Long sequences of slidings of solid blocks on an inclined rough surface submitted to small controlled perturbations are examined and scaling relations are found for the time distribution of slidings between pairs of large events as well as after and before the largest events. These scaling laws are similar to the Omori law in seismology but the scaling exponents observed are different. Log-periodicity correction to the Omori scaling is also found. It is shown that the scaling behaviors are dependent on the angle that the incline forms with the horizontal.

Longitudinal magnetoresistance measurements in Co70.4Fe4.6Si15B10 are reported for applied magnetic fields H varying from 0 to ±15 Oe. Two regimes were observed: one which increases with H2 and another which decreases with M2 for higher fields. The maximum in the magnetoresistance is observed for H=2.4 Oe for samples annealed for 15 min at 573 K in a transverse magnetic field of 2 kOe. The measured variation in R(H) from H=0 to 2.4 was about 0.2%. The value of H=2.4 is nearly the same field where a field-induced transition in the magnetization was observed. For samples annealed in the absence of the magnetic field the H2 term is the only one observed. The present results can be accounted for by the rotational model for uniaxial transverse anisotropy and the results are also in agreement with the random anisotropy model used to discuss the irreversibility and the field-induced transition.

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Apresentamos um sistema experimental simples eútil para ensino e aprendizagem em cursos de Física experimental. Desenvolvemos um sistema experimental com a ajuda de um computador pessoal que funciona ao mesmo tempo como um gerador de função e um conversor A/D, o qual pode ser usado como um osciloscópio digital de dois canais e um analisador de espectro, permitindo em tempo real a automação na aquisição de dados. Estudamos a resposta em frequência de um circuito oscilador RLC a uma excitação do tipo onda quadrada, onde a saída deáudio do PC foi usada para produzir uma forma de onda quadrada com frequência sintonizável, e sua placa de som para detectar os sinais AC de resposta do circuito. Usamos o circuito RLC como um susceptômetro AC para medir susceptibilidades magnéticas de amostras ferromagnéticas. Os dados experimentais estão em acordo com a previsão teórica da análise de Fourier. Estudamos ainda um sistema não linear constituído por um circuito caótico RLD. Palavras-chave: Aquisição de dados, instrumentos virtuais, placa de som, ressonância. In this work we present a simple and useful experimental system for teaching and learning in graduate courses of experimental physics. We developed our experiment with the help of a personal computer that works at the same time as a function generator, an oscilloscope, a spectrum analyser and an analogical/digital converter (A/D). We have performed a frequency response test to demonstrate the performance of the referred system, where we used a PC to produce a square-wave form and its sound card to make data acquisition of an oscillator circuit. We have measured the magnetic properties of ferromagnetic samples by means of a RLC circuit that works as an AC susceptometer. We analyzed and adjusted the experimental data in agreement with theoretical results from a Fourier analysis. In adittion, we have also studied a nonlinear system composed of a RLD circuit.

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