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  • Imagine that here, in front of me, is a box.

  • But this is no ordinary box.

  • It's indestructible, could be any size, and anything could be happening inside it.

  • Maybe it's making steel from iron.

  • Or maybe it's burning coal to generate electricity, or creating compounds to be used in medicines.

  • No matter what's going on inside this magical box, it's our job as engineers to make it the best most efficient box it can be.

  • And to do that, we'll need to rely on an incredibly basic rule. A law, if you will.

  • It's called the Law of Conservation, and it says that matter and energy can neither be created nor destroyed.

  • And we can use this law to figure out both, where our box isn't operating as efficiently as it can be, and how to fix it.

  • [Theme Music]

  • Whatever it's doing, our box is what engineers call a system, and what's happening inside it is a process.

  • Everything beyond the box is known as the surroundings.

  • Let's say you have an entire steel mill inside this particular box, so the process is converting iron to steel.

  • Now, iron is expensive.

  • Running steel mills is expensive.

  • So if there's something inefficient about the way your box works, you're going to want to know about itand fix it.

  • Ideally, whatever you feed into the system at the beginning, is what you get at the end.

  • Input equals output.

  • If your box is perfectly efficient,

  • the law of conservation tells you that the amount of steel you get at the end, should weigh exactly as much as the ingredients you put in.

  • So if you have 100 kilograms of iron and other ingredients going into your box every hour, you'd also have 100 kilograms of steel coming out.

  • You know you're not going to get 105 kilograms of steel, because you can't create 5 kilograms of matter from nowhere.

  • And you can't have 95 kilograms at the end, either, because everything you put into this perfect version of the box is being turned into steel,

  • and 5 kilograms of matter just can't disappear.

  • If you're running this system continuously, with ingredients always flowing in and an equal amount of steel always coming out,

  • you have what's called steady-state.

  • When a system is steady-state, that means the variables at the input and output remain constant, despite what's happening inside the system.

  • It's like water flowing through a full tank: if the system is steady-state, water comes out at the same rate it flows in, and the tank stays full.

  • Of course, we're just talking about what would happen in an ideal situationif the box is perfectly efficient at steel-making.

  • The thing is, the real world doesn't usually work that way.

  • Engineers deal with systems that are way more complex, and they're not going to be perfectly efficient.

  • But you can use the law of conservation to get a system as close to perfect as possible

  • in other words, to get as much output from your input, as you can.

  • Before we go back to our steel mill, though, let's bake a cake.

  • The pan will be our system, the oven will be our surroundings, and the cake batter will be our raw materials, or input.

  • The cake itself will be our delicious output.

  • Like most processes, we're using this one to make something we wantthe cake.

  • But, again like other processes, it's probably gonna produce some byproducts.

  • When you burn coal to get electricity, for example, you also get ash, sulfur oxides, and nitrogen oxides.

  • Steelmaking creates dust from zinc and other metals, which is considered abrasive, hazardous waste.

  • And don't even get me started on chemical waste from manufacturing pharmaceuticals.

  • Our cake will probably have some byproducts, too, like if we burn some of the batter on the pan, creating some charred bits that we wouldn't want to eat.

  • Look, not everyone's a master chef.

  • There are lots of other ways to end up with leftovers, too.

  • Sometimes you can't get all of your raw materials to react or turn into what you wantlike if there's some batter in the pan that doesn't get cooked.

  • Some of the final product can also get stuck in the system.

  • Maybe some of the batter spilled onto the oven racks.

  • Any of these examples could be waste since you couldn't simply separate them back into their initial parts to use them as input next time.

  • There's also the possibility our raw materials were contaminated, which could easily end up in our final product.

  • When we cracked open the eggs, for example, some small pieces of shell might have fallen into the batter,

  • which means we'll probably still find them when we try a slice of finished cake.

  • You know, on second thought, maybe I'll pass.

  • So, clearly our cake-baking process has a lot of room for improvement.

  • Engineers define problems like these in terms of conversion and yield.

  • Conversion describes how much of our initial input was used in the process.

  • If our system has a conversion rate of less than 100%, it means you'll have some leftover or waste.

  • Now that our cake is done, let's go back to the steel mill in a box.

  • If you had 100 kilograms of iron and other ingredients going in, and a 60% conversion rate,

  • that means you'd have 40 kilograms of iron left over at the end of the process.

  • That's a lot of wasted iron!

  • Yield, meanwhile, describes how much of the final product you were able to get from your initial input.

  • So if your box has a 30% yield, 100 kilograms of raw materials would get you 30 kilograms of steel, and 70 kilograms of waste.

  • The most basic way to think about a system is in terms ofbalance”.

  • Engineers measure the values that go in and the values that come out, and if there's a difference, they'll try to figure out what's causing it.

