Main content
Course: AP®︎/College Biology > Unit 3
Lesson 3: Cellular energy- First Law of Thermodynamics introduction
- Second Law of Thermodynamics and entropy
- The laws of thermodynamics
- Reaction coupling to create glucose-6-phosphate
- ATP and reaction coupling
- Introduction to metabolism: Anabolism and catabolism
- Overview of metabolism
- Cellular energy
© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Second Law of Thermodynamics and entropy
Entropy is a measure of disorder or randomness in a system. It represents the number of possible states or configurations that a system can take on. According to the Second Law of Thermodynamics, entropy tends to increase over time, meaning that systems naturally progress towards a more disordered or random state.
Want to join the conversation?
- If we could understand all of the bare fundamentals of how all molecules behave and interact, and we could also take a "snapshot" of every molecule and it's state in the universe,
Then from that, would we be able to technically predict everything that will ever happen, inside of our universe?(15 votes)- in quantum world things behave weirdly . "NO SNAPSHOTS"(6 votes)
- If entropy only increases in the universe, and that is meaning that there is more and more heat, how does that relate to the notion that energy can be neither created nor destroyed? This definition of entropy is making it seem like energy is being created and that there is more and more of it.(5 votes)
- No energy is being created. Energy is being converted ultimately into heat, which is dispersed in the expanding universe. Eventually, entropy increases to a point that heat energy is no longer useful for doing any work.(18 votes)
- Which exactly is the Second Law? I guess it has two parts or something?
1)https://www.khanacademy.org/science/biology/energy-and-enzymes/the-laws-of-thermodynamics/v/the-second-law-of-thermodynamics
2)https://www.khanacademy.org/science/biology/energy-and-enzymes/the-laws-of-thermodynamics/v/second-law-of-thermodynamics-and-entropy(7 votes)- The first video on your list is the one that thoroughly explains the Second Law.(5 votes)
- How do we know that the universe is a closed system if it's deemed to be infinite?(4 votes)
- Well, what is a closed system? One that doesn't exchange matter with its surroundings. If the universe was infinite, then there wouldn't be any surroundings, so there wouldn't be an exchange of matter, or anything for that matter because the surroundings don't exist. It wouldn't matter if the universe was infinite or finite for it to be a closed system, assuming that there is nothing beyond the universe for it to exchange mass with.(7 votes)
- Doesn't this disprove evolution? if something is left to itself, wouldn't it just get worse, instead of getting more complicated, as evolution says?(2 votes)
- Nope. There is source of low entropy energy that comes from the sun. The process of life and evolution is converts this low entropy energy into higher entropy waste energy.
If you look into the way complex systems that are not in equilibrium change over time their complexity will increase to a point and then degrade into low complexity as the lower entropy system transitions into a higher entropy system at equilibrium.
If you look at a starting system where you have a layer of cold cream on top of a layer of hot coffee which has low entropy and low complexity. Over time these two fluids will start to merge and mix as they transition to equilibrium. As they mix there will be complex patterns of streaks of cream and coffee but when it comes to equilibrium you have a homogenous mixture of cream and coffee with low complexity.(8 votes)
- Okie dokie, entropy is irreversible in terms of being observable - you could watch the same container of gas your entire life and never see the molecules resume their original positions/orientations simultaneously; however, is there not any probability that throughout the rest of time, there would be an instance in which this happened? Being in a theoretically, entirely closed system, wouldn't the molecules eventually achieve that, since all probable positions of the molecules and combinations will eventually occur/reoccur at some point?
Not that this is really helpful in any sense, but it's fun to think about!(3 votes)- When you look at a finite closed system from a quantum perspective there are only a finite number of possible states that a system can be in. So with random fluctuations the system will eventually occur.
There is a good book by Sean Carroll called "From Eternity to Here" about Entropy and the Arrow of time.(6 votes)
- love the increasingly impassioned speech at the end
and thank you very helpful for review(5 votes) - If we cannot create/destroy energy, shouldn't the number of states remain the same?(4 votes)
- No, because it does not rely on energy but on the space.
