Content uploaded by James R. Hofmann
Author content
All content in this area was uploaded by James R. Hofmann
Content may be subject to copyright.
Ampere, Electrodynamics, and Experimental Evidence
... Se sabía que una forma de magnetizar una barra de acero consistía en rodearla por medio de un cable en forma de solenoide.Cuando por el cable circulaba corriente, la barra quedaba magnetizada. Ampère formula la hipótesis siguiente: la corriente en la bobina provoca por "arrastre" corrientes en el mismo sentido sobre la superficie del acero y a la vez estas corrientes provocan en forma análoga otras corrientes en el interior del material (cf.Hofmann 1987).Posteriormente, Ampère junto con Fresnel intentan invertir este fenómeno: es decir, tratan de producir una corriente en un cable helicoidal enrollado alrededor de un imán permanente. Es decir, esperaban que el magnetismo del imán provocara una corriente que circularía por el cable enrollado alrededor de él. ...
... ¿Por qué? SegúnHofmann (1987), en este caso fueron los comentarios de Fresnel los que pudieron incentivar a Ampère:i. Si en el caso de la magnetización es la corriente en las espiras de la bobina la que provoca corrientes macroscópicas coaxiales en la superficie del acero, entonces la corriente en un cable conductor recto debería provocar una corriente en un hilo de acero adyacente al cable.ii. ...
... Según la hipótesis de las corrientes moleculares, si por un cable conductor recto circula una corriente, en un hilo de acero adyacente habrá corriente alrededor de cada una de sus partículas en el plano determinado por el cable conductor y el hilo de acero. En este caso no se observará ninguno de los efectos típicos de la corriente eléctrica −como por ejemplo, la descomposición del agua− si se intenta buscarlos en los extremos del hilo de acero.Por lo tanto, podemos suponer, como lo haceHofmann (1987), que Ampère diseña el famoso experimento de Ginebra de 1821-1822 incentivado por las sugerencias de Fresnel. Recordemos que el experimento fue realizado dos veces: la primera en 1821 y la segunda en 1822. ...
... Algunas de las dificultades que presentan los estudiantes, están relacionadas con los obstáculos que se debieron de superar para el descubrimiento de la inducción electromagnética. como ejemplo podemos mencionar que André-Marie Ampère diseño un programa de investigación que tenía como objetivo detectar corrientes que suponía se iban a inducir en un circuito secundario a partir de una corriente estacionaria introducida en un circuito primario (Hofmann 1987). En las investigaciones descritas aparecen concepciones alternativas de los estudiantes en las que consideran erróneamente que ENSEÑANZA DE LAS CIENCIAS, NÚM. ...
Electromagnetic Induction (EI) theory is fundamental in the physics programme for Secondary school (16-18 years old) and introductory University courses. It is not surprising that this theory and Faraday's law have their own chapter in different Spanish and international curriculums. However, although Lorentz's Force and Faraday's Law have been common topics in EI teaching for many years, work done to look at the learning achieved by students on this topic is relatively recent. This article examines different projects on alternative conceptions for students in the top years of Secondary school and at University regarding EI and Faraday's law. In accordance with our analysis, usual teaching produces little learning on scientific theory's key ideas for EI in classic physics. In general, the different studies analysed show that most students do not understand a model for EI or Faraday's law. A significant number of students do not explain the phenomena but describe them or resort to memorised knowledge presented incoherently. Another significant number of students did not know the causes associated with EI, attributing them to the stationary magnetic field or electric current.
... Many recent publications deal with Ampère's work and his force law [103,114,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157]. Further papers are quoted in these books and works. ...
... The term EE had been previously used by other scholars in the historico-philosophical literature concerning scientific experimentation, see, e.g.,Hofmann (1987);Gooding (1989);Sargent (1995). ...
In this paper, I propose an account that accommodates the possibility of experimentation’s being exploratory in cases where the procedures necessary to plan and perform an experiment are dependent on the theoretical accounts of the phenomena under investigation. The present account suggests that experimental exploration requires the implementation of an exploratory procedure that serves to extend the range of possible outcomes of an experiment, thereby enabling it to pursue its objectives. Furthermore, I argue that the present account subsumes the notion of exploratory experimentation, which is often attributed in the relevant literature to the works of Friedrich Steinle and Richard Burian, as a particular type of experimental exploration carried out in the special cases where no well-formed theoretical framework of the phenomena under investigation (yet) exists. I illustrate the present account in the context of the ATLAS experiment at CERN’s Large Hadron Collider, where the long-sought Higgs boson has been discovered in 2012. I argue that the data selection procedure carried out in the ATLAS experiment illustrates an exploratory procedure in the sense suggested by the present account. I point out that this particular data selection procedure is theory-laden in that its implementation is crucially dependent on the theoretical models of high energy particle physics which the ATLAS experiment is aimed to test. However, I argue that the foregoing procedure is not driven by the above-mentioned theoretical models, but rather by a particular data selection strategy. I conclude that the ATLAS experiment illustrates that, contrary to what previous studies have suggested, there are cases of experimentation in which exploration serves to test theoretical predictions and that theory-ladenness plays an essential role in experimentation’s being exploratory.
