knowledge, science, and the universe chapter 1 - Physical Science ...

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SECTION 1–4 The Universe Around Us 11 Nuclear Interactions: The Strong and Weak forces We start at a scale size of approximately 10 –15 m. Are there structures smaller than this There may be, but if so, we do not as yet have the ability to measure them. Atomic nuclei are this size and are comprised of protons and neutrons, which we together call nucleons. Nucleons are so small that it would take one trillion (or 10 12 ) lined up next to each other to reach across the head of a pin. They are so dense that a pinhead- size ball made of nucleons packed next to each other would weigh about a million tons. No crane could lift it! A well-tested model theorizes that protons and neutrons are each comprised of more fundamental particles called quarks. The strong nuclear force acts on quarks, binding them together into nucleons and also binding the nucleons together into atomic nuclei. The nuclear weak force, on the other hand, attempts to break nuclei apart. The electromagnetic force is also present in atomic nuclei as a repulsion between positively charged protons. Atomic nuclei do not fly apart only because the strong nuclear force is more powerful than the electromagnetic force. An additional help is that only protons feel the repulsive electromagnetic force, while both neutrons and protons feel the attractive strong nuclear force. Nuclei that are suscep tible to being changed or broken apart under the influence of the various forces are called radioactive. The Electromagnetic Interaction Surrounding atomic nuclei are electrons held in place by their attraction to the nuclear protons. Electrons and nuclei bound together constitute atoms. The average distance of the outermost electrons from the nucleus is about 10 –10 m, meaning an atom is approximately 100,000 times larger than its nucleus and is mostly empty space. If a nucleus were the size of a ballpoint pen tip, its surrounding atom would be the size of a football field! Atoms, in a variety of combinations, make up matter as we know it. The tiniest speck of dust visible to the unaided eye contains about 10 18 atoms. A sample of air the size of a sugar cube has about the same number. The atoms of each of the nearly 100 elements found in nature have a unique number of protons and electrons. The lightest element, hydrogen, has one of each. The next lightest element, helium, has two protons, two neutrons and two electrons. The atoms of each successive element increase with each step by one proton, one electron, and usually one or several neutrons. Atoms join together in small groups called molecules. The common table sugar molecule contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. Some substances, like nylon for example, have long molecular chains that may contain a million atoms or more. Molecular sizes vary from 10 –10 m to 10 –8 m or larger. Even so, the largest molecules are too small to be seen under the most powerful optical microscope. Objects from 10 –7 m to 10 4 m in size are made of large numbers of atoms and molecules arranged together. We will call these arrangements “mixtures.” Mixtures can be simple in structure like a shovel full of dirt, in which the individual molecules are just piled against each other and have no real connection beyond proximity. Other mixtures can be quite complex, such as the human body, where the individual parts not only interact in a marvelous manner but the absence of a small amount of needed material (proteins, vitamins, etc.) will cause other parts of the body to drastically change their functions. The realm of mixtures is the world we live in. The Realm of the Gravitational Interaction As molecular assemblies increase in mass, they are eventually held together by gravity instead of electromagnetism. Since gravity always pulls matter inward toward the center of the distribution, objects held together by gravity tend to have a round shape. Earth has the shape of a ball with a radius of almost 6400 km (4000 miles). This is so large we do not generally notice that the level of a lake curves downward to be about 16 feet lower five miles away than it is at our feet. Earth is a member of the solar system, which consists of the Sun, eight planets and their moons, dwarf planets, thousands of asteroids and comets, and an untold number of smaller pieces proton A composite, strongly interacting particle made up of three quarks. Protons carry a positive electrical charge and is a constituent part of the nucleus of atoms. Neutron A composite, strongly-interacting particle made up of three quarks, but which carries no net electrical charge. Neutrons are a constituent part of the nucleus of atoms. Nucleon A generic name for either a proton or a neutron. Quark The elementary particles of which protons and neutrons consist. A proton and a neutron each consist of three quarks. Radioactive A term referring to atoms whose nuclei can spontaneously change under the influence of the weak nuclear force. Electron An elementary particle in atoms having a negative charge. Electrons are located outside atomic nuclei. Element A substance composed of atoms that have an identical number of protons in each nucleus. Elements cannot be reduced to simpler substances by normal chemical means. Atom The fundamental unit of an element. Molecule A microscopic structure usually made up of more than one atom. Solar System The sun and all planets, comets, asteroids, and other bodies that orbit about it under the pull of gravity.
12 C h A p T E R 1 Knowledge, Science, and the Universe of rock and ice. The Sun’s gravity keeps all these bodies orbiting about it in a circular disk. The average distance from the Sun to the farthest planet Neptune is 4.5 ¥ 10 12 m, or the distance light can travel in just over 4 hours. Like the atom, the solar system is mainly empty space. If the sun were an orange on the 50-yard line of a football field, Earth would be a BB ten yards away and Neptune would be a marble 300 yards away in the parking lot. On this scale all the material in the entire solar system combined would form a ball smaller than a grapefruit. As large as the solar system is, it is dwarfed by the enormity of the Milky Way Galaxy in which it resides. This huge structure is a circular disk of over 200 billion stars held together by their mutual gravity. It measures about 10 21 m from edge to edge, which is the distance light can travel in 100,000 years. If the entire solar system were a marble on the 50-yard line of a football field in Provo, Utah, then the edge of the galaxy would be about 200 miles away! The nearest stars would be a half- dozen grains of sand spread throughout the football stadium. The vastness and emptiness of it all is staggering. Such distances seem unreal in part because the Milky Way Galaxy seen on a moonless night seems to be crowded with stars. That so many stars are visible attests to their great brightness and prolific abundance. Finally, we come to the realm of visible galaxies that populate the vastness of the universe itself. The deepest image from the Hubble Space Telescope suggests there are at least 100 billion galaxies averaging over 100 billion stars each. These are spread throughout the 10 26 m of visible space in clusters of 10 to 10,000 members or more. Evidence strongly suggests that the universe itself had a beginning almost 14 billion years ago in an event of nearly incomprehen sible magnitude called the “Big Bang.” As you study the realms described above, try to learn not just what they are but how we know of them. Distinguish between the wellknown parts and those for which our knowledge is more tentative and developing by paying attention to the data, reasoning, and assumptions involved. Let your own intuitive heart guide you through the huge body of authority, evidence, and reason the knowledge is built upon. And do not forget that as you learn more about the world in which we live, you are simply continuing the process of discovery you began as a child. References 1. Chuang Tsu, Inner Chapters, p. 48, translated by Gia-Fu Feng and Jane English, Vintage Books, Random House, New York, NY, 1974 . Selected Sources of Additional Reading: 1. Bryson, Bill. A Brief History of Nearly Everything. 2. Trefil, James and Robert M. Hazen. Science Matters. 3. Weisskopf, Victor. Knowledge and Wonder. 4. http://www.particleadventure.org/
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