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Learning the Twin Paradox for Freely-falling Observers

The “twin paradox” is often discussed in the introductory treatment of special relativity. Under “twin paradox” we understand the fact that if two twins start from the same place with synchronized clocks, traveling in an arbitrary way and then meet again at the same spacetime point, where they compare their clocks, in general, they find…

Relativistic Treatment of the DC Conducting Straight Wire

Introduction The direct-current-conducting infinitely long wire is often discussed in the context of relativistic electrodynamics. It is of course a completely academic discussion since for the typical household currents the drift velocity of the electrons in the wire, making up the conduction current, is tiny (of the order ##\mathcal{O}(1\,\text{mm}/\text{s})##!). Nevertheless, it is unfortunately only quite…

What is the Homopolar Generator: An Analytical Example

Introduction It is surprising that the homopolar generator, invented in one of Faraday’s ingenious experiments in 1831, still seems to create confusion in the teaching of classical electrodynamics. This is the more surprising as the problem of the “electromagnetism of moving bodies” has been solved more than 100 years ago by Einstein in his famous…

Explore Some Sins in Physics Didactics

Introduction There are many sins in physics didactics. Usually, they occur, because teachers, professors, textbook or popular-science-book writers, etc. try to simplify things more than possible without introducing errors in reasoning, or they copy old-fashioned methods of explaining an issue, leading to the necessity to “erase” from the students’ heads what was hammered in in…