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Ohm proceeded from this, starting experiments to measure the dependence of current strength on voltage. And very soon it became clear that nothing like this happens in electrical conductors: the resistance of a substance to an electric current does not at all depend on the applied voltage. This, in fact, is Ohm’s law, which (for a separate section of the circuit) is written very simply:
V = IR
where V is the voltage applied to the section of the circuit, I is the current strength, and R is the electrical resistance of the section of the circuit.
Today we understand that electrical conductivity is due to the movement of free electrons, and resistance is due to the collision of these electrons with atoms of the crystal lattice (see Electronic theory of conduction). With each such collision, part of the energy of a free electron is transferred to the atom, which, as a result, begins to oscillate more intensely, and as a result, we observe the heating of the conductor under the influence of an electric current. Increasing the voltage in the circuit does not in any way affect the proportion of heat losses of this kind, and the ratio of voltage and electric current remains constant.
However, when Georg Ohm experimentally discovered his law, the atomic theory of the structure of matter was in its infancy, and several decades remained before the discovery of the electron. Thus, for him the formula V = IR was a purely experimental result. Today we have a fairly harmonious and, at the same time, complex theory of electrical conductivity and we understand that Ohm’s law in its original form is just a rough approximation. However, this does not prevent us from successfully using it to calculate the most complex electrical circuits used in industry and everyday life. The SI unit of electrical resistance is called Ohm, after this eminent scientist.
Ernest Rutherford is a unique scientist in the sense that he made his main discoveries after receiving the Nobel Prize. In 1911, he succeeded in an experiment that not only allowed scientists to look deep into the atom and get an idea of its structure, but also became a model of grace and depth of design.
Using a natural source of radioactive radiation, Rutherford built a cannon that produced a directed and focused stream of particles. The gun was a lead box with a narrow slot, inside which radioactive material was placed. Due to this, the particles (in this case, alpha particles, consisting of two protons and two neutrons) emitted by the radioactive substance in all directions, except one, were absorbed by the lead screen, and only a directed beam of alpha particles flew out through the slot. Further along the path of the beam, there were several more lead screens with narrow slots that cut off particles deviating from a strictly specified direction. As a result, a perfectly focused beam of alpha particles flew up to the target, and the target itself was the thinnest sheet of gold foil. The alpha ray hit her. After colliding with the foil atoms, the alpha particles continued on their way and hit a luminescent screen installed behind the target, on which flashes were recorded when alpha particles hit it. From them, the experimenter could judge in what quantity and how much alpha particles deviate from the direction of rectilinear motion as a result of collisions with foil atoms.
Experiments of this kind have been carried out before. Their main idea was to accumulate enough information at the angles of particle deflection, according to which one could say something definite about the structure of the atom. At the beginning of the twentieth century, scientists already knew that the atom contains negatively charged electrons. However, the prevailing idea was that the atom is something like a positively charged thin grid filled with negatively charged raisin electrons – the model was called the "raisin grid model". Based on the results of such experiments, scientists were able to find out some of the properties of atoms – in particular, to estimate the order of their geometric dimensions.
Rutherford, however, noticed that none of his predecessors even tried to test experimentally whether some alpha particles were deflected at very large angles. The raisin grid model simply did not allow the existence of such dense and heavy structural elements in the atom that they could deflect fast alpha particles at significant angles, so no one bothered to test this possibility. Rutherford asked one of his students to re-equip the installation in such a way that it was possible to observe the scattering of alpha particles at large deflection angles – just to clear his conscience, to completely eliminate this possibility. The detector was a sodium sulfide-coated screen, a material that gives off a fluorescent flash when an alpha particle hits it. Imagine the surprise of not only the student directly conducting the experiment, but also of Rutherford himself when it turned out that some particles are deflected at angles up to 180 °!
