Reportamos el uso de la técnica fotoacústica resuelta en tiempo en el análisis de la evolución de... more Reportamos el uso de la técnica fotoacústica resuelta en tiempo en el análisis de la evolución de oxígeno fotosintético y el almacenamiento de energía en plantas verdes. Presentamos una aplicación de esta técnica al monitoreo de la fotosíntesis en la maleza acuática llamada comúnmente lirio acuático o jacinto de agua (Eichhornia Crassipes).Palabras Clave: Fotosíntesis; técnica fotoacústica; lirio acuáticoWe reported about the use of the time resolved photoacoustic technique for the analysis of the photosynthetic oxygen evolution and the energy storage in green plants. We present an application of this technique to the study of the photosynthesis in the aquatic vegetation, usually called aquatic lirium or water hyacinth (Eichhornia Crassipes). Keywords: Photosynthesis, photoacoustic technique, water hyacinth.
Starting from a brief review on the concepts of electrical resistance and impedance and the defin... more Starting from a brief review on the concepts of electrical resistance and impedance and the definition of thermal resistance, it is shown how thermal impedance can be defined from a simple situation of an ideal semi-infinite solid sample subjected to uniform sinusoidal heating at its surface. Using the thermal impedance definition, the non-stationary component of the temperature field perpendicular to the surface of the sample can be rewritten in a more compact form than as usual. It is shown that a dimensionless number dependent on thermal impedance can be defined for non-stationary phenomena to compare the efficiency between the different modes of heat transfer. The thermal impedance concept for the little-known for most people (but not least important) case of non-Fourier heat conduction is also discussed here. We show that the thermal impedance for diffusive Fourier heat transfer is a complex valued parameter with the same real and imaginary parts. However, this behavior is affected when approaching non-Fourier conditions, under which the imaginary component can become a pure imaginary parameter. These topics are not treated in heat transfer and physics textbooks. However, the subject can be of interest not only for students and teachers at undergraduate and graduate levels but also for scientists dealing with heat transfer in the presence of time-varying periodical heat sources.
ABSTRACTIn this work we report about the design and construction of a simple and cheap calorimete... more ABSTRACTIn this work we report about the design and construction of a simple and cheap calorimeter for phase transitions monitoring using Peltier elements and based in the well known inverse (front) photopyroelectric method for thermophysical characterization of materials. We describe its application for the detection of phase transitions in chocolate samples, as an alternative, for example, to the most widely used and more expensive Differential Scanning Calorimetry technique. The manufacture of chocolate requires an understanding of the chemistry and the physical properties of the product. Thus the involved problems during the confection process are those of the so-called materials science. Among them, those related with tempering are of particular importance. Because the fats in cocoa butter experience the so-called polymorphous crystallization, the primary purpose of tempering is to assure that only the best form is present in the final product. One way to characterize this is b...
We show that the front photoacoustic signal due to a sample consisting of a glass plate with a me... more We show that the front photoacoustic signal due to a sample consisting of a glass plate with a metal coated surface, at which thermal waves are generated by periodical light absorption, enhances for certain modulation frequencies when the other glass surface is covered with a liquid sample. This contradicts the intuitive expectation based on the assumption that the liquid provides a new channel for heat conduction thereby decreasing the substrate temperature. Experimental results are shown and the described effect is explained using a thermal wave interference model.
Sistema automatizado para la medición de la conductividad térmica de líquidos mediante el método ... more Sistema automatizado para la medición de la conductividad térmica de líquidos mediante el método del alambre caliente. Manakin: DSpace XMLUI Project v2. ...
ABSTRACT The implementation of an automated system based on the hot-wire technique is described f... more ABSTRACT The implementation of an automated system based on the hot-wire technique is described for the measurement of the thermal conductivity of liquids using equipment easily available in modern physics laboratories at high schools and universities (basically a precision current source and a voltage meter, a data acquisition card, a personal computer and a high purity platinum wire). The wire, which is immersed in the investigated sample, is heated by passing a constant electrical current through it, and its temperature evolution, ΔT, is measured as a function of time, t, for several values of the current. A straightforward methodology is then used for data processing in order to obtain the liquid thermal conductivity. The start point is the well known linear relationship between ΔT and ln(t) predicted for long heating times by a model based on a solution of the heat conduction equation for an infinite lineal heat source embedded in an infinite medium into which heat is conducted without convective and radiative heat losses. A criterion is used to verify that the selected linear region is the one that matches the conditions imposed by the theoretical model. As a consequence the method involves least-squares fits in linear, semi-logarithmic (semi-log) and log–log graphs, so that it becomes attractive not only to teach about heat transfer and thermal properties measurement techniques, but also as a good exercise for students of undergraduate courses of physics and engineering learning about these kinds of mathematical functional relationships between variables. The functionality of the experiment was demonstrated by measuring the thermal conductivity in samples of liquids with well known thermal properties.
