ChapterOnQuantumMechanics - Chapter 7 Atomic Physics and...

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7 - 1 Chapter 7 Atomic Physics and Quantum Mechanics At this point we stop to take stock of some of what we have learned, and to say more about where we are going and why. We have looked at the big things in the universe (Chapter 2) and discussed gravity and general relativity (Chapter 6) which describe how all the big things out there move. We have considered small things like light, electrons and protons (Chapters 3 and 5), but we have not mentioned how these small things are held together, or how they interact with each other. The time has come to talk about quantum mechanics and we will do that as well as talk about electromagnetism and atomic physics. We will finish our discussion in the next chapter when we focus on the strong force , nuclear physics, and chemistry. We might summarize Chapter 3 in one sentence: big things are made of small things - lots of small things (a human body has about 10 27 atoms). There are many questions we can ask. For example, “What keeps the electrons in an atom so close to the protons?” The standard answer is that electrons are negatively charged, protons are positively charged, and that opposite charges attract each other. There is a great deal of truth to this. The attraction between charged things is often referred to as the electromagnetic force or electromagnetism . As we will see, the way atoms are held together is more complicated than just the electromagnetic attraction between electrons and protons. To describe atoms, and other small things, we need quantum mechanics (described in this chapter) to understand how all small things interact. We also need to understand other forces, like the strong force (described in the next chapter), work. As we learn about quantum mechanics we will discover that the special way that it keeps atoms together also makes them produce photons with certain colors/wavelengths. In some ways,
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7 - 2 these special wavelengths are like fingerprints. As detectives we can use atomic fingerprinting to provide evidence that stars are made of atoms and, for reasons we will discuss in the next chapter, which kinds of atoms. The fingerprints will also come in handy to help determine both the direction that the stars move, as well as how fast they are going. Ultimately, the more we know about the stars the more we can learn about the universe itself. We are now ready to learn more about atoms and how they produce the light we see from stars in the night sky. 7.1 The fundamental building blocks of nature In the 17 th and 18 th centuries, physicists and chemists studied matter in its different forms: solids, liquids, and gases. Each seemed to have things in common: for example, gases or liquids could be combined to make other gases or liquids. Scientists hypothesized that matter is made up of different types of atoms, that atoms make up molecules, and that these molecules interact with each other to produce bigger molecules. Lots of molecules interacting together is called either a gas, liquid, or solid; which one they make depends on the type of molecules and other things, such as temperature. A good example is water, which is made of hydrogen and
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