Here is a wordy topic which also happens to be rich with
physics and foundational in almost every aspect of engineering. The 2nd law of thermodynamics states that,
you cannot build a device capable of extracting heat from something to do work
without having some residual useless heat output. Perhaps more simply stated, you cannot
convert a given amount of heat energy into exactly the same amount of
work. There will always be some
frictional type losses that reduce the total amount of energy available for work
in any energy conversion process.
These frictional losses play a critical role in something
known as entropy. Entropy is related to
energy in a similar way that temperature is related to heat. Entropy is a measure of disorder or
equivalently it is proportional to your lack of knowledge on the molecular
details inside a system. In fact, the
heat energy in a system is linearly related to both its temperature and entropy.
Another way to state the 2nd law of thermodynamics is to say
that some useless heat byproduct will always occur in any physical process. As this useless heat is linearly proportional
to the entropy, the entropy will always increase from any evolution of
events. As the universe is continually
undergoing various processes, the entropy of the universe is itself continually
increasing.
This does not mean that local order cannot be increased due
to work, it means that when local order is increased, the total disorder of all
systems must overall increase. Creating
carbon dioxide and water from combustion of fossil fuels is an example of
converting energy into both work and waste heat associated with entropy and
disorder.
The work done by running a diesel engine can increase order
in noticeable ways such as moving materials, running a generator to do
electrical work or operating some other piece of equipment. The initial order in the system present prior
to burning the oil is found in having the diesel fuel molecules concentrated in
one place and one configuration. When
the fuel is burned to do some work, the organic molecules which made up the oil
are all oxidized and converted primarily to CO2 and H2O which then diffuse into
the air in a chaotic fashion.
When a drop of red die is kept separate from a glass of
clear water, the system has a relatively high state of order. When the drop of die is placed into and mixed
with the water, the disorder increases as the two are no longer distinct in the
mix.
Even human work and
effort increases entropy and disorder even when we locally decrease disorder by
building and constructing ordered systems.
The disorder from oxidizing food and converting it in a mix to waste
products is always a process which overall increases total disorder.
The bottom line is that any useful work requires waste heat in
some form or another. This
observationally contradicts any perpetual motion machine. Even solar energy which is effectively a
continual energy supply requires maintenance for solar panels and wind turbines
due to various wear and tear on electrical and mechanical components as entropy
marches on.
