Entropy

Thermodynamic Definition; (from Clasius statement of the second law of thermodynamics)

DS = Q/T

Thermodynamic Example 1- heat transfer (eg conduction)

Decrease in Entropy DS = Q/T = -100 kJ / 373 K = -268 J/K

Increase in Entropy DS = Q/T = -100 kJ / 283 K = 353 J/K

Total Entropy Change DSnet = -268 + 353 = 85 J/K

Note: This simple heat transfer example demonstrates that entropy is always increasing. Theoretically it has been proposed that this may be true for the entire universe, the implication of a theoretical "heat death" in the universe, where eventually everything in the universe becomes a constant temperature.

Thermodynamic Example 2 - Hot an cool air mix. Entropy increases .

• Implications: No work is gained in process, but work would be required to "un mix" the hot and cold air again. (If anyone can devise a method to separate hot and cold air without work please let me know right away. ;)

This type of thinking can be extended to processes like friction. With friction, mechanical energy is converted into heat. Entropy increases. This process is irreversible. (we cannot get mechanical energy directly from one heat source)

Philosophical Definition

Entropy is a measure of disorder (randomness)

Philosophical Example 1 - Mixing two colours of paint. Again it takes no work to mix the colours but it will take work to un mix them.

• Implications
  • the randomness of the universe is constantly increasing
  • so called "Times Arrow" - entropy increase can tell us in which way time goes. (eg Watch a movie of a something breaking backward. We can tell that time is in reverse since the broken object will go from a state of more randomness to less (decreasing entropy) which we know to be impossible.
• Apparent Exceptions
  • Formation of Living Organisms - It would appear that the "stuff" we are made of has gone from a more random state as the raw materials (food) to a more ordered state (us) which would violate the law of increasing entropy.

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