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  • Showing only topics in ~science with the tag "thermodynamics". Back to normal view / Search all groups
    1. A layperson's introduction to Thermodynamics, part 3: Entropy and the heat death of the universe

      Intro Hello everyone, Today we cover entropy and the heat death of the universe. The previous chapters can be found here and here. While I recommend you read both, you should at least read the...


      Hello everyone,

      Today we cover entropy and the heat death of the universe.

      The previous chapters can be found here and here. While I recommend you read both, you should at least read the first part and skim the second.

      A collection of all topics covered can be found here: https://tildes.net/~tildes/8al/.



      Entropy describes how chaotic a system is. In thermodynamics, chaos is created from an irreversible process. We are all sort of familiar with this concept. A broken cup will not unshatter itself. As a consequence of how our universe works, (net) chaos can only increase. And this might have far reaching consequence, if we look at the effects of entropy on a cosmic scale.


      Entropy describes an amount of irreversible chaos.

      But first, let's cover cycles super quickly. In thermodynamics, a very important concept is a "cycle". A cycle is a repeating process, that returns to its initial condition. For instance, when we ride a bike. We're turning our feet around the crank shaft. Repeatedly returning to the same position we started from. As we push on the pedal, some of our work is lost and turned into heat. Primarily due to friction from the wheels and from the different mechanical parts.

      A cycle that wastes no energy is called a reversible cycle. That would mean 100% of the work in a cycle (even the work that is turned to heat) has to be returned in some way to its original state. The most famous example of this is the Carnot heat engine.[1] But in reality, the Carnot heat engine is nothing more than a theoretical engine. As we remember from before, we cannot turn 100% of heat back into work. So any heat engine, be it a car's motor, a refrigerator, a star, or the human body, will in some way contribute to this irreversible chaos.

      Now what about entropy? If we look at entropy at the molecular level, it all becomes a bit abstract. But we can think of this concept with bigger building blocks than molecules, and still be close enough. Say you have a brick house with orderly layed bricks. This house would love to come crashing down. And lets imagine it does. When the house lays in ruins, it is not likely to suddenly "fall" into the shape of the house again. So if the house has collapsed, our system is in a higher state of chaos. Our entropy has increased. And unless we supply work to the system (and waste energy trough heat), we will not get the brick house back.

      So now we understand, that on the grand scale of the universe, entropy will only increase.

      The heat death of the universe

      But what are the consequences of this? Imagine entropy going on for billions and billions of years. Everything in the universe slowly reaching a higher state of chaos. Everything that is orderly, turns into chaos. All high quality energy has turned into low quality energy. Everything has been wasted and turned into heat. Everything ripped apart until you are left with nothing to rip apart. At this point, there is no interactions between molecules any more. Everything has reached absolute zero temperature.

      At this point, entropy is at its absolute maximum. And we have reached entropic equilibrium.

      This is the heat death of the universe.


      Of course, the heat death of the universe is just one of the many theories about the end of the universe. It assumes that thermodynamics properly describes the universe, and that there are no hidden surprises.

      Frankly told, it's the best bet we have with our current knowledge. But we still know so little. So I would not panic just yet. Alternatively, this is where we could continue with "an engineer's perspective on existensial nihilism". But I think that this is something better reserved for later, and better presented by someone else.

      We have covered what I consider the absolute minimum of thermodynamics, that still gives us a basic understanding of thermodynamics. There are of course a lot of other topics we could cover, but thats it for now. I will potentially write an appendix later with some questions or things that have been asked.

      But for now, that's it. Questions, feedback or otherwise?


      [1] The Carnot heat cycle is a bit beyond the level of what we have discussed so far. It describes a system where heat is supplied and removed to have a piston expand and contract without any energy becoming waste heat.

      14 votes
    2. A layperson's introduction to Thermodynamics, part 2: Equilibrium, phase changes and steam engines

      Intro Hello everyone, Today we cover equilibriums and phase changes. Through that we will get a basic understanding of how things like pressure, temperature, density, volume, etc. are related. The...


      Hello everyone,

      Today we cover equilibriums and phase changes. Through that we will get a basic understanding of how things like pressure, temperature, density, volume, etc. are related.

      The previous chapter can be found here: https://tildes.net/~science/8ao/. I highly recommend that you read it before continuing.

      A collection of all topics covered can be found here: https://tildes.net/~tildes/8al/.



      "Equilibrium" is fancy word for "balance". A system is in equilibrium when it is in balance with the surrounding systems. Any system will naturally attempt to be in equilibrim, and will adapt its physical properties to do so.

      A phase change is the transition of matter from a state (solid, liquid, gas, or plasma) to a different state. This happens due to a change in internal energy, changing how a material is bonded.

      Now that we have it summarised, lets dig a bit deeper.


      A system always tries to be in balance with its surrounding systems. We maybe don't think about this a lot, but we are all very familiar with this principle since we observe it every day.

