Second law of thermodynamics....

W

Wiggy

Temporal Novice
I know that this law is empirical and that no proof for it exists. I also know that the web is full of papers that "real" journals refuse to publish that point out major paradoxes. One of my favourites (as it's relatively easy to reproduce) is here:

http://www.padrak.com/ine/SHEEHAN.html

But what I'm curious about is the fact that this law is alleged to have come about because somebody in the 19th century was confident that if it was possible to build a perpetual motion machine, they would have done so by now. It was therefore "no perpetual motion machine, thus the second law of thermodynamics", but later became "the second law of thermodynamics, thus no perpetual machine is possible".

Anybody else got information on this?
 
But what I'm curious about is the fact that this law is alleged to have come about because somebody in the 19th century was confident that if it was possible to build a perpetual motion machine, they would have done so by now. It was therefore "no perpetual motion machine, thus the second law of thermodynamics", but later became "the second law of thermodynamics, thus no perpetual machine is possible".
Click to expand...
I think you've been fed a story that may sound right, but is not based on the motivations for the second law. The 2nd law came about because people noticed that certain energy exchanges were unidirectional. The most classic example involves how we define heat and heat transfer: In the absence of any forcing effect, heat flows from the hi temperature to the low temperature only. Another example of unidirectional energy exchange processes is easier to visualize: Two different gases will mix until they reach uniformity, but they will not spontaneously un-mix without adding energy to the system. Your story of the perpetual motion machine was another observation, but it is a bit more strict than what you stated: You cannot build a machine that operates in a cycle that draws heat from a reservoir AND PRODUCES THE SAME AMOUNT OF WORK OUTPUT. (This would be called a perpetual motion machine of the second order...one of the first order is a machine that creates energy in violation of the 1st law of thermo.)

These last words are the key, for they set the stage for the formality of the 2nd law. For many years it was thought that heat and work were the same thing. By reading the 1st law of thermo, you might even infer that they are. But they are not. The 2nd law was developed to distinguish between the two, via entropy, and the resulting work required for the energy exchange itself.

Hope this helps,
RainmanTime
 
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