There is such a mysterious and uncatchable quantity in physics. Which is not directly measurable and it doesn’t even have a single definition. Because it changes depending on the framework in which appears.
We’re talking about Entropy ( from ancient greek: inner transformation).
Although entropy is an elusive concept, it’s curiously ubiquitous, such as an invisible dark
halo that surrounds and cloaks our universe. So that it takes different appearances and
facets from time to time. If it didn’t exist, physics and our universe would be very different.
Entropy has appeared during the history of science, in different areas and moments,
independently: it was first defined by thermodynamic during the industrial revolution,
then it was described by theoretical physics, until the 19th century, when
it was introduced in the information theory and then extended almost to any other area of science.
NOBLE ENERGY USE LESS ENERGY.
In thermodynamics, as we’re about to see, entropy measures the tendency of the energy in the universe
to degrade and to spontaneously evolve towards messier and
unusable energy forms for human activities. As a matter of fact, energy sources are not
all the same: there are different types. As well as food, there are better sources of energy than
others. In nature, the more energy is concentrated and stored in ordered structures, the more is
exploitable. Kinetic energy, potential, chemical, nuclear, wind are all usable energy sources:
these are suitable to produce forces for generating movements and doing work.
Thermal energy, by contrast, is related to heat production and is considered unusable
energy because it cannot be converted into exploitable energy. We’ll clear up why later.
ENERGY IS CONSERVED, BUT ENTROPY DEGRADES IT During the first industrial revolution,
the invention of the steam machine had generated great enthusiasm,
and scientists and engineers of the time had been working hard, attempting to improve
the efficiency of thermal machines, to produce energy for industrial purposes.
All the efforts were aimed at realizing a dream: to build the perfect thermal machine,
a device capable to convert the whole absorbed heat into usable energy, in a perpetual motion,
with no loss.
A possibility was very suggestive and debated: building a boat capable to absorb
the heat in the sea and completely convert it into continuous propulsive energy. Physicists
and engineers soon clashed with difficulties and experimental limits, which turned out to
besides of the same coin: different aspects straight related to the concept of Entropy.
Hopes were nipped in the bud by British physicist William Thompson, also known as Lord Kelvin,
who claimed an experimental principle, based on his studies on heat.
This is now called the 2nd principle of thermodynamic:
“It’s impossible to produce a transformation in which the only
the result is to absorb an amount of heat and to completely convert it in work”
This statement was also experimented with years ago by Sadi Carnot. The French engineer,
was one of the first at the beginning of the 18th century,
to systematically study the working cycles of thermal machines.
He guessed that their efficiency should have a limit impossible to overcome. Because energy loss
could never be eliminated. Carnot understood that heat and temperature were strictly related.
He realized that it was precisely the variations in temperature that triggered the heat flows
that allowed a thermal machine to do work and produce energy. He was among the first to perceive
that the energy generated by a thermal machine, only depended on its working temperatures,
while was totally independent of its technical and design features and from the substance
used to perform the transformations (it could be a gas or another one fit for purpose).
In 1864 the German physicist Rudolf Clausius, starting from Carnot’s work,
claimed an asymmetry in the flow of heat: in nature, we observe that heat always flows
from a hotter body to a colder one, while the reversal process doesn’t happen spontaneously.
This seems obvious nowadays but there wasn’t any physical law that accounted for this.
Starting from this observation Clausius stated a principle,
which is today an alternative version of the 2nd principle of thermodynamic.
“It’s impossible to perform a transformation
in which the only result is to transfer heat from a colder body to a hotter body”.
Clausius understood that to do so, energy or work from the environment was needed:
otherwise, the transformation would never occur spontaneously.
Let’s think about our fridge: we realize that it can reverse the spontaneous flow of heat to cool
down the food we put into. However, to do so, it must necessarily be connected to a power
outlet and absorb energy, otherwise, the transformation would never occur spontaneously. And our fridge
would not be able to perform the refrigerator cycle that lets us preserve our food. Our fridge
is nothing more than an inverted thermal machine. Prototypes were designed right this time
Carnot, then Kelvin and Clausius, but also Joule, Clapeyron, and other great minds, independently,
realized that, whereas heat and work are energy forms strictly related,
they are not symmetrically replaceable each other.
In attempting to assess the energy loss within thermal machines,
Clausius wanted to define a quantity to measure how much energy degrades in heat, during a
transition. Thus he defined the variation of entropy of a system during a reversible process,
as the ratio between the heat exchanged by the system.