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Batteries Posted February 6, 2001 And, as you might have guessed, I will also be taking calls in the studio at 352-3706 from those of you who have not bought a KCR membership this year. If that's you, and you're listening to the show even though you're not a KCR member, well, first of all, that's just fine. This is not pay-per-view radio. The FBI is not going to plant a bomb in your car just because you listened to Mostly Mozart without paying for it. But we are a community radio station, and that means that we exist to serve the people who live here. Like you. And if we're going to do it right, we need people like you to get involved. The easiest way to get involved, is the thing that most people think of when they want to solve a problem: "throw money at it!" You can send us cash, and you can be sure that we'll put it to good use. We don't waste dollars here, you can trust me on that one. The problem with throwing money at a problem is that it doesn't give you much in the way of instant gratification. But if it's instant gratification you're after, then don't worry, you've come to the right place! I've got instant gratification right here. During this show, and for the rest of this week, if you buy a KCR membership, we will give you a little something special from our huge collection of goodies! The people and businesses of Nelson have been very kind to you; they have donated so much good stuff that you might even end up buying two memberships just to get the stuff. Which is fine with us! You can buy as many memberships as you like. So, for the next hour, we'll be hearing some lovely Mozart pieces, and I'll be telling you about these goodies that I want to give away, and if you want any of them, all you have to do is call 352-3706 and buy a KCR membership. You get to help us do what we do, and if you don't know what we do, well, I'll just speak for myself and say that I listen to Mozart and say things about theoretical physics that you probably won't hear anywhere else. This may sound strange, but it's true: I've even been told that people like it. And it goes to show that KCR is a great place to be creative in whatever way works for you. A year and a half ago, I would have laughed at you if you said I should do a radio show. In fact, Catherine Fisher asked me that about a year and a half ago, and I laughed at her. But I ended up doing it, and here I am, every Tuesday night, and some other nights too, doing my radio show, helping other people do their radio shows, arranging and rearranging all the machinery, and generally having a grand old time. We're about to hear... We're also going to hear about batteries. I mentioned that Catherine Fisher was the one who roped me into this whole radio show idea, over a year ago; and possibly by coincidence, possibly not, she recently asked me to explain something during my show: "how is electricity stored?" And for those of you who don't already know, if you just tuned in or you weren't paying attention, that's what this show is all about. Storing electricity. Well, science topics generally. Not necessarily storing electricity. I had supernovas and black holes on last week's show. All that matters is that: today I'll tell you how to store electricity. Call in if you'd like to buy a membership; it costs a mere $40 for one year, and besides the satisfaction of seeing KCR grow and improve, that gets you... As always, I will also be happy to hear your science questions. I can usually give you a quick answer on the spot, and I'll come up with a more detailed version and share it with everyone on some future show. The number again is 352-3706, and if you have some amount of money other than $40 then I'm sure we can work something out. Especially if you have some amount of money that's considerably larger than $40. But remember, a $40 membership is less than a dime a day. And a whole lot more fun. --- (sponsorship) My name is Tom Clegg and you can hear this show every Tuesday night from 8 to 9, and every Sunday afternoon from 2 to 3. Let's get on with the show. We're here to discuss how to store electricity. But we can't store it if we don't know what it is. What is electricity? In two words or less: electron pressure. A common analogy compares electricity in a wire to water in a pipe. If you have a little water pressure, you can have a shower and wash the dishes; if you have a lot of water pressure, you can put out fires and knock things over and break windows and all kinds of fun stuff. All you need is a hose, and a high pressure water supply, like a pipe from a water tower or the top of a mountain. Let's say you wanted to take all that water pressure in your hose, and keep it in your car in case you have to put out a fire or stop a dogfight. You could pour a bunch of water into a big vat, and carry that around, but you'd only be storing the water, not the water pressure. You could hook up a hose to your big vat of water, but it wouldn't give you a whole lot of pressure. You'd get just a trickle, compared to what you got back at home. Storing electricity is even more tricky. You can carry around lots of electrons in your car; in fact, since cars contain lots of metal, you already do carry around lots of electrons in your car. But there's no reason why they should want to exert a lot of pressure. Putting them at the top of a tall tower doesn't help at all, because unlike water, electrons are extremely light. Fortunately, 201 years ago, someone called Count Alassandro Volta made an interesting discovery. And if you're wondering, no, it's not a coincidence that this man's name was Volta. Somehow he had