Why We Can't Invent a Perfect Engine: Crash Course Engineering #10

We’ve introduced the 0th and 1st laws of thermodynamics, so now it’s time to move on to the second law and how we came to understand it. We’ll explain the differences between the first and second law, and we’ll talk about the Carnot cycle and why we can never design a perfectly efficient engine.

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RESOURCES:
Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. 8th ed., McGraw-Hill Education.

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[syedtamsilhussain]

I am a mechanical engineering graduate and this is the first time I understood Carnot cycle clearly

[waleedghayas]

I love how you defined entropy as not simply randomness, but how energy ends up being converted to useless forms.

[evanjones]

Crash Course is legitimately better than the thermodynamics course that I am taking at my university.

[cyrilyago]

Did they just portray nickleback as unusable form of energy 😂 at 2:10

[SaeedAcronia]

I am an aerospace engineer and I have been mocked by these "physicists" for being an end-user of their theories and equations. Guess what?!
Sadi Carnot, the father of Thermodynamics, was actually a mechanical engineer. On behalf of all mechanical and aerospace engineers in the world:






IN YOUR FACE Sheldon Cooper!

[Lucky]

The way I see it, the 2nd law isn't _fundamentally_ about energy at all. Rather, it's all about probability. It's not that we _can't_ make a perfectly efficient engine, it's just the the probability of it happening is practically 0, but actually slightly higher. What's really going on is that entropy is a measure of the number of states a system can exist in. We talk about macrostates vs microstates. For example, if our system consists of a single vase, one potential macrostate would be "broken". A microstates is an exact configuration of particles. There are FAR, FAR more microstates that correspond to "broken vase" than "not broken vase", so a system consisting of a broken vase has higher entropy than one consistenting of a whole vase. This is where the misleading description of entropy as being a measure of "disorder" comes from. Most macrostates we'd consider disordered are higher entropy simple because there are more microstates that correspond to them. But this isn't what entropy is about. So what does this have to do with energy and efficiency? Well anytime we apply energy to a system to do work on it, we're changing the state of the system. Even if we intentionally apply the energy in a very specific manner, there are simply SO MANY ways for some of the energy to be wasted compared to the number of ways for it to all be used for useful work. There's nothing _stopping_ a 100% efficient heat engine from existing, per se, but the probability of it even a single perfectly efficient heat engine cycle to occur is such that we'd never expect it to happen in any way imaginable amount of time. _However, _ given infinite time, it's virtually _guarenteed_ to happen, not just once, but an infinite number of time. That's because _anything_ with a non-zero probability of occuring, no matter how tiny, will eventually occur given a sufficiently long period of time. However, the probability is such a thing is probably on par with the probability of all the particles in your body quantum tunneling at the exactly the same time in such a manner to teleport you someplace. It theoretically _could_ happen, but don't count on it. I think the amount time needed for it to have even a 50% chance of occuring is longer than the age of the universe, though I'm going on memory.

[RobertNeyrinck]

Can we talk about how she said “unusable energy” and the graphic was a nickel back album

[skydivekrazy]

We should find Carnot's burial site...

[bidaubadeadieu]

I'm in my final year as a physics undergrad, but due to chance and schedule conflicts, I won't get to take Thermal Physics and thermodynamics until next spring, my very last semester. I'm really loving these episodes, they feel so fresh and exciting compared to the topics I'm more familiar with.

[je-fqve]

Thanks, in my home we always obey the the four laws of thermodynamics.

[mvmcali]

Entropy is such a cool concept....it took a genius to define it

[Marylandbrony]

The 2nd Law of Thermodynamics is that you don't talk about Thermodynamics.

[robertbilling]

Thanks for that, filled me with nostalgia for Dr Cohen's lectures in Cambridge 40 years ago.

[lewisdavison]

thank you for putting it in such an easy to understand and clear video, very well-spoken. Scary how bad my thermodynamics teacher is.

[Pooskipie]

Shes a great teacher Keep em coming
Wonderful show guys love these videos

[sylys]

Thanks, finally a clear exposé of what entropy is !

[ceasetheday]

finally, a definition of entropy that is useful. simply saying “disorder” or “randomness” gives no insight into why entropy would matter.

[Tsagan]

Dr Shini Samara is the best ! Loving the series thus far, continue the great work

[selahattinkaracay]

Perfect explanation. Carnot is a real genius.

[Dayglodaydreams]

There should be another rectangle at the end of the episode that says "Want to know what an Existential Crisis is click here" and the CC Philosophy episode is hyperlinked.