In the framework of the established model of the atom, the result could not be interpreted: there is simply nothing in the raisin grid that could reflect a powerful, fast and heavy alpha particle. Rutherford was forced to conclude that in an atom, most of the mass is concentrated in an incredibly dense substance located in the center of the atom. And the rest of the atom turned out to be many orders of magnitude less dense than previously thought. It also followed from the behavior of the scattered alpha particles that in these superdense centers of the atom, which Rutherford called nuclei, the entire positive electric charge of the atom is also concentrated, since only the forces of electrical repulsion can cause the scattering of particles at angles greater than 90 °.
Years later, Rutherford was fond of making such an analogy about his discovery. In one South African country, customs have been warned that a large shipment of smuggled weapons for the rebels is about to be smuggled into the country, and the weapons will be hidden in bales of cotton. And now, after unloading, the customs officer finds himself in a warehouse full of bales of cotton. How can he determine in which bales the rifles are hidden? The customs officer solved the problem simply: he began to shoot at the bales, and if the bullets ricocheted off any bale, he would use this indicator to identify the bales with contraband weapons. So Rutherford, having seen how alpha particles ricocheted off the gold foil, realized that a much denser structure was hidden inside the atom than was expected.
The picture of the atom drawn by Rutherford based on the results of the experiment is well known to us today. An atom consists of a superdense, compact nucleus that carries a positive charge, and negatively charged light electrons around it. Later, scientists have provided a reliable theoretical basis for this picture (see Bora Atom), but it all started with a simple experiment with a small sample of radioactive material and a piece of gold foil.
The word "quantum" comes from the Latin quantum ("how much, how much") and the English quantum ("quantity, portion, quantum"). It has long been customary to call the science of the motion of matter "mechanics". Accordingly, the term "quantum mechanics" means the science of the motion of matter in portions (or, in modern scientific language, the science of the motion of quantizing matter). The term "quantum" was coined by the German physicist Max Planck (see Planck’s constant) to describe the interaction of light with atoms.
Quantum mechanics often contradicts our common sense. And all because common sense tells us things that are taken from everyday experience, and in our everyday experience we have to deal only with large objects and phenomena of the macrocosm, and at the atomic and subatomic level, material particles behave quite differently. The Heisenberg Uncertainty Principle outlines the meaning of these differences. In the macrocosm, we can reliably and unambiguously determine the location (spatial coordinates) of any object (for example, this book). It doesn’t matter if we use a ruler, radar, sonar, photometry or any other measurement method, the measurement results will be objective and not dependent on the position of the book (of course, provided that you are careful in the measurement process). That is, some uncertainty and inaccuracy are possible – but only due to the limited capabilities of measuring instruments and observation errors. To get more accurate and reliable results, we just need to take a more accurate measuring device and try to use it without errors.
Now, if instead of the coordinates of the book we need to measure the coordinates of a microparticle, for example, an electron, then we can no longer neglect the interactions between the measuring device and the object of measurement. The force of the action of a ruler or other measuring device on the book is negligible and does not affect the measurement results, but in order to measure the spatial coordinates of an electron, we need to launch a photon, another electron or other elementary particle of energies comparable to the measured electron in its direction and measure its deviation. But at the same time, the electron itself, which is the object of measurement, as a result of interaction with this particle, will change its position in space. Thus, the very act of measurement leads to a change in the position of the measured object, and the measurement inaccuracy is due to the very fact of the measurement, and not to the degree of accuracy of the used measuring device. This is the situation we have to put up with in the microcosm. Measurement is impossible without interaction, and interaction – without affecting the measured object and, as a consequence, distortion of the measurement results.
Only one thing can be said about the results of this interaction:
uncertainty of spatial coordinates × uncertainty of particle velocity> h / m,
or, in mathematical terms:
Δx × Δv> h / m
where Δx and Δv are the uncertainty of the spatial position and velocity of the particle, respectively, h is Planck’s constant, and m is the mass of the particle.
