Physics · Ch 8 — Atomic and Nuclear Physics
Nuclear Fusion
Nuclear Fusion
Nuclear fusion is the process by which two or more light nuclei (typically each with mass number ) combine to form a single, heavier nucleus. As in fission, the mass of the resulting nucleus is less than the sum of the masses of the original light nuclei, and this mass difference is released as energy.
Why fusion needs extreme temperature. Unlike fission, fusion never occurs spontaneously at room temperature: when two light, positively charged nuclei approach each other, they are strongly repelled by their mutual Coulomb repulsion long before they get close enough for the (very short-range) attractive nuclear force to take over. To overcome this repulsion, the two nuclei must be moving with enough kinetic energy - equivalently, the surrounding matter must be at an extremely high temperature, of order K or more - for them to be forced close enough together that the strong nuclear force can bind them; a fusion reaction occurring at such extreme temperature is called a thermonuclear fusion reaction.
Energy generation in stars. The natural setting where fusion routinely occurs is the core of stars, whose interior temperatures reach of order K - indeed, thermonuclear fusion is the only energy source powering every star, including our Sun. A star's life begins as a collapsing cloud of gas and dust: gravitational attraction pulls the cloud inward, converting gravitational potential energy into kinetic energy and ultimately heat, until the core becomes hot enough to ignite thermonuclear fusion, which then releases enormous energy that stabilises the star against further gravitational collapse. The Sun's interior temperature is about K, and it converts roughly kg of hydrogen into helium every single second - a rate it can sustain for another roughly 5 billion years given its remaining hydrogen supply. Once the Sun eventually exhausts its hydrogen fuel, it will enter a "red giant" phase, in which helium itself begins fusing into carbon, and the Sun will expand dramatically, engulfing its inner planets.
The proton-proton chain. According to Hans Bethe, the Sun is powered by the proton-proton chain of fusion reactions, whose first two steps are …
Worked out. This worked example scales up the per-fission energy release to a macroscopic sample: since 235 g of contains Avogadro's number of atoms, 1 kg (1000 g) contains about 2.56e24 uranium-235 nuclei. Multiplying by the 200 MeV released per fission event gives a total energy of about 5.12e26 MeV, which converts to roughly 8.19e13 joules, or equivalently about 2.27e7 kilowatt-hours - an amount of energy so large that it is enough to keep a 100-watt light bulb burning for about 30,000 years, and would require exploding around 20,000 tons of TNT to match thro …
Elementary particles
Until roughly the 1960s, protons, neutrons and electrons were all believed to be truly fundamental "building blocks" of matter, with nothing smaller inside them. In 1964, physicists Murray Gell-Mann and George Zweig independently and theoretically proposed that protons and neutrons are, in fact, not fundamental - they are themselves built from smaller constituents, which came to be called quarks. Quarks were confirmed experimentally in 1968 at the Stanford Linear Accelerator Center (SLAC) in the USA.
There are six known types ("flavours") of quark - up, down, charm, strange, top and bottom - each with a corresponding antiparticle, and all quarks carry electric charges that are simple fractions of the elementary charge : the up quark carries charge , while the down quark carries charge . According to the quark model, a proton is built from two up quarks and one down quark, giving a net charge of , matching the proton's known charge exactly; a neutron, meanwhile, is built from one up quark and two down quarks, giving a net charge of , matching the neutron's electrical neutrality. …
What this figure shows. This figure shows the internal quark composition of the two nucleons side by side: the proton is drawn made of two up quarks (each carrying charge +2/3 e) and one down quark (carrying charge -1/3 e), which sum to the proton's net charge of +e, while the neutron is drawn made of one up quark and two down quarks, which sum to a net charge of zero. Each quark inside the diagram is individually labelled with its type (u or d) and its fractional electric charge, making visually explicit the quark model's central claim that the familiar integer charges of the proton and neutron arise from combinat …
Fundamental forces of nature
Nature is now understood to be governed by exactly four fundamental forces, each responsible for a different domain of physical phenomena:
Gravitational force. A universal attractive force acting between any two masses; it is the force that keeps the planets, including Earth, bound in orbit around the Sun, and the reason we ourselves stay on the surface of the Earth rather than floating away.
Electromagnetic force. The force that acts between electric charges (and, through moving charges, between magnetic sources); it plays the dominant role in almost all everyday physical and chemical phenomena, including the force that lets us physically stand on the ground - it is the electromagnetic repulsion between the atoms of our feet and the atoms of the Earth's surface, not direct contact in any deeper sense, that supports our weight.
Strong nuclear force. As discussed in section 8.5, this is the short-range but extremely powerful attractive force acting between nucleons (protons and neutrons) that holds the nucleus together against the mutual electrostatic repulsion of its protons; it is this force that keeps the atoms of our own bodies stable.
Weak force. A fourth fundamental force, even shorter in range than the strong nuclear force, responsible for processes such as beta decay (section 8.6.2) and for the nuclear reactions that generate energy inside stars; the fusion of hydrogen into helium in the Sun's core, for instance, proceeds via the weak force (in the proton-to-neutron conversion of the proton-proton chain's first step), producing both the neutrinos and the enormous radiated energy that ultimately reaches Earth as sunlight. A full, detailed treatment of the weak force is beyond the scope of this textbook. …