About Perpetual Machines

About Perpetual Machines

Were it not for the first law of thermodynamics, we could build perpetual machines of the first kind, as they are called, whose driving power comes from nowhere. Were it not for the second law of thermodynamics, we could build perpetual motion machines of the second kind, which would draw their energy from anywhere. For instance, a motor at room temperature could be driven by the air molecules that happen to collide with is piston. The impossibility of this is not immediately obvious, which is why it was not established until the 19th century and is still overlooked in many science fiction stories.

Numerous people who understand the second law as a principle governing heat engines can get bewildered about the wider applications. These involve entropy, about which science fiction has also perpetrated a great deal of nonsense. Actually entropy is a measurable quantity, though you need calculus to describe it mathematically. In any thermodynamic process where an amount of heat Q is exchanged at a temperature T (which may vary throughout the volume and during the time in which things happen), the increase of entropy is equal to the integral of dQ/T.

Now “increase” can be negative, that is, represent a decrease. When something occurs thermodynamically in a system, entropy can and often does decrease somewhere. However, it increases elsewhere, and the second law states that the total gain in entropy is always positive. That is, whenever a change involving an energy transfer takes place in a system, entropy always is greater at the end of the process than it was in the beginning.

A “system” can be anything: an atom, a molecule, a machine, a living organism, a galaxy, the cosmos as a whole, anything. But we must consider the entire system, not just a selected part of it.

An increase in entropy corresponds tp, or measures an increase in disorder, or a decrease in the orderliness of the system. Therefore, whenever something changes, we find there is less order afterward than there was before.

Here is a very rough example or analogy. Think of a house whose lady has brought it to absolutely perfect arrangement and cleanliness – not a single item of furniture out of place, not a speck of dirt or dust anywhere, Then her children come home from school and her husband home from the office, and the family starts using the place. Things happen in it The immaculate condition doesn't last long, does it?

True, next morning the lady can restore her dwelling to its former orderliness. However, to do so she must expend energy, both her own and the energy of whatever appliances she uses, such as a vacuum cleaner. That energy comes from the conversion of food in her body – or fuel in an electric generator somewhere – into disorganized gasses and masses. The house may become neat again, but the environment as a whole is more chaotic than it was.

This is not an argument against good housekeeping! It is simply a reminder that everything has its price.

All biological processes require entropy increase. Illustration by Elena.