Newton's third law is often misunderstood by students and even their professors, as has already been pointed out in the literature.1,2 Application of the law in the context of electromagnetism can be especially problematic, because the idea that the forces of “action” and “reaction” are equal and opposite independent of the medium through which they act can be muddied by the concept of “action at a distance.” While some experiments have been described3,4 illustrating Newton's third law in magnetic situations, these do not offer the student a clear way of evaluating his/her own preconceptions. The experiment we present shows how easily the student, and even the graduate student, can fail to apply the third law correctly in an electromagnetic situation. The experiment described here employs a magnetic levitator and shows the difficulty in recognizing action and reaction forces.
An electronic system for the conversion of a liquid level into voltage is shown. This allows the student to carry out basic hydrodynamic experiments where fast liquid level changes occur. The use of this system is illustrated with the classic draining tank experiment.
Charging a capacitor with a photovoltaic module is an experiment which reveals a lot about the modules characteristics. It is customary to represent these characteristics with an equivalent circuit whose elements represent its physical parameters. The behavior of a photovoltaic module is very similar to that of a single cell but the electric energy involved is much higher, which represents a major challenge for their characterization. In this work, an experiment is conducted to test out this behavior and in addition to obtain the module I-V characteristic curve. The experiment is simple and only common instrumentation is required.
In hydrostatics, pressure measurement with U-gauges and their relationship to density is a well-known experiment. Very little is studied or experimented with the dynamics of the movement of a liquid in a U-tube probably due to its theoretical complexity but, after all, it is a simple damped oscillating system. In this paper we present a relatively simple experiment that allows studying in some detail the dynamics of the movement of a liquid in a U-tube when an initial pressure gradient is applied. In order to record the information of the column displacement as a function of time we have developed a position sensor system based on a solar cell that allows the recording of the experiment using a simple data acquisition system.
A simple and economical system that allows one to obtain the characteristic I-V curve in solar cells is presented. It is a computer-controlled system suitable as an instrumentation project for students who are learning about photovoltaic systems. This system is based on the use of an R-3R ladder circuit as a variable resistive load for a solar cell and is controlled by a microcontroller system connected to a PC. The system allows us to generate the I-V curve and calculates the parameters I sc , V oc , P max .
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