      If you have a cup of hot cocoa, it will cool down until it has reached ambient temperature. At this point, the cocoa is considered to be in "thermal equilibrium". If we fill a balloon with air, it will expand. It will do so until the air inside the balloon has the same pressure as the air outside the balloon. At this point, the balloon is considered to be in "barometric (pressure) equilibrium".

      Just like when we talk about energy, there is a relationship when we talk about equilibriums. We have something we call (you may remember this from basic chemistry) an "ideal gas". An ideal gas is a good way of looking at this principle. Since the temperature, volume and pressure have a direct relationship.

      Pressure-volume-temperature diagram for ideal gases.

      In the diagram above we can see that if we change one of the three variables, then one (or both) of the other two variables has to change too. For instance, if we heat some air in a canister, the air will try to expand. But being unable to change in volume, it will instead increase pressure. [1]

      Phase changes

      Any material has a set of phases. The ones we'll discuss are the solid, liquid and gaseous phases. Unless we control the material's environment very carefully, materials will always follow this order when energy is added. Solid becomes liquid, liquid becomes gas, and vice versa. For instance water; ice (solid) becomes water (liquid), water becomes steam (gas). So each of these transformations is a phase change.

      So when water is solid (ice), the molecules are in a grid. The molecules do not move around much, maybe a little bit where they stand. But they all still stand in a grid.

      When the water gets heated up, the molecules will start to move. Molecules have a natural attraction to each other due to subatomic forces like the van der Waals force. So the molecules will no longer stay in a grid, but they will still keep each other close due to this attraction. So a material that sticks together but freely moves around is called a liquid.

      Once the material overcomes this natural attraction, the molecules can go anywhere they want. And that's when we get a gas. Or steam, in the case of water. All of this applies even for materials we don't usually imagine would melt or evaporate, for instance steel.

      Here is a visual representation of the three states.

      Now comes the fun part. Ice is water that is at 0 degrees Celcius or below. Liquid water is water that is 0 degrees and above. But wait! Does that mean that water can be both solid and liquid at the same temperature? Yes, indeed. A material requires a certain amount of internal energy to become liquid. That is why internal energy and temperature is often used interchangeably, but is not exactly the same.

      The water molecules in ice will use the supplied energy to get excited and start moving around. This continues until the solid-liquid water reaches a point where all molecules move around. At that point it has completely become a liquid. While water is in solid-liquid state, the amount of internal energy dictates how much is liquid and how much is solid. The exact same thing happens with water at 100 degrees. It can be steam or liquid, but not fully either until it reaches a certain amount of internal energy.

      Here is a diagram of this process.

      Another fun tidbit that makes water special: Water has a lower density as a solid than it has as a liquid, when both are at 0 degrees Celcius. This means that per unit of volume ice weighs less than (liquid) water. Therefore ice floats on top of water. This is the only material that behaves in this way. And thats extremely important to our existince, since it helps regulate heat in the ocean.

      Steam engines (and implication)

      We have learned a few new things today. But there is one really important wrinkle to all of this. A system always will try to be in balance. And this we can exploit. Pressure is a type of "pushing". So thats a type of work! And an increase in thermal energy can lead to an increase in temperature. We remember that from the ideal gas. So if we cleverly organize our system, we can create work from heat! This is the basis behind most heat engines (simplified a ton). We supply thermal energy to some gas or fluid, and extract work from this gas or fluid.

      A classical example is the steam engine. We have water inside a closed system. When we heat up the water, it will turn into steam. And this steam will want to be much less dense than water. As a consequence, the pressure inside the water tank increases drastically. We release a small amount of this steam into a closed piston.

      Here is an animation of this in action.

      The piston suddenly gets a high pressure level. As we remember, it will want to be in equilibrium with its surroundings. Currently the pressure inside the piston is much higher than outside the piston. As we remember from the ideal gas law, a higher volume will mean a lower pressure. So the piston will be moved, as the steam expands to reach a pressure balance. The movement from the piston will drive something, like a wheel. The steam is removed from the expanded piston, and the piston will return to its closed position.[2] Then the process is repeated again and again, to have the piston continously move something.

      All that from a bit of water in a tank and some supplied heat.

      Whats next?

      Next time we will talk about another important property. Entropy! In the previous topic I had a lot of questions regarding the quality of energy types, and what specifies heat from work on an intrinsic level. Entropy is the big answer to this. From that we will also cover the heat death of the universe, which would be a good introduction to "a laypersons introduction to nihilism" if we have any philosophers here.


      [1] For solid and fluid materials (as well as non-ideal gassess) this becomes a lot more complicated. If we ever do a "layperson's intro to fluid mechanics" we will cover it then.
      [2] This described design is very inefficient and very simplified. Usually the piston is made so steam is supplied in turns supplied to either side of the piston. Then the work will both removed the steam that already performed work as well as move the piston. That way you can have continous movement in both directions.

      See for instance this image.

      17 votes
    3. A layperson's introduction to Thermodynamics, part 1: Energy, work, heat

      Intro Hello everyone, @wanda-seldon has been giving us an introduction to quantum physics. For now, she will be given a short break to prepare new stuff. In the meantime I will be covering some...


