<p>La ecuación de Bernoulli, que bajo ciertas condiciones relaciona la presión de un fluido ideal en movimiento con su velocidad y su altura, es un tema central en los cursos de Física General para estudiantes de Ciencias e Ingeniería. Frecuentemente, en los libros de texto utilizados en cursos universitarios, al igual que en diversos medios de divulgación, se suele extrapolar este principio para explicar situaciones en las que no es válido. Un ejemplo habitual es suponer que, en cualquier situación, mayor velocidad implica menor presión, conclusión correcta solo en algunas circunstancias. En este trabajo, reportamos los resultados de una investigación con estudiantes universitarios, sobre las concepciones alternativas presentes en dinámica de fluidos. Encontramos que muchos estudiantes, incluso después de haber transitado por los cursos de Física General, no han elaborado un modelo adecuado acerca de la interacción de un elemento de un fluido con su entorno y extrapolan la idea que mayor velocidad implica una menor presión en contextos donde no es válida. Mostramos también que un enfoque de la dinámica de fluidos basado en las leyes de Newton resulta más natural para confrontar estas concepciones alternativas.</p>
While Bernoulli’s equation is one of the most frequently mentioned topics in physics literature and other means of dissemination, it is also one of the least understood. Oddly enough, in the wonderful book Turning the World Inside Out, Robert Ehrlich proposes a demonstration that consists of blowing a quarter coin into a cup, incorrectly explained using Bernoulli’s equation. In the present work, we have adapted the demonstration to show situations in which the coin jumps into the cup and others in which it does not, proving that the explanation presented in Ehrlich’s book based on Bernoulli’s equation is flawed. Our demonstration is useful to tackle the common misconception, stemming from the incorrect use of this equation, that higher velocity invariably means lower pressure.
The estimation of the electric field in simple situations provides an opportunity to develop intuition about the models used in physics. We propose an activity aimed at university students where the electric field of a finite line of charge is compared, analytically or numerically, with the fields of an infinite line and of a point charge. Contrary to intuition, it is not necessary to get very close for the line charge to be considered infinite, nor to move very far away for the finite line field to resemble that of a point charge. We conducted this activity with a group of students and found that many of them have not yet developed an adequate intuition about the approximations used in electromagnetism.
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