Apna phone number register karein. Ab aap Whatsapp pe solutions paa saktey h, hum aapko message karenge. Ab aap Whatsapp pe solutions paa saktey h, hum aapko ping karenge.
Study Materials. Why use Doubtnut? Instant Video Solutions. Request OTP. Updated On: The greater the kinetic energy and the higher the particles get up the barrier or the lower the barrier , the more likely the tunneling.
You might think that, in the core of our Sun, nuclei are coming into contact and fusing. However, in fact, temperatures on the order of 10 8 K are needed to actually get the nuclei in contact, exceeding the core temperature of the Sun.
Quantum mechanical tunneling is what makes fusion in the Sun possible, and tunneling is an important process in most other practical applications of fusion, too. Since the probability of tunneling is extremely sensitive to barrier height and width, increasing the temperature greatly increases the rate of fusion.
The closer reactants get to one another, the more likely they are to fuse see Figure 4. Thus most fusion in the Sun and other stars takes place at their centers, where temperatures are highest. Moreover, high temperature is needed for thermonuclear power to be a practical source of energy. Figure 4. The probability of tunneling increases as they approach, but they do not have to touch for the reaction to occur. The principal sequence of fusion reactions forms what is called the proton-proton cycle :.
The energy in parentheses is released by the reaction. Note that the first two reactions must occur twice for the third to be possible, so that the cycle consumes six protons 1 H but gives back two. The overall effect of the cycle is thus. The solar interior is dense, and the reactions occur deep in the Sun where temperatures are highest.
It takes about 32, years for the energy to diffuse to the surface and radiate away. However, the neutrinos escape the Sun in less than two seconds, carrying their energy with them, because they interact so weakly that the Sun is transparent to them.
Negative feedback in the Sun acts as a thermostat to regulate the overall energy output. For instance, if the interior of the Sun becomes hotter than normal, the reaction rate increases, producing energy that expands the interior. This cools it and lowers the reaction rate. Conversely, if the interior becomes too cool, it contracts, increasing the temperature and reaction rate see Figure 5.
Stars like the Sun are stable for billions of years, until a significant fraction of their hydrogen has been depleted. What happens then is discussed in Introduction to Frontiers of Physics.
Figure 5. Nuclear fusion in the Sun converts hydrogen nuclei into helium; fusion occurs primarily at the boundary of the helium core, where temperature is highest and sufficient hydrogen remains. Energy released diffuses slowly to the surface, with the exception of neutrinos, which escape immediately.
Energy production remains stable because of negative feedback effects. Theories of the proton-proton cycle and other energy-producing cycles in stars were pioneered by the German-born, American physicist Hans Bethe — , starting in He was awarded the Nobel Prize in physics for this work, and he has made many other contributions to physics and society.
Neutrinos produced in these cycles escape so readily that they provide us an excellent means to test these theories and study stellar interiors. Detectors have been constructed and operated for more than four decades now to measure solar neutrinos see Figure 6. Although solar neutrinos are detected and neutrinos were observed from Supernova A Figure 7 , too few solar neutrinos were observed to be consistent with predictions of solar energy production.
After many years, this solar neutrino problem was resolved with a blend of theory and experiment that showed that the neutrino does indeed have mass. It was also found that there are three types of neutrinos, each associated with a different type of nuclear decay. Figure 6. This array of photomultiplier tubes is part of the large solar neutrino detector at the Fermi National Accelerator Laboratory in Illinois.
In these experiments, the neutrinos interact with heavy water and produce flashes of light, which are detected by the photomultiplier tubes.
In spite of its size and the huge flux of neutrinos that strike it, very few are detected each day since they interact so weakly. This, of course, is the same reason they escape the Sun so readily. Figure 7. Supernovas are the source of elements heavier than iron. Energy released powers nucleosynthesis. Spectroscopic analysis of the ring of material ejected by Supernova A observable in the southern hemisphere, shows evidence of heavy elements.
The study of this supernova also provided indications that neutrinos might have mass. The proton-proton cycle is not a practical source of energy on Earth, in spite of the great abundance of hydrogen 1 H. This is why our Sun will last for about ten billion years. However, a number of other fusion reactions are easier to induce. Among them are:. Deuterium 2 H is about 0. In addition to an abundance of deuterium fuel, these fusion reactions produce large energies per reaction in parentheses , but they do not produce much radioactive waste.
Questions from Nuclei. The approximate mass of this element in the sample left after two mean lives is. Some of this liquid is now discarded. Assertion : The whole mass of the atom is concentrated in the nucleus Reason : The mass of a nucleus can be either less than or more than the sum of the masses of nucleons present in it. Physics Most Viewed Questions. The phase difference between displacement and acceleration of a particle in a simple harmonic motion is: NEET Oscillations. The energy equivalent of 0.
Two cylinders A and B of equal capacity are connected to each other via a stop clock.
0コメント