How is the age of antiquities found in archaeology measured? Why can't people live in the basement for a long time? At 12 o'clock on December 26, the fifteenth issue of "Zhang Chaoyang's Physics Class" began broadcasting. Zhang Chaoyang, founder, chairman of the board of directors and CEO of Sohu, sat in the Live Sohu video broadcast room, from the qualitative-energy relationship of relativity, talked about the decay of carbon-14, the structure of the atomic nucleus, the strong interaction between the nucleons, and popularized the decay process of the atomic nucleus, such as the decay of α, the decay of β, etc. Finally, application examples of decay are introduced: the decay of radium in soil and the dating of antiquities.
Review the classical mass-energy relationships of physics introduce the concept of "energy ladder"
Zhang Chaoyang first reviewed the classic mass-energy relationship E=mc. The scientific community recognizes that this formula is applicable at different energies and different scales, but the mass-energy conversion is not obvious in general physical and chemical changes, and is only more obvious in the nuclear process. Subsequently, he introduced the concept of "energy ladder", that is, physical changes, chemical changes, nuclear processes, and a stepwise increase in energy orders.
In physical changes, the structure of the microscopic world does not change, such as the process of cutting the cake in half, raising the temperature of the room, digging and building a house; in the chemical change, the reorganization of atoms, the transfer of electrons, etc., the chemical bonds are broken or formed, but the nucleus does not change, such as paper burning into ashes, eating things are digested, etc. In the nuclear process, there are changes in the nucleus, and its microscopic energy is very high, often reaching the order of MeV, at this time, the conversion relationship between mass energy is very obvious, such as the explosion process of the atomic bomb.
(Zhang Chaoyang on the "Energy Ladder")
The Stability of the Nucleus: A Game of Repulsion and Attractiveness
Regarding nuclear processes, Zhang Chaoyang supplemented the knowledge of atomic nuclei. The nucleus is made up of protons and neutrons. The interior of the atom is actually quite empty. If you compare an atom to a football field, the nucleus is only the size of an apple. Although the nucleus is small, the mass of the atom is mainly concentrated here.
The force between neutrons and protons is the nuclear force. This is a strong interaction force whose potential energy is related to the distance between the nucleons as follows:
(Zhang Chaoyang introduced strong interactions and nucleus structure)
It can be seen in the figure that when the distance between the nuclei r is short, it is the repulsive force; when the distance reaches the lowest point of potential energy, it is in equilibrium; when the distance continues to increase, the nuclear force becomes attractive again, and the strength tends to decay exponentially with the distance. In short, it manifests itself as a short-range force.
(When the atomic number reaches 82 or more, the curve of the stable kernel is biased toward more neutron numbers.) ）
Zhang Chaoyang introduced that the stability of the atomic nucleus is maintained by the strong interaction of the nucleons attracting each other and the balance of coulomb repulsion. Since the strong interaction force is a short-range force, the nucleon is only attractive to the neighboring nucleon; but the Coulomb force is a long-range force, and all protons have a strong repulsive force between them. Therefore, when the number of nuclei is small, protons and neutrons each account for half to maintain equilibrium; when the atomic number reaches 82 or more, more neutrons are needed to maintain the stability of the nucleus.
Dismantling the atomic bomb explosion process Neutron slows down into a uranium fission relay to produce a chain reaction
Then, Zhang Chaoyang introduced that the atomic bomb explosion is the use of neutrons to bombard uranium-235 to produce fission, releasing more neutrons to bombard other uranium nuclei, which is a chain reaction. This process releases a tremendous amount of energy. But the nucleus of atoms, like "apples" vs. "football fields," is empty in between. Neutrons are difficult to hit the nucleus accurately, so "slow neutrons" are needed. Zhang Chaoyang likened slow neutrons to "fat people" - "slow action" is easy to bump into.
The neutrons produced by fission are very fast, and the "action force" needs to be reduced to ensure the continuation of the chain reaction. How to reduce the "mobility" of neutrons? This requires a neutron moderator , heavy water. It is an important material for the manufacture of atomic bombs. Speaking of this, Zhang Chaoyang also introduced a historical story: during World War II, Hitler wanted to develop an atomic bomb, and needed to extract a large number of raw materials from nature for the synthesis of heavy water, so he built a factory in the Norwegian mountain forest. The United States received intelligence and destroyed the factory continuously, forcing Hitler to move the factory back to Germany. The process of transport passed through the mountain lake, and the Allied agents successfully blew up the ship, causing Hitler's plan to fail. The United States successfully developed the atomic bomb and changed the world pattern.
What are the scientific applications of decay? Uncover antiquities age measurement methods
What are the applications of decay in life? Zhang Chaoyang talked about the cases of α decay and β decay.
Zhang Chaoyang explained with examples when introducing α decay, the first is the decay of uranium-238, the uranium-238 atom decays into thorium-234, and emits α particles; the second example is the decay of radium, the radium-226 atom decays into radon-222, and emits α particles.
