The chemical potential is a quantity for which students hardly have an intuitive feeling in contrast to other intensive quantities like pressure or temperature. Some students may believe that this is not really an insufficiency because the chemical potential seems to be essentially a quantity for chemists. We will try to show that the chemical potential does not merit its reputation as a difficult to understand quantity. Not only is it easy to grasp, it is a particularly intelligible quantity for which even the layman can develop a feeling. Moreover, this quantity is not only important for chemists. It is just as useful for describing physical phenomena and processes, such as phase transitions, the stratification of gases in a gravitational field and electric currents in semiconductor junctions and nuclear reactions, to mention just a few.

It is customary to say that energy exists in different forms which are transformed or converted into one another during physical processes. However, a careful analysis shows that thinking in and speaking of energy forms is inappropriate and conceptually even misleading. Since most textbooks use the term ‘‘energy form’’ without spelling out a clear procedure by which different ‘‘forms’’ of energy can be categorized, rigorous criteria for categorizing flowing and stored energy are discussed in this paper. These criteria show that the term ‘‘energy form’’ for the respective categories is unsatisfactory because it easily leads to the misinterpretation that there are different kinds of energy, rather than emphasizing the simpler and physically more correct picture of energy as an unalterable substance. Taking into account the well-known but little recognized natural law that energy always flows simultaneously with at least one other physical quantity, the concept of energy carrier is introduced. This concept provides a clear picture of how energy is transported, exchanged, and stored. This picture is scientifically accurate, yet simple and easy to present even at an elementary level.

Introduction: The German PID-NET registry was founded in 2009, serving as the first national registry of patients with primary immunodeficiencies (PID) in Germany. It is part of the European Society for Immunodeficiencies (ESID) registry. The primary purpose of the registry is to gather data on the epidemiology, diagnostic delay, diagnosis, and treatment of PIDs. Methods: Clinical and laboratory data was collected from 2,453 patients from 36 German PID centres in an online registry. Data was analysed with the software Stata® and Excel. Results: The minimum prevalence of PID in Germany is 2.72 per 100,000 inhabitants. Among patients aged 1–25, there was a clear predominance of males. The median age of living patients ranged between 7 and 40 years, depending on the respective PID. Predominantly antibody disorders were the most prevalent group with 57% of all 2,453 PID patients (including 728 CVID patients). A gene defect was identified in 36% of patients. Familial cases were observed in 21% of patients. The age of onset for presenting symptoms ranged from birth to late adulthood (range 0–88 years). Presenting symptoms comprised infections (74%) and immune dysregulation (22%). Ninety-three patients were diagnosed without prior clinical symptoms. Regarding the general and clinical diagnostic delay, no PID had undergone a slight decrease within the last decade. However, both, SCID and hyper IgE- syndrome showed a substantial improvement in shortening the time between onset of symptoms and genetic diagnosis. Regarding treatment, 49% of all patients received immunoglobulin G (IgG) substitution (70%—subcutaneous; 29%—intravenous; 1%—unknown). Three-hundred patients underwent at least one hematopoietic stem cell transplantation (HSCT). Five patients had gene therapy. Conclusion: The German PID-NET registry is a precious tool for physicians, researchers, the pharmaceutical industry, politicians, and ultimately the patients, for whom the outcomes will eventually lead to a more timely diagnosis and better treatment.

A well-known collision experiment can be carried out with an arrangement of several elastic balls suspended in a horizontal row. As we have shown in a previous article a necessary condition for the observed, simple behavior of this arrangement during collision is that the perturbation propagates throughout the system without dispersion. In the present paper, we show that the arrangement can be described by a series of spatially separated masspoints and springs of a special type: the exponent of the force law of the springs is 1.5 according to a theory of H. Hertz. It follows that the first collision sequence of such an experiment is not completely dispersion free. Indeed, slight dispersion during the first collision sequence creates the conditions for the total absence of dispersion in all the subsequent collisions.

A well-known collision experiment can be carried out with an arrangement of several identical elastic balls each suspended by two threads and in contact with one another: a certain number of the balls is displaced from its equilibrium position and then released, so as to collide with the remaining balls at rest. After the collision, the same number of balls moves away to the other side as had initially been displaced. It is shown that, contrary to common belief, the conservation laws of energy and momentum alone are not sufficient to explain this behavior. Indeed, a further condition must be satisfied by the system of balls; namely, it must be capable of dispersion-free energy propagation.