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  • From the earliest steam machines to modern muscle cars, engines have long powered engineering.

  • I mean, the field is literally named after them.

  • Engines provide us with energy we can use for all kinds of things, and how to optimize them is one of engineering's central problems.

  • One of the keys to solving it is the concept of reversibility: the more reversible a system is, the less work it needs from its surroundings.

  • In other words, a more reversible system needs less fuel to keep going.

  • So if you're trying to build the best engine you can, reversibility is a good place to start.

  • [Theme Music]

  • Engines drive our society and many of the machines we use every day.

  • And what's inside many of our engines, are pistons.

  • Pistons are disks or cylindrical parts that move back and forth in a cylinder against a liquid or a gas.

  • In an internal combustion engine, like the one that's under the hood of most cars,

  • the pistons are pushed by the expanding gases in the cylinder, which turns a type of shaft or wheel.

  • This turns their repetitive, linear motion into a rotation that helps power the engine.

  • When you're dealing with a simple machine or mechanism, like a piston, there are two main types of energy you can put into or take out of the system: heat and work.

  • You probably have a pretty good idea of what heat is. Simply put, it's thermal energy.

  • It's what you feel when you step out in the sun or stand by a warm fire.

  • In engineering, we define heat with the letter Q.

  • Work, represented by the letter W, is a little more complex.

  • It's essentially any type of energy other than heat that crosses into a system.

  • One way to measure it is by how much force is being applied over a distance.

  • There are a different types of work you're likely to encounter as an engineer, but with all of them,

  • the idea is that by putting work into a system, you can get some work out of it, usually in a more useful form.

  • One type is pressure-volume work.

  • This has to do with the expansion and compression of matter.

  • Squeezing a stress ball is a good example.

  • Based on how much pressure you apply to the ball, the change in volume tells you how much work you were able to achieve.

  • When the stress ball is expanded, we'd say the work is negative, because it's being done by the ball.

  • When it's compressed, the work is positive, because it's being done on the ball.

  • Pistons involve pressure-volume work, too.

  • In a car, for example, the work done on the system comes from the fuel, which heats up gas in the cylinder when it's ignited.

  • As the gas expands, it does work, with the increased pressure pushing the piston up.

  • Then the gas cools again and the piston moves back down.

  • This cycle produces work that turns the shaft or wheel.

  • Another type of work is shaft work, which is when a shaft or propeller rotates through a liquid or gas.

  • There's also electrical work, which is the work done on a charged particle by an electric field.

  • You can think of it like the discharge from a battery.

  • As you're watching this video, there's electrical work going on in your phone or computer.

  • A lot of engineering is about optimizing your machines and processes to produce the most amount of work with as little input as possible.

  • The more work a machine uses up, the more you need to get out of it, to have it be worth your while.

  • It's like a job; the more time and effort you put in, the more you'll want to get paid.

  • That's why work is well...work.

  • With machines, optimizing work is all about reversibility.

  • You'll never get more energy out of a system than what was put into it.

  • That would violate conservation of energy.

  • But if a process is reversible, that means it can go back to its initial state and start over with no additional work input.

  • In other words, when a process is reversible, you're maximizing the amount of work you get for your input.

  • But reversible processes are impossible in real life.

  • They require slow, steady, incredibly small changes

  • to make sure you don't permanently change the system in a way that you can't reverse without putting some additional work in.

  • Which...would require an infinite amount of time.

  • So in the real world, all processes involving work are irreversible.

  • They can be reset, to some extent, but you need to put in a bit of elbow grease to get them there.

  • A reversible process is more like the best-case scenarioone you can get close to, but never actually reach.

  • In engineering, it's not so much about whether a process is reversible, but how reversible it is.

  • The closer you can get to reversibility, the more efficient and optimal the process will be.

  • To see what I mean, let's go back to that piston.

  • You want the piston to move up and down in the cylinder, to turn a crank, and generate power.

  • There's also gas in the cylinder.

  • Bringing the piston up expands the gas and pushing it down compresses the gas.

  • When the gas is under compression, it will expand on its own.

  • But it won't compress again unless a force is applied to it.

  • It's like the stress ballafter you squeeze it, it will expand back to its original size.

  • But the ball won't randomly crumble back in on itself without an outside force.

  • Say this piston is designed so the force compressing the piston comes from a brick.

  • When you remove the brick, the gas below the piston will expand will expand freely, and the piston will rise.

  • But to get the gas to compress again and the piston to go back down, you need to lift the brick to put it back on top of the piston.

  • Since the system needs outside work to get it back to where it was, with the gas compressed, this process is irreversible.

  • Next, let's say you trying to break the brick in two.

  • The system starts out like before, with the gas compressed by the weight of a full brick.

  • Then you remove one half-brick, leaving it right where the piston was, near the bottom of the cylinder.

  • The gas still expands, but it doesn't push the piston as far, since there's still half a brick's worth of weight holding it down.

  • Then you remove the other half-brick, and like the first one, you leave it at the same height, let's say on a shelf right next to the piston or something.

  • This allows the gas to expand as much as it did when you were using one brick, pushing the piston all the way up.