In the present work, anomalous distortions occurring in the current-voltage characteristic of perovskite solar cells (PSCs), usually called J-V curve hysteresis, are studied by several methods. This includes dynamic direct current (DC) mode J-V experiments and impedance spectroscopy (IS) analyses in dark and under illumination. Initially, the CH3NH3PbI3 absorber material is characterized by alternating current (AC) and transient techniques showing ionic-related features. Subsequently, dark J-V curves of PSCs measured under different conditions are shown to exhibit capacitive hysteretic currents. This is related with low frequency excess capacitance in the dark IS spectra. These two features are correlated with the response of mobile ions in space charge regions close to the interfaces. The ion-related low frequency capacitance is shown to hinder the evaluation of deep trap and shallow doping concentrations from IS analyses as a function of temperature and DC bias, i.e. TAS and Mott-Schottky analysis, respectively. The hysteresis of the J-V curve under illumination was checked at faster scan rates after pre-bias in different device structures. The results were simulated by drift diffusion methods, suggesting that the formation of ionic dipoles can create large hysteresis. The light IS analyses at open-circuit allowed to identify different recombination mechanisms via ideality factor parametrization and revealed different exponential trends for the low-frequency capacitance. The low frequency capacitance was also studied at short-circuit under light and forward bias in the dark. The large values of capacitance in the sub-Hz regime were explained in terms of mobile ions space charges and chemical capacitances assuming a proportionality between the number of ionized/activated mobile ions and the concentration of charge carriers and photon fluence. Finally, a new method of characterization of photo-sensitive devices was introduced, named light intensity modulated impedance spectroscopy (LIMIS). This is based on the evaluation of photo-impedance from both, the individual photovoltage and photocurrent signals, under small AC light perturbation at DC open circuit. The impedance difference between IS and LIMIS informs on recombination velocity in traditional photovoltaics. Preliminary measurements of LIMIS in PSCs reveal significant impedance differences as light intensity increases and provide improved measurements of charge carrier lifetimes.
German translation by H. Härtel of the book The Electric Force of a Current: Weber and the Surface Charges of Resistive Conductors Carrying Steady Currents (Apeiron, Montreal, 2007).
Well before 1790 Paris had become the social and intellectual center for scientific life in Europe. It remained at the center until after 1830. Because this era in the scientific life of France has been seen as the source of modern physics, we need to examine the workings of Parisian scientific institutions and the practices of mathematicians and experimental physicists. What, precisely, did this band of intensely competitive men change in their mathematical and physical heritage from the eighteenth century? After examining the social and political structures of scientific Paris and their workings, we will turn to a series of problems and prize-essay questions of the era. In France the solutions to these problems were the occasions for fierce contests over the practices and future of both physics and mathematics. The solutions also disclose what was accomplished in this era in terms of changing the relationships between mathematics and experimental physics. The mathematization of electrostatics by Poisson, and Fourier’s work on the conduction of heat through solids, will help us distinguish the technical mathematician of the early nineteenth century from the theoretical physicist of a later era. Similarly, the development of the wave theory of light and subsequent work in France on elasticity will separate mathematicians from experimentalists and reveal the changes in physics by 1830.
John Herschel's discussion of hypotheses in the Preliminary Discourse on Natural Philosophy has generated questions concerning his commitment to the principle that hypothetical speculation is legitimate only if warranted by inductive evidence. While Herschel explicitly articulates an inductivist philosophy of science, he also asserts that "it matters little how [a hypothesis or theory] has been originally framed" when it can withstand extensive testing and empirical scrutiny. This evidence has convinced some that Herschel endorses an early form of hypothetico-deductivism. I aim to clarify this interpretive puzzle and adduce evidence in support of the inductivist interpretation of Herschel's philosophy of science by examining his published account of a series of experiments in the domain of electromagnetism.
