it. So let me quote:

In an electrical system, increasing either the current or the voltage will result in higher power. Let’s say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation I = P/V, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.

You know that P = 100 W, and V = 6 V. So, you can rearrange the equation to solve for I and substitute in the numbers.

I = 100 W/6 V = 16.67 amps

I’m having no trouble with these calculations, but I’ve been thrown by the idea of a 6-volt light bulb. I thought they were measured in watts.

Jacinta: Okay, so now we’re moving away from all the historical stuff, which is more of our comfort zone, into the hard stuff about electrickery. Watts and Volts. Next time.

References

https://www.britannica.com/biography/Charles-Francois-de-Cisternay-Du-Fay

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

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

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

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

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

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

https://science.howstuffworks.com/environmental/energy/question501.htm

https://byjus.com/physics/difference-between-watts-and-volts/

Written by stewart henderson

December 19, 2021 at 8:33 pm