# 2nd law- entropy

#### icespeed

hi, im new and im dumb, so this is probably a really obviously answered question, but:

everyone knows the second law of thermodynamics, right? that is, entropy in a closed system can only be increased. if you take the universe as a closed system (a little like the ultimate closed system, i suppose) then the usual example is that entropy is increased by the release of radiation whenever something happens. like, the sun explodes and that increases entropy because the sun, a thing of high order, explodes into lots of radiation, which is a thing with low order.
er, did i explain that right? anyway, here's my question:
if the universe started out (post big bang)as a dense sea of energy/radiation (are they the same?) and that's usually said to be high order, which then clumps into low order- galaxies and stars and things- which then explode or implode into more radiation and gases- which is lower order... how is the radiation things explode into different from the radiation they're made from?

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#### marcus

Gold Member
Dearly Missed
Originally posted by icespeed
something happens. like, the sun explodes and that increases entropy because the sun, a thing of high order, explodes into lots of radiation, which is a thing with low order.

It looks to me like a reasonable question, not a dumb one at all.
You did not consider the *density* of the energy.

Maybe radiation is not intrinsically "low order" (which I guess means high entropy) but is often involved with an increase in entropy because it *spreads out* to fill the available space.

While a bit of matter just sitting by itself does not spread out.

If the concentration of it----the energy per unit volume----is the same (as with extremely high temperature radiation in early universe) then I do not see that radiation is inferior to matter on any "high order/low order" scale.

But I dont claim to be expert in thermodynamics! Someone else should give an authoritative reply. I think it is a good question.

#### Tyger

Thermodynamicaly speaking

the Universe doesn't act like a closed system because it is expanding. That keeps the entropy low. If it had stopped expanding 1,000 years after the big bang it would basically be a uniform mush of hot gas in a state of maximum entropy by now. Also the thermonucleur reactions in the stars provide it with fresh energy to keep the entropy low.

#### Alexander

Hold on, expansion IS an increase of entropy. What is entropy? Nothing more than a measure of spread of energy. And energy always spreads to all available states. Say, you have a system with 1 state. All energy is in that state. Now make 10 more states available, what happens then? Because states are indistinguashable, energy will simply spread to all 11 of them and a probability to find it in one state drops 11 times. Negative logarithm of this probability is what we label as "entropy", so entropy will increase.

1 kg of gas (or photons) in 1 m3 has less entropy than same 1 kg of gas (or photons) in 1 km3.

Same with the whole universe: more space for the same amount of energy means more entropy.

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#### Tyger

Yes but Alexander

it takes time for that spread to occur, it is not instantaneous. So that the average temperature on Earth can be 275o while the average temp. in the universe at large is more like 2.75o, and that is how the low entropy is maintained in our own neighborhood.

#### Alexander

Entropy is not temperature - these are different quantities.

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#### newton1

you 1st assume that the universe is a close system
this is not true....
and what you mean "how the low entropy is maintained in our own neighborhood."??

#### Alexander

why is universe open and to what?

#### Tyger

OK, let's make some analogies.

To get work out of a thermal system you need a temperature difference. Suppose you lived in a box. If something in that box was hot and everything else was cold you could get work out of the hot item, use it to run a steam engine or somesuch. If everything in the box was the same temperature you would not be able to do any work. That's the state of maximum entropy. How could you lower the entropy to get to do some work? One way would be to use one side of the box as a piston to expand the air in the box, cooling it. Then you would have a temperature difference you could exploit. The expansion of the Universe cools it so that the temperature of the background radiation is only a few degrees, allowing us to get work out of thermal systems on earth.

The notion of entropy was developed by the French engineer Sadi Carnot who was interested in designing the most efficient steam engines.

#### Hurkyl

Staff Emeritus
Gold Member
One way would be to use one side of the box as a piston to expand the air in the box, cooling it.
But everything is still the same temperature, just cooler than it was before...

#### Tyger

c'mon Hurkyl

You know better than that. The air in the box will cool down first, it will take time for the other items to tranfer their heat to the air. Time that you can use to run an engine. Don't confuse these folks, they're trying to understand.

#### Alexander

Yes. Also, I wonder if gas expanding in vacuum can cool at all. It should not.

Let's say, 100 fast moving atoms are let to expand in a vacuum, say into the volume 1000 times more than the original one (say, a small container inside of a big one has a hole). Now we have the SAME 100 atoms (but moving in 1000 times bigger volume). If each atom lost ANY portion of its kinetic energy, then total kinetic eneggy in the system is less than before when atoms were jammed in small volume.

But where does it (energy) go?

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#### Tyger

Why is this difficult?

If you take the air out of a room, what do you have? A vacuum, that's how you make a vacuum. If it weren't for the air you would be in a vacuum. However in the universe it isn't the air that is important, it's the background radiation, it determines the temperature. And it's the difference between that temperature and the temperature of your surroundings that keeps the heat from the Sun from continually building up, and the lets all the engines on the earth, the wind, water cycle, etc., that keep things going. Your very life depends on that background temperature being low.

And when you're letting out the piston the air is doing work against it, and that is why the air is cooling, energy is being taken out to push the piston.

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