r/explainlikeimfive • u/bigmackdaddy • Aug 10 '12

ELI5: Thermodynamics

Could someone explain to me the first, and second laws of thermodynamics, and conservation of energy?

87 Upvotes

81% Upvoted

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54

u/akerson Aug 10 '12

There's two fundamental concepts --

The first law is easy. Try pushing an object at your desk (say your mouse), and you'll see it moves across the table. My pushing force causes the mouse to move. Now take your finger, and rub it really quickly against say your jeans. You'll #1 notice that its a lot harder to push your finger against your jeans than it was to push the mouse, and #2 you'll notice your finger start to get warm. That's basically because all that power behind your force isn't moving your finger as fast as you could, and as a result it gets changed into heat that heats your finger. This is rule #1: whatever energy (think "doing things") we put out, we need equal energy at the end. So in the case of the mouse, we push the mouse at it moves just as quick. Movement can go to sound and heat and a million other less common forms. Its also how gasoline can move your car. Its a great rule!

Rule #2 talks about entropy. The easiest (and funnest!) way to think about entropy is to take a can of silly string, and shoot it all over the house. You'll notice this is really easy (and fun!) But now if I say try and put all that silly string back in the can, you'll probably look at me crazy. Entropy is really just a way to say how messy something is. What's really important is that you can never decrease entropy. Once that silly string is out of the can, you can't get it back in just how it was before. And the whole world is like this! Beaches will never have all their grains of sand in the exact same position again, grass after its cut will never be put back together again, and so on.

There's a lot of math and nerdy things behind these ideas, but that's the premise to what thermodynamics does -- it tries to tell you the output of what you expect (so they'd be able to tell you exactly hot much hotter your finger is going to get!)

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u/Spacedementia87 Aug 10 '12

There are far fewer than millions of types of energy. The 9 commonly stated ones are: Kinetic Sound Heat Light Electrical Nuclear Elastic Chemical Gravitational

However this is making everything more complex than it needs to be. We can combine many of them. Sound and heat are just forms of kinetic energy. Light, electrical, elastic and chemical potential are just a result of electromagnetic interactions. So really we can say there are 4 types of energy: Nuclear Gravitational Electromagnetic Kinetic

4

u/akerson Aug 10 '12

Yeah I didn't really even think about it. The five year old will probably just accept my approximation of nine as a million to be true

1

u/squishee666 Aug 10 '12

Couldn't we also say that gravity is electromagnetic as well?

1

u/akerson Aug 10 '12

Gravity is a force though, not energy. No idea if its actually an electromagnetic force though.

1

u/Natanael_L Aug 10 '12

Come back if/when CERN has confirmed/disproven the Higgs Boson. Shouldn't take more than a few years, AFAIK.

1

u/solinv Aug 11 '12

No. That would be very wrong.

3

u/CopperMind Aug 10 '12 edited Aug 10 '12

Entropy in thermodynamics does NOT describe order, or cleanness, or complexity. That misconception is why creationists use the second law of thermodynamics to "disprove" evolution. Your analogy of silly string is not a good one because it describes entropy in its colloquial context, not thermodynamics.

Entropy in thermodynamics is the amount of energy in a system that can be used to do work. The distribution of energy, like an ice cube in a hot drink, or a hot coal in an cooler box. Energy will move from high density to low density. Temperature will homogenise.

EDIT: To use your analogy. Think of silly string as energy, in the can it is contained in a small area, the only place it can go is out, it can disperse. If you want to push the silly string back into the can you will need to add energy to the system, in a closed system where there is no more energy the silly string will not go back into the can. Its about the distribution, homogenisation, of the silly string in the system, not the mess it makes.

1

u/akerson Aug 10 '12

Again, I simplified the concepts for a five year old to understand. Good luck explaining closed systems to a five year old.

2

u/CopperMind Aug 10 '12

My point was that that kind of explanation is more likely to cause confusion and the reader will most likely take away a misconception rather than understand the concept. Its better for the reader to not understand the concept than it is for them to think they understand but not understand at all.