  • But conversion and yield only cover the beginning and end of the process.

  • Engineering, like life, is usually not that simple.

  • If you have a system that's not in steady-state, then you probably don't have the same amount of mass coming in and out.

  • Real-life processes are messy, and stuff gets stuck along the way.

  • That's accumulation, and as engineers, we can use it to keep track of the differences between what's coming in and what's going out.

  • The basic idea is pretty simple: if you subtract your output from your input, you get your accumulation.

  • If you're measuring all this in terms of mass, that simple equation is probably all you need.

  • Mass goes in, and whatever mass doesn't come out at the end, is stuck inside the system.

  • But when there are chemical reactions happening inside the systemwhich there often are

  • it can be more useful to think about accumulation in terms of molecules.

  • During a process, your raw materials might go through a chemical reaction that generates some molecules that don't end up in your product.

  • Or a reaction might consume some molecules that are hanging around inside the system and weren't part of the input.

  • To keep track of where all your molecules are, and which of them are accumulating inside the system, you need two more terms: generation and consumption.

  • To calculate accumulation, you'd take your input and subtract the output like usual.

  • But then you'd also add the molecules being generated within the system by chemical reactions, and subtract any molecules being consumed.

  • Whatever's left over is what's accumulated inside the system.

  • This equation is essential to engineering.

  • Even in its simplest form, engineers can use this equation to figure out how to improve a system.

  • So let's see how we can improve ours!

  • The steel mill in our box has some raw materials flowing in and steel flowing out.

  • As engineers, our goal is to make as much steel as possible at the end of the process.

  • Let's say we begin with 100 kilograms of raw materials with a 70% conversion rate,

  • which means we're only making 70% of what's possible, or 70 kilograms of steel.

  • Not bad, but not great.

  • We'd be losing out on 30 kilograms of raw materials, and probably a good bit of money!

  • This is where we'll need to problem-solve as an engineer and figure out how to make the process better.

  • How about a recycling system?

  • We can introduce a separator at the end of our system to sort out any leftovers.

  • Then we can run that stream of unused raw materials back into the beginning of our process.

  • We already have 70 kilograms of steel and 30 kilograms of iron and other ingredients that weren't used.

  • Let's send those 30 kilograms back.

  • Now, instead of 100 kilograms being fed into the system, we have 100 kilograms plus the leftovers from the previous cycle.

  • So, even though we still have a 70% conversion rate, we're still getting more steel from the same amount of raw material, than we were before.

  • But what if our raw materials aren't perfectly clean?

  • What if we have contaminants?

  • Sometimes contaminants can lead to big problems, literally ruining machines and halting production.

  • It can be very serious and costly.

  • For example, soda bottling plants have had to shut down production because of possible chlorine contamination.

  • With our steelmaking example, let's assume the contaminants are relatively harmless and won't react with anything.

  • We'll keep using our recycling system from before, but now we'll also need a 'purge system' to filter out the contaminants.

  • If we don't get rid of them, we'll be passing them back while adding new contaminants during the next cycle.

  • As the contaminants build up, we'd run into more and more problems with our machines and tools.

  • Remember, just because we're following the Law of Conservation, or any other principles of engineering for that matter,

  • it doesn't mean everything is working out the way we want it to.

  • As engineers, we'll encounter limitations like these and many others.

  • But with clever designs, we can work around them.

  • Engineering is all about testing our limits and pushing them as far as we can until we reach something truly extraordinary.

  • So today we learned all about the law of conservation, beginning with simple, steady-state systems.

  • We then introduced the terms conversion and yield, showing how they apply to a system.

  • We ended with accumulation and covered how generation and consumption can affect how much accumulation there is in a system.

  • I'll see you next time, when we'll learn all about reversibility and irreversibility and how they affect engineering.

  • Thank you to CuriosityStream for supporting PBS Digital Studios.

  • CuriosityStream is a subscription streaming service that offers documentaries and non-fiction titles from a variety of filmmakers, including CuriosityStream originals.

  • For instance, CuriosityStream hasStephen Hawking's Favorite Places

  • a series where renowned physicist Stephen Hawking takes a flight of epic proportions to visit his favorite places in the Universe.

  • You can learn more at curiositystream.com/crashcourse and use the code crashcourse during the sign-up process.

  • Crash Course Engineering is produced in association with PBS Digital Studios.

  • You can head over to their channel to check out a playlist of their latest amazing shows,

  • like The Origin of Everything, Deep Look, and Eons.

  • Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

  • And our amazing graphics team is Thought Cafe.

Thank you to CuriosityStream for supporting PBS Digital Studios.

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保全の法則。クラッシュ・コース・エンジニアリング #7 (The Law of Conservation: Crash Course Engineering #7)

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    林宜悉 に公開 2021 年 01 月 14 日
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