      Hello everyone,

      @wanda-seldon has been giving us an introduction to quantum physics. For now, she will be given a short break to prepare new stuff. In the meantime I will be covering some classical mechanics, more specifically thermodynamics. In part 1, we need to work our way through some of the more dry concepts, so we can understand and appreciate the horrifying implications of the fun parts. So I promise, this will be the most verbose one.

      Some of you may have briefly seen a version of this posted, that was due to me misunderstanding the schedule with @wanda-seldon. If you saw that one, I will mention I rewrote nearly all of it to be more readable.

      Now, on today's agenda: The basics of heat, work and energy and how it's all related.

      Previous posts can be found here: https://tildes.net/~science/8al/meta_post_for_a_laypersons_introduction_to_series

      Important note

      If @wanda-seldon in her posts mention "energy", it's most likely in the context of energy operators, which is a concept in quantum physics. I'm not going to pretend I understand them, so I will not be explaining the difference. We will cover what energy is in classical mechanics. So keep that in mind if you read something from either of us.



      What is heat? Using a lot of fancy words we can describe it as follows. Heat is an energy that is transferred between systems by thermal interaction. And what is work? Work is an energy that is applied in a way that performs... work. The combined energy in a system is called internal energy. This type of energy can be transformed or applied to other systems.

      These are a lot of new words, so lets break that down a bit.


      A system is just a catch-all term for something that can be defined with a boundary of sorts. Be it mass, volume, shape, container, position, etc. A canister, your tea mug, the steam inside a boiler, your body, a cloud, a room, earth, etc. They are all systems because you can in some way define what is within the boundary, and what is beyond the boundary.

      In theory, you could define every single nucleid in the universe as an unique system. But that would be counter-intuitive. In thermodynamics we tend to lump things into a system, and treat it as one thing. As opposed to Quantum stuff that looks at the smallest quantity. Calculating every single water molecule in my coffee would be pure insanity. So we just treat my mug as the boundary, and the tea inside the mug as the system. And just so it's mentioned, systems can contain systems, for instance a tea mug inside a room.


      Energy is some quantifiable property that comes in either the form of heat, work. It can be transferred to other systems, or change between the different energy types. An example of transfer is my coffee cooling down because it's in a cold room. That means heat has been transferred from one system (my mug) to another system (the room). Alternatively you could say my hot coffee mug is warming up the room, or that the room is cooling down my coffee. Thermodynamics is a LOT about perspective. An example of transforming energy types is when we rub our hands together. That way we convert work (rubbing) into heat. It's really not more complicated than that. An interaction in this case is just a system having an effect on a different system. So a thermal interaction means it's an interaction due to heat (like in the mug example).

      This brings us to an extremely important point. So important, it's considered "law". The first law of thermodynamics even. Energy cannot be destroyed, it can only change forms.

      Your battery charge is never really lost. Neither is the heat of your mug of coffee. It just changed form or went somewhere else. The combined energy of all types that is residing inside a system is called internal energy.

      Heat and work

      Let's say we have a system, like a room. And all windows and doors are closed, so no energy can leave. In this system, you have a running table fan connected to a power line, getting energy from outside the system. The table fan is making you feel cool. Is the fan cooling down the room, heating up the room, or doing nothing? Think about it for a moment.


      The first thought of many would be to think that this fan would cool the room down, it sure makes you feel cooler! But it's actually heating up the room. As we remember, internal energy is the energy inside a system (room, in this case). The fan is getting energy from outside, and uses this energy to perform work. The fan accelerates the air inside the room, and this accelerated air will evaporate some of your sweat, so you feel cool. But as we remember, energy cannot be destroyed. So we are importing energy into the system, increasing the internal energy. Some of the work from the fan is also directly converted to heat, since the motor of the fan will get hot.

      So if we are not getting rid of any of this excess energy, we are increasing the internal energy. And therefore actively increasing the temperature of the room.


      To use a more tangible example: Simplified, this phenomena is why green house gases are bad. Lets define earth as a system. Earth gets a lot of energy from the sun. And a lot of this energy will be reflected and sent back to space. Green house gases will reflect back some of this energy trying to leave earth. So instead of having a roughly equal amount of energy enter the system (from the sun, from us doing stuff, etc) that leaves out in space, we have an increasing amount of energy on earth. This, as a consequence, increases temperature.


      Now, what are the maybe not so obvious implications of this?

      Waste heat, from supplied energy or inefficient work is a constant headache in engineering. If we cannot remove enough heat, we will actively heat up objects until they are destroyed. Thats why good cooling systems are important in cars, computers, etc.

      Whats next?

      Now this was not so bad. In the future we will cover phase changes, equilibriums, entropy, the heat death of the universe and briefly touch upon engines. So thats most likely two more parts after this. After that @wanda-seldon will take over again.

      I plan on doing one main part per week, but if something is asked that warrants a small topic I might do smaller ones inbetween.


      Something unclear? Got questions? Got feedback? Or requests of topics to cover? Leave a comment.

      19 votes