the idea of touching a piece of a frog -- that's right, I said a frog -- with two different pieces of metal, and measuring the difference in electron pressure between the two metals. Sure enough, electricity was being generated by a dead frog. Pretty soon, Count Volta had found some chemicals that could take the place of the frog, made a bunch of these little current generating units, piled them up, and connected them end-to-end so as to generate more current at once. Thus, the first battery was created, although for a while it was called a voltic pile; the French word for battery is still pile, and we now refer to a difference in electron pressure as Voltage. This is Mostly Mozart, so you probably expect to find out how it works, right? But I don't know how it works! For that matter, nobody does. It's still a mystery. OK, OK, I'm just pulling your leg. What goes on inside a battery is a simple bit of chemistry. It seems almost too simple to work, but it really does work. Which is fortunate, because we haven't done much to improve on the idea in the last 200 years. You need three things to make a battery. You need a bit of lead; some weak sulphuric acid; and another bit of lead, this one severely corroded, or oxidized, which means it's covered with lead oxide or PbO2. The bits of lead are called electrodes, and the sulphuric acid is called... sulphuric acid. Dip the two electrodes into the acid, and you have a battery. You'll get a steady supply of electrons crowded around the tip of the corroded electrode, pushing their way out; and you'll get a nearly total absence of electrons around the tip of the other electrode. Put an electrical circuit between the two tips, and those electrons will quickly find their way through it, moving away from the crowded area on one electrode to the empty area at the other electrode. If the electrons are leaving through the first electrode and coming back in through the second, you'd expect that they must have some way to get back to the first electrode inside the battery, so that they can start the process again. At least I hope that's what you expect. But what's the difference between the two electrodes? How do the electrons all agree on which direction they should go, and what incentive do they have to move in any direction at all? The answer, of course, is chemistry. Both electrodes undergo chemical reactions when they're exposed to sulphuric acid. By the way, I hope you've figured out that if I'm talking about sulphuric acid, lead, and chemical reactions, this is not something you should attempt at home. OK, I didn't think you would, but I'm just making sure. In a jar of sulphuric acid, there is lots of water, lots of H2SO4, some H2 with a charge of +2, and some SO4 with a charge of -2. If you stick a piece of lead in there, the SO4 molecules start to react with it, to form PbSO4. Left over is an extra H2 with a charge of -2, which means it's carrying around two electrons that it wants to get rid of. The only place for these electrons to go is into the lead electrode. So the lead electrode quickly fills up with these spare electrons, and when it can't take any more, there's nowhere for the electrons to go, so the lead stops reacting with the SO4. If we left it at that, we'd have lots of extra electrons, but no matter what we plugged in, it would fill up with electrons and the PbSO4 reaction would stop. You guessed it: we need some way to absorb the electrons at the other electrode. Remember, the other electrode is corroded, which means it's covered with lead oxide, PbO2. And remember that there's a lot of H2SO4 in that sulphuric acid. That's what makes it sulphuric acid, and not water. Conveniently, H2SO4 and PbO2 readily react with each other to form PbSO4, the same stuff produced at the first electrode. If you're counting all the H's and Pb's and O's and S's, you'll notice a couple of differences: one difference is that the atoms left over after that reaction amount to 4 H's and 2 O's, and they come out in the form of two H2O's, which is to say water. The other difference is that this reaction consumes two electrons. Guess where the electrons come from? That's right, we had a circuit that would have a steady flow of electrons, if only it had somewhere to put them all when it was finished with them. Well, there it is, on the other electrode. So, to build a battery, you need lead; you need more lead; and you need acid. You've probably heard the words "lead-acid battery" before, haven't you? Well, there you go. That's what it means. At this point you might be wondering, "why make a pile?" Or, for those of you who are more interested in technology than history, "do they still make piles, or do they just make a single big battery?" Yes, they still make them as piles. In fact, you're making your own pile when you put two or three batteries in a row in your flashlight. And why? Remember Count Volta? If you just tuned in, and you don't know who Volta was, don't worry. His name is Volta, that's all that counts. The number of volts produced by a given battery turns out to depend on which chemical reaction it uses. In the case of a lead-acid battery, the voltage, or difference in electron pressure, is 2 volts. The corroded electrode, which is called the positive electrode, contributes 1.6 volts, and the other electrode contributes 0.4 volts. 2 volts isn't enough. We want 12 volts. So we put six of these contraptions in a row, hooking up each cell's positive electrode to the next cell's negative electrode. And we get 6 x 2 = 12 volts. So we wrap up this pile really tight, so the sulphuric acid doesn't get out, and we put it in a car, and we turn the key, and |