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Thermodynamics

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Thermodynamics

Postby Miko » Wed Mar 09, 2005 10:34 pm

I'm trying to figure out a way to easily understand the second law of thermodynamics. Does anyone have any interesting analogies that may help? Thank you!!!
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Postby Jelanen » Fri Mar 11, 2005 3:05 am

Basically, the 2nd Law of Thermodynamics states that an isolated system will move from a state of order to a state of disorder over time. Take a solution of sucrose (mmm...sweetness) in a beaker. That sucrose in solution will never de-diffuse into one corner of the beaker, that would decrease the entropy of the system and without intervention, systems do not get more orderly.
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Postby mith » Fri Mar 11, 2005 5:31 am

A spilt cup of water does not spontaneously fly back into the cup.(ok not never but the chances are small enough to be a good parlor trick :D)
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
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Postby biostudent84 » Fri Mar 11, 2005 7:51 am

Jelanen wrote:Basically, the 2nd Law of Thermodynamics states that an isolated system will move from a state of order to a state of disorder over time. Take a solution of sucrose (mmm...sweetness) in a beaker. That sucrose in solution will never de-diffuse into one corner of the beaker, that would decrease the entropy of the system and without intervention, systems do not get more orderly.
-Jelanen


Note that the reason for this diffusion is that the system (glass of water) is because the more spread out the sucrose molecules are, the less energy they have. The less energy the molecules have, the more stable the molecules are. It would take some form of energy to keep all those molecules in one area of the glass.

Just a little pet peeve of mine, and I think it must be stated here.

Everything said above about entropy is absolutely true. However, think about the water you put the sucrose in. Why is that liquit not a gas? Silly question, right?

Not neccessarily. If only entropy is looked at, it is perfectly logical that water becomes a gas. If placed in a closed system, this could never happen. I say closed system because in the Planet's atmosphere, the glass of water would evaporate...not part of the theory, as evaporated water eventually rains down in liquid form again. Any matter has a higher entropy when in a gaseous form.

So why isn't everything a gas if it would follow the 2nd Law?

Well, the reason for this is that there is a force directly opposing Entropy. This is called enthalpy. Enthalpy is heat energy. And matter requires enthalpy to move from solid to liquid to gaseous [to plasma] state.

Just as Entropy needs to be high for there to be low energy in the system, Enthalpy needs to be low.

And since high Entropy usually means high Enthalpy, and Enthalpy tries to go down while Entropy tries to go up, there are two conflicting forces in every system.

Back to our glass of water.

Since Entropy and Enthalpy are constantly acting against each other, they eventually reach an equilibrium state. In water, this equilibrium state (at room temperature anyway), forces the water to move to liquid form. This is dependant on the water molecules' properties. Sucrose molecules are in solid form at room temperature when equilibrium between Entropy and Enthalpy are reached. Oxygen molecules are in gaseous state.

Thanks for letting me add that bit of information in.

*drinks the sugar-water*

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Postby biostudent84 » Fri Mar 11, 2005 7:57 am

As a side note, there are mathematical equations for determining the equilibrium. I'll post them here when I find that chemistry book.

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analogy

Postby 2810712 » Mon Mar 14, 2005 11:11 am

see this-
If you make some naughty children sit quetly in the class they will have a tendancy to change their state i.e. they will try to get out the class and play , get despersed i.e. get disordered. They will get disordered if they are nopt forced to sit there.
But if theey are in the playground and plaing some game, they will not tend to go and sit in the class unless forced to do so. The degree disorder is callled entropy.
But , entropy doesn't solely decide if something happens or not. It also depends on the energy released during that process. If The process is favoured by one of these factors , then their magnitude desides what happens .
Let energy releaased = force by teacher on students
Now, as students tend to go out and teacher tends to stop them , the one who is stronger will susceed .

hope this helps

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