Since molecules take up greater space, the number of possible states increases.(2 votes)
- If energy cannot be created or destroyed and only converted, then why does entropy of the universe only increase? Shouldn't entropy be conserved as well?(3 votes)
- Conserved quantities like energy and momentum come from symmetries in systems. For example since the physical laws at one position in space are the same at another position is where the conservation of momentum comes from. Also since the physical laws today are the same as tomorrow is where the conservation of energy comes from.
There is no such symmetry for entropy to be conserved.(3 votes)
- For an exothermic reaction which is true ssystem is more than zero or surrounding is more than zero(3 votes)
- For an exothermic reaction, the energy wants to "exit", so the surroundings are more than zero because the energy is "exiting" the true system.(3 votes)
Video transcript
- [Voiceover] The Second
Law of Thermodynamics, one statement of it is that the entropy of the
universe only increases. And, I put an exclamation mark here, because it seems like a
very profound statement. And, on a lot of levels, it is. And, just to get us into
the right frame of mind, I have this image here
from the Hubble telescope of the night sky. And, each of these dots, these are not stars. These are galaxies. That's a galaxy. That's a galaxy there. That's a galaxy. So, hopefully this gets
you into little bit more of a cosmological scale. But, let's think about what
this is actually telling us. The entropy of the
universe only increases. So, entropy, we can define
that as the disorder of a system. And, we're really talking about the number of states that a system could take on. And then, we're saying the universe. But, we could also say the entropy of a closed system only increases. A system that is fully contained, that's not interacting
with its surroundings, because the universe is
the ultimate closed system. There's nothing for it to,
outside of it to interact with thermodynamically. And, I'll do a quick review
of open and closed systems, just so we make sure we understand that. So, if I had a campfire, so I have some logs and I had my, the flame going right over here. So, that's the campfire. If I were to just look
at the logs and the fire, that's going to be an open system. Because, it's clearly
interacting thermodynamically with its surroundings. It's releasing heat. It's warming up the air
molecules around it. It's releasing light
out into the universe. There could be interactions from the rest of the universe into the system. So, it isn't isolated from
the rest of everything else. But, a closed system, it is isolated. And, there are, it's very hard to create a true closed
system in our everyday life. But, we can approximate it. And, the one that you've
probably experienced in the not too distant
past is an ice cooler. And, an ice cooler,
we're at least attempting to thermodynamically isolate, isolate the inside of the cooler from the outside, from
the rest of the universe. So, this is, and the way we
do it is we have some type of an insulating material. Maybe some styrofoam. And, we could put, you know, we'd use it to maybe store ice. But, it's not a perfect closed system, because eventually, the heat from the rest of the universe will warm
up the walls of the cooler. And eventually, that heat will warm up, will be transferred to the ice, and it will warm it up. And, it will melt it. So, it's not a perfect closed system, but it's a good approximation, because we're at least attempting to isolate it thermodynamically from the rest of the universe. And, I can even make
a little cover of this to show that we really
wanted to isolate it. And, in research labs, you'll see things that are
much better approximations of closed systems. But, even those at some level
are, they're going to interact with the rest of the universe. The ultimate closed system, so this is a closed system, is really the universe. Nothing to interact with
outside of it thermodynamically. So, let's think a little
bit about this definition. The entropy of the
universe only increases. Why does this make intuitive sets? Well, the best example I can think of is just straight up diffusion. So, if I were to have, let's
say I have a container. So, I have a container, and I'll make it a, I'm gonna make it a closed container. We'll say this is some type of theoretical ideal closed system here. Now, let's say I had some ideal gas. So, I had some ideal gas
molecules right over here. They have some average temperature, but that means they all each
have their own individual, their own individual kinetic energy. They're all bouncing
around in different ways. What's going to happen over time? Well, over time, the
ones on the left here, they're gonna bounce off this wall. And then, they're eventually gonna go in this direction. And so, over time, you're
gonna have a situation where the system is going to
look something more like this. So, the system is going
