6.3. Friction

Perhaps some readers will be surprised to find a section on friction on a web page that is very chemically oriented, because friction is often considered a mechanical problem.
Reibung ÜbersichtLook at the above picture series:

  • The prehistoric transition from primate to man as Homo sapiens is characterized by the mastery of fire. With the fire drill, it was possible to generate by friction the temperature required to ignite a fire. In contrast to all species of animals, these primates were able to consciously trigger a chemical reaction, to keep it under constant control and then to stop it when it was no longer needed:
    Developing chemical skills is linked to the transition from primates to human being.

  • The invention of the wheel facilitated the creation of large buildings by minimizing friction. Sliding friction was replaced by rolling friction. The construction of the pyramids is one of the seven wonders of the world in the classical antiquity.

  • When talking about friction in the context of physics, it is usually mentioned that electrostatic charges may occur. However, the fact that this charge represents an electron transfer and thus forms the bridge to the area of the oxidation chemistry is often suppressed. A glance at a charred section through a wooden slat, caused by a blunt saw blade, confirms the chemical aspect of this everyday sawing experiment.

  • The match optimizes the fire drill as a technical tool by minimizing the technical-chemical effort.

  • The modern wheel provides low friction on the axle through ball or roller bearings and at the same time by the chemical and physical design of the tread for a large stiction to protect the transported persons or goods.

The key to understanding all these phenomena is the relationship between the friction and the temperature as a quantum phenomenon. Therefore, we want to describe the friction phenomenon with the means of energy quantization and show that the thermal differences between static and rolling friction can be well understood with the shelf model.

Video with a soundtrack: Comparison of Sliding and Rolling Friction

6.3.1. Details on sliding friction

In the video we have focused on the field strength components that are parallel to the sliding surface (to the direction of motion). The following picture shows how you can imagine the field line course close to reality. This also leads to realistic ideas about the components of the fields. When two charge centers are orthogonal with respect to the friction surface, the component of the E field parallel to this surface is zero and it becomes a maximum in the middle between two such positions.For the quiescent particle, this results in a field strength wave, which acts like the alternating field strength of a photon. This is shown in the following image series in detail.

Zero crossing 1
Rollreibung1
Wave crest
Rollreibung2
Zero crossing 2
Rollreibung3
Wave trough
Rollreibung4
Zero crossing 3
Rollreibung5

From wave crest to wave trough, the direction of the field vector reverses in the opposite direction. The blue-drawn static particle undergoes an alternating electric field with approximately sinusoidal alternating field strength parallel to the sliding friction surface and the relative movement.

Reibung-Sinusfeld

Quantum thermodynamically, this works like stimulating photons: the particles in the 'resting' material change to other stationary states of higher thermal energy. This results in a different distribution of the thermal energy over the achievable energy states for the particles. The different energy distribution represents a different (higher) temperature with greater half-value energy.

To reduce friction, lubricants are used. These are predominantly non-polar and electrically non-conductive substances. The video makes this usage understandable: Due to their high dielectric constant, such substances weaken the electric fields of the components, which rub against each other and cause their temperature to increase. By non-polar lubricants thus results a lower temperature increase. If - as in the car engine - the lubricant is liquid, a pump can lead it away to the cooling trough. After release of thermal energy, it is recirculated at lower temperature.

6.3.2. Details on rolling friction

The following image series shows the rolling material passing a resting particle of the base and illustrates the main difference between rolling and sliding friction. Here, too, only the field strength vectors lying parallel to the relative movement, which lie directly on the friction surface, are considered.

before
Rollreibung1e
passing
Rollreibung2e
after
Rollreibung3e

It is easy to see that the parallel component does not completely reverse the direction. In the shown example the original vector rotates in the passage area of the particle marked by the red arrow by about 60° (from -30° to + 30°). Then there is a much smaller fluctuation for the parallel component, which also depends on where the passage is situated. A smaller proportion of the kinetic energy is converted into thermal energy, leaving the temperature virtually unchanged.
In addition, it should be noted that in the case of rolling friction, the transition from kinetic energy to thermal energy can always take place at a single point and this point shifts in a time-dependent manner. As a result, both surfaces can cool down again.

Relativbeweg.

In the sliding friction, however, both surfaces heat each other, because of course, the stationary plane relative to the moved surface performs as well a relative movement and thus stimulates the particles of the moving body. Thus, both bodies expose each other over a large area to a "radiation influence". The above sketch illustrates this phenomenon. If both bodies are made of the same material, the temperature change in both will be nearly the same. This is not to be expected for different material composition.