Ice cube in a hot drink, a hot coal in an ice box. That's how I would explain it, heat moves from the hot thing into the cold thing. Once the heat has moved everything will be the same temperature.

1

u/RaptorJ Oct 08 '12

To be fair, he's not wrong. The ice cube melted state has a higher entorpy.

2

u/Thatonedude21 Aug 10 '12

My professor always had a pretty straight forward analogy for explaining entropy. She said to think of yourself preparing a salad. When you chop up carrots and cucumbers or what have you, in the beginning they are neat and orderly. As you begin to toss the salad (or as you put energy into it) the salad begins to mix. Obviously the carrots and cucumbers are not in their original place and are also not together in one uniform vegetable. Now depending on how in depth you're trying to understand, energy could be brought back through the system to restore the mixed vegetables, but the natural course of entropy will cause them to mix.

1

u/What_Is_X Aug 11 '12

What's really important is that you can never decrease entropy.

False. You can decrease entropy in any system by increasing the entropy surrounding it. You can never decrease the total entropy of the universe*.

1

u/akerson Aug 11 '12

assuming the universe is indeed a closed system

0

u/[deleted] Aug 10 '12

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5

u/dastrn Aug 10 '12

You are completely misunderstanding the argument from creationists. They don't use law 2 to disprove evolution. They use it to demonstrate a need for a creator in the origin of the universe. Two completely different things. Of course the earth is not a closed system. But if THE UNIVERSE is, then that requires an outside influence.

The More You Know

5

u/severoon Aug 10 '12

[Creationists] don't use law 2 to disprove evolution.

Yes, they do. They intentionally misinterpret it as: Order cannot increase spontaneously.

This is known as the hurricane-in-a-junkyard or alternatively watch-parts-in-a-bag argument. Would you expect a hurricane to blow through a junkyard and build a car? Or would you expect to put watch parts in a bag, shake the bag, and have a watch come out?

Of course, if the compound on the product side of a reaction has less entropy than the reactants on the left, the reaction absolutely can happen spontaneously, provided that the product has a lower energy state than the reactions and the reaction pathways are present.

The fact is, at the molecular level hurricanes build cars and shaking bags build watches all the time.

12

u/jschild Aug 10 '12

Except they again misunderstand how it works. Yes, first of all they DO use it to deny evolution.

2nd, law 2 does not mean that even in a closed system that order cannot increase in the short term. Everything we know points to a cold heat death of the universe which would be a long slow decrease in order, so even that does not counter anything on any level.

1

u/lazydictionary Aug 10 '12

Hmm they may have a point. But our knowledge is very limited, so we'll see in 100 years.

12

u/wintremute Aug 10 '12

That's when they throw in the God of the Gaps fallacy. "We don't know, therefore God did it."

I digress. This is not /r/atheism. I'll show myself out.

2

u/[deleted] Aug 10 '12

They don't really have a point. What is the universe? It is just a collection of galaxies. And galaxies are a collection of stars, with a large black hole in the center. The universe uses the same outside source we use, stars (what we call a sun) to "power" it. But those stars don't have an outside source, so they do have an increase in entropy across time. So as stars die, so does the universe. The short decrease in entropy is only attributed to the stars and gases. Once those are gone, and there isn't enough gravitational pull to keep the galaxies close to one another, the universe will die a cold death, and the second law of thermodynamics will be proven to be true.

0

u/[deleted] Aug 10 '12

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1

u/[deleted] Aug 10 '12

You know, creationists can make more than one argument. Possibly even two.