Reportamos el uso de la técnica fotoacústica resuelta en tiempo en el análisis de la evolución de... more Reportamos el uso de la técnica fotoacústica resuelta en tiempo en el análisis de la evolución de oxígeno fotosintético y el almacenamiento de energía en plantas verdes. Presentamos una aplicación de esta técnica al monitoreo de la fotosíntesis en la maleza acuática llamada comúnmente lirio acuático o jacinto de agua (Eichhornia Crassipes).Palabras Clave: Fotosíntesis; técnica fotoacústica; lirio acuáticoWe reported about the use of the time resolved photoacoustic technique for the analysis of the photosynthetic oxygen evolution and the energy storage in green plants. We present an application of this technique to the study of the photosynthesis in the aquatic vegetation, usually called aquatic lirium or water hyacinth (Eichhornia Crassipes). Keywords: Photosynthesis, photoacoustic technique, water hyacinth.
Starting from a brief review on the concepts of electrical resistance and impedance and the defin... more Starting from a brief review on the concepts of electrical resistance and impedance and the definition of thermal resistance, it is shown how thermal impedance can be defined from a simple situation of an ideal semi-infinite solid sample subjected to uniform sinusoidal heating at its surface. Using the thermal impedance definition, the non-stationary component of the temperature field perpendicular to the surface of the sample can be rewritten in a more compact form than as usual. It is shown that a dimensionless number dependent on thermal impedance can be defined for non-stationary phenomena to compare the efficiency between the different modes of heat transfer. The thermal impedance concept for the little-known for most people (but not least important) case of non-Fourier heat conduction is also discussed here. We show that the thermal impedance for diffusive Fourier heat transfer is a complex valued parameter with the same real and imaginary parts. However, this behavior is affected when approaching non-Fourier conditions, under which the imaginary component can become a pure imaginary parameter. These topics are not treated in heat transfer and physics textbooks. However, the subject can be of interest not only for students and teachers at undergraduate and graduate levels but also for scientists dealing with heat transfer in the presence of time-varying periodical heat sources.
ABSTRACTIn this work we report about the design and construction of a simple and cheap calorimete... more ABSTRACTIn this work we report about the design and construction of a simple and cheap calorimeter for phase transitions monitoring using Peltier elements and based in the well known inverse (front) photopyroelectric method for thermophysical characterization of materials. We describe its application for the detection of phase transitions in chocolate samples, as an alternative, for example, to the most widely used and more expensive Differential Scanning Calorimetry technique. The manufacture of chocolate requires an understanding of the chemistry and the physical properties of the product. Thus the involved problems during the confection process are those of the so-called materials science. Among them, those related with tempering are of particular importance. Because the fats in cocoa butter experience the so-called polymorphous crystallization, the primary purpose of tempering is to assure that only the best form is present in the final product. One way to characterize this is b...
We show that the front photoacoustic signal due to a sample consisting of a glass plate with a me... more We show that the front photoacoustic signal due to a sample consisting of a glass plate with a metal coated surface, at which thermal waves are generated by periodical light absorption, enhances for certain modulation frequencies when the other glass surface is covered with a liquid sample. This contradicts the intuitive expectation based on the assumption that the liquid provides a new channel for heat conduction thereby decreasing the substrate temperature. Experimental results are shown and the described effect is explained using a thermal wave interference model.
Sistema automatizado para la medición de la conductividad térmica de líquidos mediante el método ... more Sistema automatizado para la medición de la conductividad térmica de líquidos mediante el método del alambre caliente. Manakin: DSpace XMLUI Project v2. ...
ABSTRACT The implementation of an automated system based on the hot-wire technique is described f... more ABSTRACT The implementation of an automated system based on the hot-wire technique is described for the measurement of the thermal conductivity of liquids using equipment easily available in modern physics laboratories at high schools and universities (basically a precision current source and a voltage meter, a data acquisition card, a personal computer and a high purity platinum wire). The wire, which is immersed in the investigated sample, is heated by passing a constant electrical current through it, and its temperature evolution, ΔT, is measured as a function of time, t, for several values of the current. A straightforward methodology is then used for data processing in order to obtain the liquid thermal conductivity. The start point is the well known linear relationship between ΔT and ln(t) predicted for long heating times by a model based on a solution of the heat conduction equation for an infinite lineal heat source embedded in an infinite medium into which heat is conducted without convective and radiative heat losses. A criterion is used to verify that the selected linear region is the one that matches the conditions imposed by the theoretical model. As a consequence the method involves least-squares fits in linear, semi-logarithmic (semi-log) and log–log graphs, so that it becomes attractive not only to teach about heat transfer and thermal properties measurement techniques, but also as a good exercise for students of undergraduate courses of physics and engineering learning about these kinds of mathematical functional relationships between variables. The functionality of the experiment was demonstrated by measuring the thermal conductivity in samples of liquids with well known thermal properties.
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