Modelization and analysis of the electric arc in low voltage circuit breakers

246 p.

Das Problem „Siebkrabben“ in der Garnelenfischerei

Along with consumption shrimp, brown shrimp fisheries also land some shrimp too small for human consumption which are sieved out on land and have to be degenerated (“crushed shrimp”). This share is recorded in German official statistics besides a third fraction called “industrial shrimp”. In view of the MSC certification processes, sustainable fishing and the fishermen’s voluntary obligation to - in future - limit the shares of “crushed shrimp”, landings data from 2010 have been analysed. Depending on seasons the goal of less than 20% of “crushed shrimp” in German landings was not met in several months in 2010, especially in August, when more than 40% of the landings contained more than the proposed amount. The processing procedures aboard the vessels are shortly discussed and the proposal is made to rather increase net selectivity than use wider sieves for the cooked fraction before landing. Cooking and processing small and later rejected shrimp is an uneconomic action. Reducing the share of “crushed shrimp” serves the idea of sustainable use of shrimp stocks as well as of sound economics.

Addressing students' difficulties with Faraday's law: A guided problem solving approach

In traditional teaching, the fundamental concepts of electromagnetic induction are usually quickly analyzed, spending most of the time solving problems in a more or less rote manner. However, physics education research has shown that the fundamental concepts of the electromagnetic induction theory are barely understood by students. This article proposes an interactive teaching sequence introducing the topic of electromagnetic induction. The sequence has been designed based on contributions from physics education research. Particular attention is paid to the relationship between experimental findings (macroscopic level) and theoretical interpretation (microscopic level). An example of the activities that have been designed will also be presented, describing the implementation context and the corresponding findings. Since implementing the sequence, a considerable number of students have a more satisfactory grasp of the electromagnetic induction explicative model. However, difficulties are manifested in aspects that require a multilevel explanation, referring to deep structures where the system description is better defined.