First law of Thermodynamics animation [Mechanical engineering]

First law of Thermodynamics animation

The Second Law of Thermodynamics

For simple mechanical operations on macroscopic objects, the First Law, conservation of energy, is all we usually need to determine such things as how many joules of energy is required to lift a weight or to boil some water, how many grams of glucose you must metabolize in order to climb a hill, or how much fuel your car needs to drive a given distance.

But if you think about it, there are a number of "simple mechanical operations" that never occur, even though they would not violate energy conservation.

  • Suppose you drop a book onto a table top. The kinetic energy contained in the falling book is dispersed as thermal energy, slightly warming the book and the table top. According to the First Law, there is no reason why placing pre-warmed book on a warmed table top should not be able to propel the book back into the air. Similarly, why can't the energy imparted to the nail (and to the wood) by a hammer not pop the nail back out?
  • One might propose a scheme to propel a ship by means of a machine that takes in seawater, extracts part of its thermal energy which is used to rotate the propellor, and then tosses the resulting ice cubes overboard. As long as the work done to turn the propellor is no greater than the heat required to melt the ice, the First Law is satisfied.
  • Because motion of the air molecules is completely random, there is no reason why all of the molecules in one half of a room cannot suddenly "decide" to move into the other half, asphyxiating the unfortunate occupants of that side. (To the extent that air behaves as a perfect gas, this doesn't involve the First Law at all.)

What do all these scenarios that conform to the First Law but are neverthless never seen to occur have in common? In every case, energy becomes less spread out, less "diluted". In the first two examples, thermal energy (dispersed) gets concentrated into organized kinetic energy of a macroscopic object— a book, a propellor. In the third case, the thermal energy gets concentrated into a smaller volume as the gas contracts.

The second law of thermodynamics says in effect, that the extent to which any natural process can occur is limited by the dilution of thermal energy (increase in entropy) that accompanies it, and once the change has occurred, it can never be un-done without spreading even more energy around.

This is one of the most profound laws of nature, and should be a part of every educated person's world view. It is unfortunate that this law is so widely misrepresented as simply ordaining the increase in "disorder".



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FAQ

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What are the consequences of the first law of thermodynamics?

According to the first law of thermodynamics energy can neither be created nor be destroyed and any change in energy of a system is accounted for by work done.
The work-energy relationship is the foundation on which all engines and motors operate, including the human engine. Energy gets converted into other forms of energy or it gets used to do work.

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