Jürgen W. Precker

Simple experimental verification of the relation between the band-gap energy and the energy of photons emitted by LEDs

The wavelength of the light emitted by a light-emitting diode (LED) is intimately related to the band-gap energy of the semiconductor from which the LED is made. We experimentally estimate the band-gap energies of several types of LEDs, and compare them with the energies of the emitted light, which ranges from infrared to white. In spite of essential simplifications of the theoretical model involved in the adopted experimental procedure, the relation between the band gap energy and the wavelength of the emitted light is clearly demonstrated. The experiment is easily carried out, and no sophisticated equipment is needed.

The Speed of Sound in Air: An At-Home Experiment

This paper describes a simple experiment that students can do at home to determine the speed of sound in air. A set of audio files with selected frequencies is first created and stored on the students computer. Resonances are obtained in a straight pipe partially inserted into a bucket of water. The length of the air-filled part of the pipe is changed by raising and lowering the pipe. Resonances are detected by ear. We present some typical results obtained by students at home. All are in good agreement with the expected value.

Measurement of specific heat and Newton's law of cooling: a refinement in the analysis of the experimental data

One of the techniques for the determination of the specific heat of solids and liquids use the Newtons law of cooling for the analysis of the experimental data instead of the usual calorimeter method. The success of this technique depends on the possibility in determining the temperatures of the system immediately before and after the internal heat transfer due to the immersion of the sample (whose specific heat one wants to measure) with small uncertainties in a container filled with hot water. In this paper a refinement of this technique is proposed using two curve fittings for the function that describes the law of cooling and small extrapolations to determine those temperatures, as well their uncertainties. This allows to determine not only the value of the specific heat, but also the uncertainty of this value by error propagation. The fitting of the function that describes the law of cooling (and not of polynomials) to the data allows the comparison of the parameters obtained from the experiment with those predicted by the theory, and there is good agreement. The application of this refinement to the determination of the specific heat of aluminium indicates that the procedure is good in spite of the use of low cost apparatus.

Influência da temperatura do ar de secagem no calor latente de vaporização de água em feijão macassar (Vigna unguiculata (L.) Walp.), variedade sempre-verde

In order to determine the energy needed to artificially dry an agricultural product the latent heat of vaporization of moisture in the product, H, must be known. Generally, the expressions for H reported in the literature are of the form H = h(T)f(M), where h(T) is the latent heat of vaporization of free water, and f(M) is a function of the equilibrium moisture content, M, which is a simplification. In this article, a more general expression for the latent heat of vaporization, namely H = g(M,T), is used to determine H for cowpea, always-green variety. For this purpose, a computer program was developed which automatically fits about 500 functions, with one or two independent variables, imbedded in its library to experimental data. The program uses nonlinear regression, and classifies the best functions according to the least reduced chi-squared. A set of executed statistical tests shows that the generalized expression for H used in this work produces better results of H for cowpea than other equations found in literature.

Calculation of the Uncertainty in the Determination of the Equilibrium Moisture Content of Pumpkin Seed Flour

Empiric equations for the determination of the equilibrium moisture content as a function of temperature and relative humidity of air are normally obtained from experimental data by curve fitting. Once the fitting parameters of a function for a given product are determined, the value of that function for specific values of temperature and humidity of air is obtained by substituting these values into the function, which represents an indirect measurement. Generally, the results of these modelings are stated without the uncertainties inherent in the measurement. This article proposes a method for the determination of these uncertainties. For that, a computer code was developed which calculates the error propagation in first order approximation from the obtained fitting parameters and their covariances. The code was used for the determination of the uncertainties of the equilibrium moisture content of pumpkin seed flour, both for adsorption and desorption. This procedure allows a report of the measurement in agreement with the rules established by international standardization organizations.

A low-cost method for measuring the specific heat of aluminium

One of the standard experiments in basic thermodynamics is the determination of the specific heat of solids and liquids using a water calorimeter. Recently, the measurement of the specific heat of aluminium without the use of a calorimeter has been proposed, where the cooling curves of the systems container/water and container/water/sample are analysed. Here we discuss an application of this method that simplifies Newtons law of cooling. In spite of the use of only low-cost instruments, this application yields consistent results in determining the specific heat of aluminium

A capacitância de um condensador com placas planas não paralelas

We analyze in this article three different methods to obtain the capacitance of a capacitor with not parallel plane plates: a solution by integration, a solution by limits and a proposed solution. In spite of the fact that each of the solutions involves different approximations, the three capacitances obtained are coherent.

Esfera em plano inclinado: conservação da energia mecânica e força de atrito

This paper proposes an experiment seeking to demonstrate that for the static frictional force there is conservation of energy. The experiment consists of a sphere rolling down in a rough inclined plane, for which the slope can be varied. For each given angle the mechanical energy in the beginning and at the end of the movement was determined. The analysis of the results showed that the conservation of the mechanical energy occurs up to a maximum angle, starting from the one which the sphere begins to slide. The determination of this angle by graphic inspection allowed the calculation of the coefficient of static friction between the bodies.