This blog aims to provide a conceptual understanding of physical and philosophical issues for cognitive and other scientists.
I will begin with a general topic, which interests me as neuroscientist – the concepts of energy and its different types. I will avoid the formulas but emphasize their physical content.
Energy is an abstract quantity that is used to describe interactions between different objects and processes. The advantage of the notion of energy is that it permits comparison of totally different processes (e.g. one can compare climbing a hill and boiling a can of water in terms of their energy requirements). Another important advantage of energy notion is that it permits linking totally different processes using the idea of energy transformation between them (e.g. to link energy in a piece of bread with energy needed for the muscles to lift an object).

Thus, energy permits obtaining a general idea of something in common between totally different processes or drawing a link between them without going into details of exact interactions and transformations, which are always quite complicated.
For example, one can generally say that energy of glucose is partly transformed to the energy of electric fields in neural cells. This statement is true even if we do not provide any details about the mechanisms of this transformation. Even if we know the principal steps of the transformation, we do not describe it precisely in terms of how atoms and electrons interact with each molecule and between the molecules. Thus, the notion of energy permits following the general mechanism without getting lost in complicated molecular and quantum details at each step.

Interactions between objects are usually due to the fact that they are in motion or they have a special position in space. When an object is in motion, it has kinetic energy. When it is in a certain position, which may potentially lead to motion, it has potential energy. For example, different parts of a complex molecule have a certain position with respect to each other that defines the potential energy of the molecule—the potential of its parts to move with respect to each other.
If we hold something in our hand without moving, this object has a potential to fall; thus, it has potential energy due to its position. If we release the object, it falls and acquires kinetic energy due to its movement. In this way, potential energy is transformed into kinetic energy. Importantly, as the two types of energy mutually transform, their sum remains unchanged. The same concerns, for example, gas molecules in a certain closed and isolated container—the total sum of potential and kinetic energies of molecules remains the same.
That is why the term internal energy was proposed, which is the sum of kinetic and potential energies of all molecules in the container under the given conditions. The way to change the internal energy of the container would be to change conditions, e.g. to heat it. Heating would increase internal energy of the container.
Without changing the surroundings, i.e. without putting energy into the system or taking energy from it, the internal energy of the isolated object does not change with time, but rather is conserved. This is known as energy conservation principle.
Energy can be used to do work; in this case, one can say that energy is converted into work. For example, if there is a piston in the container, heating the container will move the piston due to the molecules of the gas producing work on it. Part of the internal energy will be converted into work. In general, part of the amount of heat supplied to a closed system changes its internal energy, and another part is converted to work done by the system on its surroundings. This is the first law of thermodynamics, which is closely linked to the energy conservation principle: if no heat is supplied, the internal energy in a closed system is conserved.

However, most of the systems, including the brain, are not insulated; they dissipate heat in the environment. In this case, if we supply energy, part of it does work, part heats up the system, and part is just dissipated to the surroundings as heat.
It turns out that the more disordered the system is, the less its internal energy can be converted into work. Intuitively, this is because disordered motion is less effective to do work compared with ordered motion. To take this into
account, the free energy concept was introduced. Free energy is the internal energy minus the unusable energy related to the disordered motion of parts of the system. “Free” means it is freely available (though in reality we usually pay money for energy). At least, physically it is free to consume, no further work is needed to obtain it.
The entropy (measure of disorder) of a system multiplied by temperature mathematically defines the unusable energy. Thus, the free energy reflects the maximal part of internal energy the system can convert into work.
Enthropy is also a measure of…information. Intuitively, this is because there is zero information in total disorder. When we add some order to the system, we provide some information about our actions. For example, a book on the table means that somebody has put it there. Hence, there is a link between energy and information, which is very tempting to apply to the brain. This will be the topic of some other posts here.

Kuzma Strelnikov