Este livro apresenta o modelo planetário para o átomo desenvolvido por Wilhelm Weber (1804-1891) na segunda metade do século XIX, antes do modelo de Bohr. Ele é baseado na força eletrodinâmica de Weber de 1846 que depende da distância entre as cargas que estão interagindo, da velocidade relativa entre elas, assim como da aceleração relativa entre elas. Weber mostrou que duas partículas eletrizadas com cargas de mesmo sinal, interagindo entre si, poderiam se comportar como se possuíssem massas inerciais negativas. Para que isto ocorresse elas teriam de estar aceleradas uma em relação à outra, além de estarem muito próximas, abaixo de uma certa distância crítica r_c. Quando esta condição era satisfeita, estas duas cargas de mesmo sinal iriam se atrair, em vez de se repelir como fazem usualmente. Ele previu então que os átomos poderiam ser compostos de partículas negativas descrevendo órbitas elípticas ao redor de um núcleo positivo, sendo atraídas pelo núcleo, enquanto que as partículas positivas compondo o núcleo também iriam se atrair devido às suas massas inerciais negativas. Enfatizamos aqui três aspectos notáveis de seu trabalho:
1) A previsão de Weber foi feita antes da descoberta do elétron (1897) e também antes das experiências de espalhamento de Rutherford (1911). Seu modelo também foi desenvolvido antes da descoberta das séries de Balmer (1897) e Paschen (1908) descrevendo as linhas espectrais de emissão de um átomo de hidrogênio. O modelo atômico de Bohr (1913), por outro lado, foi inventado de forma a ser compatível com estes dados experimentais. Enquanto o modelo de Bohr foi inventado com esta finalidade, o modelo de Weber representou uma previsão real de como os átomos deveriam ser compostos.
2) No modelo de Weber não são necessárias forças nucleares para estabilizar o núcleo eletrizado positivamente. Afinal de contas, as partículas positivas compondo o núcleo são mantidas unidas por forças puramente eletrodinâmicas. Na física moderna, por outro lado, os cientistas tiveram de postular a existência de forças nucleares já que não estavam mais cientes da existência da eletrodinâmica de Weber. Portanto, após ter sido estabelecida a existência de um núcleo atômico positivo, surgiu o problema de explicar a estabilidade deste núcleo já que havia a força repulsiva Coulombiana atuando entre suas partículas eletrizadas positivamente. Para explicar a estabilidade nuclear postularam então a existência de forças nucleares. Já com o modelo planetário de Weber temos uma unificação do eletromagnetismo com as forças nucleares, antes mesmo que estes dois ramos da física fossem separados. Mesmo a estabilidade do núcleo foi prevista e explicada apenas com a eletrodinâmica de Weber.
3) Quando Weber desenvolveu seu modelo entre as décadas de 1850 e 1870, o elétron e o pósitron ainda não eram conhecidos, já que estas partículas só foram descobertas em 1897 e 1932. Portanto, ele só conseguiu fazer cálculos algébricos qualitativos com relação ao valor de sua distância crítica r_c. Mas quando utilizamos, por exemplo, o valor conhecido atualmente da massa e da carga de dois pósitrons, calculando a distância crítica de Weber abaixo da qual estas duas partículas começam a se atrair, encontramos um número da ordem de grandeza de 10^{-15}m, ou seja, essencialmente o tamanho conhecido dos núcleos atômicos. Esta similaridade entre o valor de r_c de Weber para estas partículas fundamentais e o tamanho dos núcleos não deve ser uma coincidência. Acredito que a eletrodinâmica de Weber apresenta a essência da explicação correta não apenas da estabilidade nuclear, mas também uma justificativa para o tamanho medido destes núcleos.
RÉSUMÉ. — Dès ses premières recherches électrodynamiques, au cours du mois d'octobre 1820, Ampère recherche l'expression de la force s'exerçant entre deux éléments de courant. Ces premières tentatives, quelques semaines seulement après l'annonce de la découverte d'Œrsted à l'Académie des Sciences par Arago, sont révélées par un manuscrit inédit d'Ampère (appendice III). La rédaction de ce manuscrit peut être située dans la succession de ses découvertes expérimentales et de ses réflexions théoriques grâce à un texte peu connu (appendice I) où Ampère précise la chronologie des lectures successives qu'il fit à l'Académie entre le 18 septembre et le 13 novembre 1820. La genèse de cette formule apparaît ainsi beaucoup moins linéaire et « uniquement déduite de l'expérience » qu'il ne l'a prétendu six années plus tard dans sa Théorie mathématique des phénomènes électrodynamiques.
In 1832, after Michael Faraday had announced his discovery of
electromagnetic induction, Andre-Marie Ampère claimed that he had
actually discovered the induction of one current by another in 1822. In
fact, he had, but did not really publish the fact at that time. This
article explores the reasons for Ampère's failure to lay claim to
a discovery that would have guaranteed him scientific immortality.