FÍSICA QUÂNTICA PARA PRINCIPIANTES 2 - A constante de Planck

This video is an integral part of the lesson on the website: https://sites.google.com/view/profess... Planck's constant, represented by the letter h, is one of the fundamental constants of physics. This number plays a fundamental role in quantum mechanics, always appearing in the study of phenomena where explanation through quantum mechanics is relevant. But what is Planck's constant? Where did it come from? What does it mean? At the turn of the 19th century to the 20th century, physicists were very excited. Physics had advanced so much that it seemed it had reached the point of describing and explaining all phenomena. Mechanics was very well established and had unified gravitation (the movement of the planets) with the movements that occur on Earth. Electromagnetic theory had discovered that light is an electromagnetic wave (and that other forms of radiation existed like light, albeit invisible), thus unifying optics with electricity and magnetism. This electromagnetic theory was also fully intertwined with mechanics. The kinetic model (which describes the internal energy of a body as the agitation of its particles), also entirely based on mechanics, explained heat. Joule had discovered the equivalence between kinetic energy (motion) and heat. Everything was going well, and the theories fit together and complemented each other. However, two problems were troubling physicists. In 1901, Lord Kelvin (the same person who proposed the Kelvin scale) wrote: "The beauty and clarity of the dynamic theory, which posits heat and light as modes of motion, is presently obscured by two clouds. I. The first appeared with the wave theory of light, developed by Fresnel and Dr. Thomas Young; involving the question of how the Earth can move through an elastic solid, such as the luminiferous ether is essentially? II. The second is the Maxwell-Boltzmann doctrine of the equipartition of energy." These two problems proved unsolvable by the physics equations of the time, and it was because of these problems that a new physics (or two new physics) was born: The first problem gave rise to relativity: One of the great questions of science at the end of the 19th century was to understand the medium in which electromagnetic waves propagate, since it was inconceivable to admit that these waves lacked a propagation medium. Therefore, at the time, it was assumed that the luminiferous aether was the propagation site for electromagnetic waves. The problem is that this luminiferous aether was never detected! This was a serious problem for the theory at the time. Today we know that electromagnetic waves do not need a medium to propagate. But it was a long road to reach this understanding. This problem was only resolved with the advent of Einstein's THEORY OF RELATIVITY. The second problem mentioned by Lord Kelvin gave rise to QUANTUM PHYSICS. To better understand, let's turn to the concept of blackbody thermal radiation. This is the name physicists give to describe the thermal radiation emitted by a body that emits radiation at the same intensity as it absorbs it. Electromagnetic theory states that moving electric charges generate electromagnetic waves. Therefore, the thermal agitation of bodies generates electromagnetic waves. We call these waves THERMAL RADIATION. So far, so good; the physics of the time was quite good. However, when this radiation was measured in the laboratory (detected and plotted on a graph), the results predicted by this theory did not match the results obtained from these measurements. In classical theory (thermodynamics and electromagnetics), the predicted results describe an exponential curve (in black in the graph above), meaning that the higher the frequency of the radiation, the more intense it becomes! In this case, THEORY does not match the EXPERIMENTAL RESULTS. This was one of the major problems in physics at the time and could not be explained with the theories available at the time. When we look at the graph of blackbody radiation, we see that for long wavelengths (low frequencies), the Rayleight-Jeans formula (classical theory) fits well. However, for short wavelengths (high frequencies), this formula is far from what happens in reality. This is the first indication of the quantum physics scale, which we discussed in the last lesson. Because of this, this problem became known as the ULTRAVIOLET CATASTROPHE (ultraviolet radiation has a high frequency, therefore, a short wavelength), so this discrepancy is most evident in ultraviolet radiation. In 1901, the German physicist Max Karl Ernst Ludwig Planck (let's just call him Planck) proposed a mathematical solution to this problem. That's the subject of this video.