to look more like this, where instance, let's see,
this is six particles. These six particles are gonna diffuse throughout the container. So, they're gonna diffuse
throughout the container. They're going to take up more
of the space of the container. Now, what just happened in that process? Well, when you knew that
the particles were confined to this little section of the container, there were fewer possible states. You had lower entropy
than when you are here, when you know that it's
filled up the container. There's more possible locations, more possible orientations for it. And so, you are going to have more states. You have higher entropy, higher, higher, higher, entropy. And, in general, these processes
where you have the entropy increasing, we call these
irreversible processes, irreversible, irreversible processes. And, why is it irreversible? Well, there's some probability that these molecules
might just gather back into this corner of it. But, it's very, very low probability. And, this is when we're dealing with six molecules. But, in real systems, we'd
be dealing with much larger than six molecules. We'll be dealing with millions of millions of millions of millions of molecules. So, things with, between 20
and 30 zero's of molecules. And there, it's very unlikely that
they just all bump together in the right way to start
taking a smaller volume, when they could actually
fill the container. And so, that's why you don't see, that's why you don't see
smoke just naturally turn into to some type of shaped particle, or take up less space, as opposed to filling its container. So, this is irreversible, because you went from,
you went from fewer number of potential states, as a smaller volume, to a higher number of potential states. And, the universe is
constantly doing this. That's why the entropy of the
universe is only increasing. Now, there's some processes that it feels like the entropy
isn't increasing that much. So, if you were to take one
billiard ball right over here, and you were to roll
it, you were to roll it into another billiard
ball right over here, and transfer the momentum to that one, it feels like that could
go the other way around. Like, that other billiard
ball could hit this one and go backwards. And, at a macro level, it feels like this is
a reversible process, and people will tend to
call this reversible. But, if you really were to
go on a microscopic level, and it looks like the entropy
isn't increasing that much, but if you were to look at
it on a microscopic level, and just to be clear, the entropy, you know, when this ball is moving and this is stationery, going to a state where this is moving and this is stationery, it doesn't look like the
entropy is increasing that much. And so, that's why they tend
to call this irreversible, because you tend to observe things where maybe this one,
it could go backwards. This could hit this one and then, this one could go, you can kind of run the film in rewind. But, even there, if you were
to look at a microscopic level, you would see that some
heat is being generated, and that some molecules in
the ball are getting excited as they collide, and as they
have friction with the air, and as they, roll on the ground over here. And, you're never going
to get those molecules to go back into the state
that they were before, that you actually do have
the entropy increasing in the system. So, even when in our every day lives, in thermodynamics, people talk about reversible processes. They're only approximately reversible, and that the entropy's only
increasing a little bit. It's not like there's
zero increase in entropy. Irreversible reactions,
these are the ones. Diffusion is a very obvious one, where it's very clear
that you have an increase in entropy, and it feels like it's a very, very low probability, or
almost zero probability of this thing ever going
back to where it was. And, you won't observe it, because we were talking
about that many molecules. Something with 20 or
30 zero's of molecules. The odds of all of them
just doing the right thing, you could wait around a very long time and never actually observe that happening. And so, hopefully this makes sense that the disorder in this way, the number of states only increases as you have more and more interactions. And, a lot of that is coming from heat. Everything you're doing right now, when I'm making this video, my body is generating heat. That heat is dissipating
into the universe. That is adding to the number of states that the universe can actually take on. As I move my hands up, my little digital pencil that
I'm using is causing friction. That's releasing heat into the universe. My computer is running and releasing heat into the universe. You watching this, releasing
heat into the universe. The electrons traveling on
the wire to your computer, releasing heat into the universe. And, all of that is increasing
the number of states. So, if you're thinking on a
molecular level of everything.