A “Perpetual Motion Machine” Powered by Electromagnetism

• Autores: Hollis Williams
• Localización: The Physics Teacher, ISSN 0031-921X, Vol. 62, Nº. 1, 2024, págs. 47-49
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • “Perpetual motion” is a hypothetical type of motion that continues forever without any external energy input contributing to the system. Students should know that this is generally impossible because of energy losses due to friction or other nonconservative forces, or because some assumption has been made that violates the first or second law of thermodynamics (or both). The first law states that the total energy of an isolated system remains constant over time, and one of the most common formulations of the second law states that the entropy of an isolated system over time cannot decrease. Note that the laws apply to an isolated system (i.e., one that cannot exchange mass or energy with its surroundings), whereas a closed system can still exchange energy even if it cannot exchange mass.1 There have historically been many purported perpetual motion machines.2,3 A close analysis of the machine invariably finds either that there is some assumption that violates the laws of thermodynamics or that there is some input from an external energy source, which means that the system is not isolated.4,5 In this article, we will consider an amusing example of such a machine, in which a ball starts on a platform, falls through a hole, and slides down a track formed from two side-by-side stainless steel rods.6 I explain how the machine can be used in class to illustrate physical principles, outline the possible mechanisms, and end with an explanation of the correct mechanism.