  • But, think about the amount of work it will take to reverse this process and get the piston to go back down.

  • Before, you had to lift the entire brick all the way from where the piston started to where it stopped when the gas was done expanding.

  • But this time, half the brick is already part of the way up the cylinder, because that's where you removed it.

  • So you start by lifting that half-brick up to the top, which compresses the gas and pushes the piston part of the way down.

  • Then, you lift the other half-brick to where the piston is now, pushing the piston down all the way back to where it started.

  • So, instead of having to lift the whole brick all the way up the piston, now you effectively only have to lift half a brick all the way up the piston;

  • it just took two steps instead of one.

  • That's a lot less work, which means the process is that much less irreversible than it was with one brick.

  • And now you can start problem-solving as an engineer.

  • If breaking the brick in two makes the process more reversible, how can you make it even better?

  • A simple answer is to keep breaking the brick into smaller and smaller pieces.

  • Eventually, you'd turn it into infinitely tiny grains of sand.

  • This time, you start with a pile of sand with the same weight as the full brick, pushing the piston down.

  • Then you remove one grain of sand at a time, leaving each grain at the same height that the piston was when you removed it.

  • Gradually, the piston rises, producing work.

  • But each movement is so small that to reverse the process and move the piston down, all you really have to do is shift each grain of sand sideways.

  • Remember, we're talking about increments that are infinitely small, so you effectively aren't lifting anything.

  • You can keep the shifting grains of sand sideways, and slowly, the weight on the piston will increase,

  • compressing the gas and bringing you right back to where you started.

  • Which is definitely less work than lifting a whole brick, or even a half brick.

  • In fact, apart from that one grain you've lifted from the bottom to the top,

  • the amount of work required to put each grain of sand back on the piston is exactly the same as the work 'produced' when you take it off.

  • Everything is happening so slowly and gradually that you aren't losing energy as heat, which means you don't need to 'add' work to replace that lost energy.

  • So you don't need to put in any external work to push the piston back down,

  • and you can use the same amount of work produced by the system to get it right back to where it started.

  • And there you have it: a reversible process.

  • Again, this would be pretty much impossible to accomplish in real life.

  • For one thing, you can't actually have infinitely tiny grains of sand.

  • Even a single molecule isn't infinitely tiny.

  • Plus, you'd need an infinite amount of time to get through all these infinitely small steps.

  • But as you break the brick into smaller pieces, you can get closer and closer.

  • It's also worth noting that the closer you get to the reversible version of this process,

  • the longer it will take, which would not be useful for most applications.

  • If this type of piston was in the engine of your car, you'd be better off walking.

  • So reversibility sounds good, but you need to work with irreversible systems if you really want to achieve something.

  • As an engineer, the goal is to figure out how close you can get a system to being reversible,

  • while still keeping it time, effort, and cost effective.

  • Which brings us to efficiency.

  • In general, the efficiency of any system is the ratio of what you get out of it, compared to what you have to put into it.

  • It'll have a value ranging from 0% to 100%, with 100% being maximum efficiency.

  • In this case, efficiency helps quantify how close a system is to perfectly reversible.

  • It's the amount of work produced by the system you're looking at,

  • as a percentage of the amount of work that would be produced by the idealbut impossiblereversible system.

  • The result is η, the efficiency.

  • If something is 100% efficient, that means it's a completely reversible system.

  • If it has 0 efficiency, it's totally irreversible.

  • You can see how important efficiency is by going back to cars and engines.

  • The more efficient your vehicle is, the more energy you can get out of your fuel, and the farther you can go on a tank of gas.

  • That's why you'll want to keep efficiency relatively high in most engineering systems.

  • It's especially important whenever you want to sustain a process for a long time, like a cross-country road trip.

  • But sometimes you'll need to sacrifice efficiency to accomplish your goals.

  • You might need to put a lot of work into a system to do something quickly or get a big output.

  • Think of a drag race.

  • Converting between types of energy might also be more important than getting a big output for your input,

  • like turning a hand crank to get a small amount of electricity when the power is out.

  • In these situations, converting energy might be worth the low efficiency.

  • Engineering is all about trade-offs.

  • It's awesome when you can have great marks all around, but that's probably not going to happen very often.

  • In today's lesson, we learned all about how to design the most efficient machines and processes.

  • We began by going over heat and work: the two main types of energy.

  • We then moved on to reversibility and irreversibility and found that most processes are somewhat irreversible in nature.

  • Putting all of this together, we worked through a problem with a piston and learned how to use efficiency to measure a system.

  • I'll see you next time, when we'll learn about the first law of thermodynamics and the conversation will really heat up.

  • 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 amazing shows, like

  • PBS Space Time, Above the Noise, and Physics Girl.

  • Crash Course is a Complexly production.

  • And this episode was filmed in the Dr. Cheryl C. Kinney Studio with the help of these wonderful people.

  • And our amazing graphics team is Thought Cafe.

From the earliest steam machines to modern muscle cars, engines have long powered engineering.

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可逆性和不可逆性。速成工程#8 (Reversibility & Irreversibility: Crash Course Engineering #8)

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    林宜悉 發佈於 2021 年 01 月 14 日
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