Zhang Chaoyang mentioned that there are still a small amount of radium in soil, underground rocks and building materials, but the α particles produced by radium decay will be blocked by human skin, so that they will not cause great harm to the human body. However, radon, the product of decay, is also a radioactive substance, and it will also undergo α decay. Radon in the air is absorbed by the human body and then emits α particles from the decay of the lungs can cause lung cancer. He stressed that radon is the cause of lung cancer second only to smoking, and "it is certainly not possible to inhale radioactive material directly into the lungs, which will cause harm to the body." Therefore, you can't stay in the stone structure, closed basement for a long time, you should often open the window to breathe, this is medical knowledge. ”
(Decay of uranium and radium)
Regarding the decay of β, Zhang Chaoyang cited the decay of carbon-14 as an example. Carbon-14 decays β become nitrogen-14 and releases an electron. Carbon-14 in the atmosphere exists in the form of carbon dioxide, derived from the collision of high-energy neutrons in the solar wind, cosmic rays and nitrogen, so the content is maintained at a stable level.
In addition, Zhang Chaoyang elaborated on the application of carbon-14 decay in determining the dating of cultural relics. When archaeologists find mummies or ancient artifacts, how do they measure the age of these artifacts? "The age of a dead organism is generally inferred from its carbon-14 remnants." Organisms need to breathe carbon dioxide when they survive, and the carbon-14 content in the body is unchanged; and after death, they stop breathing, do not exchange carbon with the outside world, and the carbon-14 in the body decays β decays and gradually disappears with a half-life of 5730 years, so as long as the remaining radiocarbon-14 content is determined, its age can be inferred.
Zhang Chaoyang demonstrated detailed calculations for carbon dating. Since β decay, the change in the number of atoms in n atoms in t time is N=-Nλ t, and solving this equation can obtain the exponential decay relationship between the number of decaying atoms over time. At the same time, Zhang Chaoyang introduced the concept of activity, that is, the amount of radiation per unit time, and the activity also follows the change relationship of exponential decay. The unit activity of carbon-14 is 0.25Bq/g, where Bq is the radioactive activity unit "Becco". For example, by detecting the actual activity of carbon-14 in Egyptian antiquities samples of 0.64Bq, combined with the exponential decay formula of activity, the age of the antiquities can be obtained to be 3300 years. This is an example of the application of β decay in archaeology.
Sohu video power knowledge live broadcast spread knowledge to play science
Since November 5, Zhang Chaoyang's Physics Class has been broadcast live for 15 issues. In the first and second lessons, Zhang Chaoyang popularized "force" and "speed" to calculate the number of daily flight circles around the earth by Musk's spacecraft and the Chinese space station; the content of the third and fourth lessons was related to "vibration", popularizing the basic knowledge of visible light, explaining why the color of the sun is different in the morning, middle and evening, and the secret of the blue sky; the fifth and sixth lessons triggered a big discussion about the speed of sound and temperature.
In the seventh, eighth and ninth periods, the two dark clouds of classical mechanics are revisited, leading netizens to touch quantum mechanics; the tenth and eleventh lessons focus on reviewing the blackbody radiation curve and its application, trying to return to the universe 13.7 billion years ago; the twelfth, thirteenth and fourteenth periods try to enter Einstein's world of thought, derive the famous formula "E=mc", and try to argue the effects of "time expansion" and "length contraction". The fifteenth issue deals with the structure of atoms and the decay of atomic nuclei.
Observing life phenomena + explaining the principles behind + deducing the formula, combined with the fourteenth physics class, it can be seen that "Zhang Chaoyang's Physics Class" is introduced from daily phenomena to help netizens understand phenomena, enhance interest, and then "see the essence through phenomena", explain knowledge points, deduce the process, and in turn solve problems in life.
In a follow-up course, Zhang Chaoyang will further explain the common physical phenomena in life. Through the topics that netizens are most familiar with, they can go deep and simple, explain phenomena, and explore the principles of the material world. Achieve the effect of professional recognition, ordinary people can understand, and thus stimulate the enthusiasm of scientific learning. The course is broadcast live on Sohu video every Friday and Sunday at 12:00.
In addition to "Zhang Chaoyang's Physics Class", Sohu Video also invited a number of general education broadcasters to popularize science knowledge and convey value. Dr. Chen Zheng, a teacher at the School of Science of Beijing Jiaotong University, played a "strange scientific experiment" and walked into the "wave-particle duality of light"; Bao Kun, a doctor of physical chemistry at Cornell University, incarnated as "Bao Daren play science" to teach ordinary people to understand the 2021 Nobel Prize; and Liu Boyang, a doctor of astrophysics, popularized "How a total solar eclipse is produced". In the future, more knowledge broadcasters will settle in and play science together.