Thermodynamics

The statement concerning the 2nd law applying to a closed system must be seen in the light of the original message to which I answered. My response implied that a closed system cannot lower its entropy.

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Mihkel

Although i still have doubts i am pretty happy with your answers. My whole purpose with this discussion was not to prove anyone's ideas wrong with my own, but to hear a professional opinion regarding the subject. Most of what you guys have said does make sense now, (thanks for being patient) although i am not totally convinced, my curiousity on the 2nd law has been satisfied. We can move on from the second law.Now onto the Law of Energy Conservation. Assuming my definition of the law is correct (energy can only be converted, not created or destroyed) how did the universe create itself?

You have the 1st Law stated correctly but before I give you an answer tell me why you think the 1st Law applies or even exists?

This is a general question to anybody.How to you state the Second Law of Thermodynamics without reference to a closed system? That's just the way I've always learned it, and I can't think of any other way to state it.If "net entropy cannot decrease in closed system" is a valid way to state the Second Law, then would it by definition not apply to an open system?

Im a little confused by your question. Do you mean what purpose does the law have for the universe?

Here are some of the standard expressions of the second law of thermodynamics, taken from the hyperphysics website. These statments are equivalent, in that each form can be derived from the others.

• Heat will not flow spontaneously from a cold object to a hot object. (The Clausius statement of the law.)
• Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. (This refers to "closed system", with the notion of being free of external influences.)
• You cannot create a heat engine which extracts heat and converts it all to useful work. (The Kelvin-Planck statement of the law.)
• There is a thermal bottleneck which contrains devices which convert stored energy to heat and then use the heat to accomplish work. For a given mechanical efficiency of the devices, a machine which includes the conversion to heat as one of the steps will be inherently less efficient than one which is purely mechanical.

The Clausius statement of the law does not refer to "closed systems", but to "spontaneous" processes. The Kelvin-Planck statement refers to heat engine efficiency without making reference to closed systems.Cheers -- Sylas(If I have the above wrong, I'm sure there is a physicist in the house to correct me.)

I understand much better now. Thanks.

I can think of two rhetorical responses:1) What makes you think that a law that operates within the universe applies to the universe?2) What is the total net energy content of the universe? How do you know its more or less than zero?

"how did the universe create itself?" I will answer with a question: How did God create himself?
*edit to add quote.This message has been edited by 1.61803, 02-17-2005 12:19 AM

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"One is punished most for ones virtues" Fredrick Neitzche

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What makes you think that a law that operates within the universe applies to the universe?

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To assume that the whole must share the characteristics of its constituent parts is a fallacy, but I can't remember the name of. I don't think it's quite the same as "Hasty Generalization".This is also the fatal weakness of the Argument of Contingency: just because everything in the universe has a cause (which may not even be true) it doesn't follow that the universe as a whole must have a cause.

Isn't that a fallacy of composition, that because parts have a particular makeup the whole must also?

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Asgara
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I cant answer that. Ill have to ask a creationist some day. But while im in a forum full of evolutionists i may as well question you guys first.

It may not necessarily apply to the universe itself. But then where did the energy come from that is present WITHIN the universe?Or is the universe itself the energy?(And about your second question, i would have thought there must have been more than zero right? I mean, we do use this energy surely???)

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But then where did the energy come from that is present WITHIN the universe?

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The best analogy that I know of is addition.We all know that 0=0. However, 1-1=0 as well, as does 2-2, 3-3, and 1000000000-1000000000. The yin and yang of the universe is energy and gravity. Current theories state that gravity is actually negative energy. Therefore, the net energy of the universe may actually be zero.

Potential energy is represented as a negative. For example, what is the energy in a body in a circular orbit?Let M and m be the two masses involved, and R be the orbital radius. The formula for the gravitational potential energy of this system is -GMm/R. Note the minus sign. The kinetic energy of motion of GMm/2R; half the magnitude and the opposite sign.The net energy of the system is therefore negative, and can be calculated as -GMm/2RThere is a speculation that the total energy of the universe is zero; although accelerating expansion seems to indicate that this speculation is false; and that there is a net positive energy. Where did it come from? We don't really know; physics has not solved ultimate origins.Cheers -- SylasThis message has been edited by Sylas, 02-17-2005 21:31 AM