Accordingly, uncertainty arises in determining the spatial coordinates of not only an electron, but also any subatomic particle, and not only coordinates, but also other properties of particles, such as speed. The measurement error of any such pair of mutually correlated characteristics of particles is determined in a similar way (an example of another pair is the energy emitted by an electron and the time interval during which it is emitted). That is, if we, for example, managed to measure the spatial position of the electron with high accuracy, then at the same moment in time we have only the most vague idea of its speed, and vice versa. Naturally, in real measurements, these two extremes do not reach, and the situation is always somewhere in between. That is, if we managed, for example, to measure the position of an electron with an accuracy of 10–6 m, then we can simultaneously measure its speed, at best, with an accuracy of 650 m / s.
Due to the principle of uncertainty, the description of objects of the quantum microcosm is of a different nature than the usual description of objects of the Newtonian macrocosm. Instead of spatial coordinates and velocities, which we used to describe mechanical motion, for example, a ball on a billiard table, in quantum mechanics, objects are described by the so-called wave function. The “wave” crest corresponds to the maximum probability of finding a particle in space at the time of measurement. The motion of such a wave is described by the Schrödinger equation, which tells us how the state of a quantum system changes over time.
The picture of quantum events in the microcosm, drawn by the Schrödinger equation, is such that the particles are likened to individual tidal waves propagating over the surface of the ocean-space. Over time, the crest of a wave (corresponding to the peak in the probability of finding a particle, such as an electron, in space) moves in space in accordance with the wave function that is the solution to this differential equation. Accordingly, what we traditionally think of as a particle, at the quantum level, exhibits a number of characteristics inherent in waves.
Coordination of the wave and corpuscular properties of the objects of the microworld (see de Broglie’s relation) became possible after physicists agreed to consider the objects of the quantum world not as particles or waves, but as something intermediate and possessing both wave and corpuscular properties; there are no analogues of such objects in Newtonian mechanics. Although even with such a solution there are enough paradoxes in quantum mechanics (see Bell’s theorem), no one has yet proposed the best model for describing the processes occurring in the microcosm.
The discovery of the chemical basis of life was one of the greatest discoveries of biology in the 19th century, and received a lot of confirmation in the 20th century. There is no vital force in nature (see Vitalism), just as there is no essential difference between the material from which living and nonliving systems are built. A living organism is most like a large chemical plant, in which many chemical reactions take place. The loading platforms receive raw materials and transport finished products. Somewhere in the office – perhaps in the form of computer programs – are stored instructions for managing the entire plant. Likewise, the nucleus of the cell – the "governing center" – stores the instructions that govern the chemical business of the cell (see Cell Theory).
This hypothesis was successfully developed in the second half of the 20th century. Now we understand how information about chemical reactions in cells is transmitted from generation to generation and implemented to ensure the vital activity of the cell. All information in the cell is stored in the DNA molecule (deoxyribonucleic acid) – the famous double helix, or "twisted ladder". Important operational information is stored on the rungs of this ladder, each of which consists of two molecules of nitrogenous bases (see Acids and bases). These bases – adenine, guanine, cytosine, and thymine – are usually denoted by the letters A, G, C, and T. Reading information from one DNA strand gives you the sequence of the bases. Think of this sequence as a message written in an alphabet with only four letters. It is this message that determines the flow of chemical reactions in the cell and, therefore, the characteristics of the organism.
The genes discovered by Gregor Mendel (see Mendel’s Laws) are actually nothing more than sequences of base pairs on a DNA molecule. And the human genome – the totality of all 123helpme.me of its DNA – contains approximately 30,000-50,000 genes (see Human Genome Project). In the most advanced organisms, including humans, genes are often separated by fragments of “meaningless,” non-coding DNA, while in simpler organisms, the gene sequence is usually continuous. In any case, the cell knows how to read the information contained in the genes. In humans and other highly developed organisms, DNA is wrapped around a molecular backbone, with which it forms a chromosome. All human DNA is located on 46 chromosomes.
In the same way that information from a hard disk stored in the office of a plant must be broadcast to all devices in the plant’s shops, information stored in DNA must be translated using cellular hardware into chemical processes in the "body" of the cell.