Entropy: what is it?

The term entropy denotes, in classical thermodynamics, an extensive state function. In other words, a state function proportional to the amount of matter in the presence.

Entropy was introduced in 1865 by Rudolf Clausius. It is denoted S. In the international system, it is measured in joule by kelvin (JK^-1). It is always a difference in entropy which is measured, in a reversible transformation, as the quotient of the variation in the quantity of heat transferred to a system by the absolute temperature of this one: dS= dQ/T.

the third law of thermodynamics commands that the entropy be zero at absolute zero.

Entropy, a measure of disorder

Statistical thermodynamics makes it possible to specify that entropy characterizes the microscopic disorder of a system, its degree of disorganization. The entropy of a system therefore accounts for the degree of dispersion of theenergy (thermal, chemical, etc.) within the system itself. And according to the second law of thermodynamics, the energy of an isolated system tends to disperse as much as possible. Its entropy therefore also tends to increase.

Moreover, according to Boltzmann’s law, entropy is proportional to the natural logarithm of the number of microstates of a system: S = k . lnΩ. The state of equilibrium being the one which offers the most possibilities of microscopic realizations, the entropy is maximum at equilibrium.

The entropy of black holes

In 1970, John Wheeler, the physicist American inventor of the term “black hole” notes that if you throw your cup of coffee into a black hole, it will disappear forever, taking with it its entropy. Thus, the entropy of the universe will have decreased. This contradicts the second law of thermodynamics.

It was a few years later that stephen hawking and Jacob Bekenstein, one of the founders of black hole thermodynamics, propose that any black hole has an entropy proportional to its area. L’black hole entropy — which is colossal — tells us about the amount of information contained in the black hole in question.