![]() The Sun produces energy by fusing protons or hydrogen nuclei 1H (by far the Sun’s most abundant nuclide) into helium nuclei 4He. The probability of tunneling increases as they approach, but they do not have to touch for the reaction to occur. (b) At higher energies, the two nuclei approach close enough for fusion via tunneling. (a) Two nuclei heading toward each other slow down, then stop, and then fly away without touching or fusing. Moreover, high temperature is needed for thermonuclear power to be a practical source of energy.įigure 4. Thus most fusion in the Sun and other stars takes place at their centers, where temperatures are highest. The closer reactants get to one another, the more likely they are to fuse (see Figure 4). Since the probability of tunneling is extremely sensitive to barrier height and width, increasing the temperature greatly increases the rate of fusion. 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. ![]() However, in fact, temperatures on the order of 10 8K are needed to actually get the nuclei in contact, exceeding the core temperature of the Sun. You might think that, in the core of our Sun, nuclei are coming into contact and fusing. The greater the kinetic energy and the higher the particles get up the barrier (or the lower the barrier), the more likely the tunneling. Tunneling through the barrier is important in practice. If the nuclei have enough kinetic energy to get over the Coulomb repulsion hump, they combine, release energy, and drop into a deep attractive well. Potential energy between two light nuclei graphed as a function of distance between them. One way to accomplish this is to heat fusion fuel to high temperatures so that the kinetic energy of thermal motion is sufficient to get the nuclei together.įigure 3. If the nuclei are given enough kinetic energy to overcome the electric potential energy due to repulsion, then they can combine, release energy, and fall into a deep well. A ball rolled from the right must have enough kinetic energy to get over the hump before it falls into the deeper well with a net gain in energy. The graph is analogous to a hill with a well in its center. Figure 3 shows an approximate graph of the potential energy between two nuclei as a function of the distance between their centers. Since the attractive nuclear force that can fuse nuclei together is short ranged, the repulsion of like positive charges must be overcome to get nuclei close enough to induce fusion. The major obstruction to fusion is the Coulomb repulsion between nuclei. ![]() The larger BE/A is, the less mass per nucleon, and so mass is converted to energy and released in these fusion reactions. Fusion of light nuclei to form medium-mass nuclei destroys mass, because BE/A is greater for the product nuclei. The temperatures and pressures sustained within a hydrogen bomb for instance can briefly maintain periods of fusion.įor a visual idea of what happens during fission, the youtube video above is worth watching.Figure 2. the atomic bomb), it is fission that is more often associated with this type of nuclear power use. Find out why fission is only worthwhile for certain elements.Īlthough both fission and fusion can have roles in atomic weaponry (e.g. So, why do binding energies change if each nucleus is made up of the same 'stuff'? Well, it's to do with the fundamental forces acting on the component nucleons (protons and neutrons) in the nucleus. Because of this, for fission to produce energy the total binding energy ( def) of the resulting elements must be greater than that of the original nucleus - i.e., the amount of energy needed to break the nucleus apart must be less than the energy we will gain from breaking it. To break a nucleus apart, we need to put energy in to separate the bonds formed between the protons and neutrons. Nuclear fission (the breaking up of atoms) is the reaction used in power stations to produce energy. Credit to youtube user digitalcircuit36939 for the nuclear fission animation.
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