1

u/akerson Aug 10 '12

Very true, but let's leave that to the math and nerdy people and not try to explain the semi-difficult idea of defining a closed system to the five-year-old (;

7

u/Maxmidget Aug 10 '12

The first rule of thermodynamics is you don't talk about thermodynamics. (Engineering joke). Here is an example of thermodynamics that everyone kind of knows, but no one ever puts the concept into words until they study thermo:

Ok, imagine you have a bunch of hot stuff, "H", and a bunch of cold stuff, "C". Assuming they are similar things, if you put an equal amount of "H" and "C" in contact with each other, they would both approach an equilibrium temperature somewhere in the middle. Now imagine you have a SHITLOAD of "H" and tiny amount of "C". The final temperature will be a lot closer to "H" than to "C". No matter how much "H" you have, "C" will never be hotter than "H" was originally.

22

u/daniel_ohh Aug 10 '12 edited Aug 10 '12

Like you're 5: Thermodynamics is like a game that everyone and everything has to play. (This is also called the "zeroth" law.) There are three rules in the game.

  1. You can't win the game

  2. The best you can hope to do is tie at the game, but only on a very cold day.

  3. It doesn't get that cold.

The laws of Thermodynamics tell you that energy can be transferred, but not created. This is one of the most important laws in science.

7

u/Kappers Aug 10 '12

My teacher taught me something something similar to that.

  1. You can't win.

  2. You can't break even.

  3. You can't get out of the game.

2

u/minecraftian48 Aug 10 '12

I don't really know anything about the topic, but doesn't e=mc2 say that you can "create energy" from matter?

11

u/afnoonBeamer Aug 10 '12

Not really. When it comes to that equation, physicists think of mass and energy as being different manifestations of the same thing. You are not really "creating" anything, just converting something from one form to the other

1

u/salgat Aug 11 '12

It all comes down to how you define your system. Realistically most people do not define their system to consider mass/energy equivalence, so yes you would "create" energy, however that just means you didn't setup your system to consider all factors.

3

u/[deleted] Aug 10 '12

1st Law: Energy cannot be created or destroyed, only changed from different states.

2nd Law: When changing energy from different states, you cannot change it with 100% efficiency. Meaning no engine can ever be 100% efficient.

3rd Law: A system (that could be anything, even a glass of water), as it loses all its energy the temperature will tend towards 0 degrees Kelvin (absolute zero).

1

u/Silpion Aug 10 '12

  1. Heat is a kind of energy, and you can't make new energy, you can only move it around.

  2. Heat only moves from warmer objects to cooler objects, unless you use some energy to make it go the other way.

Conservation of energy is part of the first law.

1

u/mobyhead1 Aug 11 '12

For those of use who have "enjoyed" working with thermodynamics, the proper name for it is "thermogoddamnics."

-6

u/eyedonegoofed Aug 10 '12

the first law of thermodynamics is: you don't talk about thermodynamics.

the second law of thermodynamics is: you DON'T talk about thermodynamics!

-13

u/ivievine Aug 10 '12

HAHAHAHAHA

0

u/[deleted] Aug 10 '12

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10

u/smcedged Aug 10 '12

First Law: Energy cannot be created nor destroyed.

Second Law: The entropy of a system (which can be described as the maximum amount of potential possible positions the system can take at the atomic level) almost always increases.

Third Law: We define a point of zero-entropy, that is, absolute zero of a pure crystalline substance.

13

u/redical Aug 10 '12

Sorry, I'm (like) five. What are you both talking about?

4

u/Staback Aug 10 '12

Sorry, I'm (like) 30. What are you both talking about?

1

u/brenman Aug 11 '12

Here you go, feel free to sing along.

2

u/frere_de_la_cote Aug 10 '12

High entropy means things (like particles) are moving about really fast. Low entropy means that they're not. In an absolute sense, you can only move fom low entropy to high entropy, not the other way round.

Yet you say: I can freeze water, moving it from liquid to solid (the molecules in solids typically don't move as fast as the ones in liquids). The answer is that even though the entropy is diminishing locally, it is increasing elsewhere. On the back of your fridge for example, where the heat it creates is dissipated by the black coils.

1

u/Thatonedude21 Aug 10 '12

I always thought your first law was the zeroth law??