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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