Energy and entropy

This article is a synopsis of the material on energy and entropy from the book "The Battle of the Black Hole".

Energy is able to change its shape. Kinetic, potential, chemical, electrical, nuclear and thermal are just some of the forms of energy. It constantly changes from one form to another, but one thing is unchanged: the total sum over all forms of energy never changes.

Let's look at an example. Raising a massive object, for example, a stone to a hill. Before you start raising, you can refresh yourself with food. After raising the stone, release it and, under the influence of gravity, it rolls down. Consequently, chemical energy (food) is converted into potential energy, and then into kinetic energy. But what happens to kinetic energy when the stone rolls down and stops? It turns into heat, some of which has gone into the atmosphere, some into the ground. Full cycle: chemical => potential => kinetic => thermal.

Einstein showed that mass is energy. By stating this, he meant that every object contains latent energy that can be extracted by changing its mass. For example, a uranium nucleus spontaneously decays into a thorium and helium nucleus. Together, they weigh slightly less than the original uranium. This excess mass is converted into kinetic energy of thorium and helium nuclei, as well as several photons. When atoms slow down and photons are absorbed, the excess energy becomes heat.

Thermal energy is the most mysterious of all. Before the advent of the modern molecular theory of heat, physicists and chemists considered it to be a substance that behaves like a liquid. It was imagined that it flows from hot objects to cold ones, cooling hot ones and heating cold ones.

But heat is a form of energy. To illustrate this, let's do a thought experiment. If you shrink to the size of a molecule in hot water, you can see how water molecules randomly and quickly collide with each other. If the water starts to cool, the molecules will move more slowly. If cooled to the freezing point, the molecules combine to form a solid crystal. Even so, they hesitate. If all the energy is withdrawn, the molecules stop vibrating (if you do not take into account quantum fluctuations) and in this case the temperature reaches absolute zero Kelvin or minus 273.15 degrees Celsius.

The principle of conservation of energy during its transformations between different forms is called the first law of thermodynamics .

Entropy

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