Assignments
• Watch Cosmos: Possible Worlds, Episode 10.
• Questions or comments? Use the form at the bottom of this (or any) page, to submit them for inclusion on this page (see Questions for Discussion, below) and for possible discussion in class. As time goes on, I hope that more and more of the class entails your questions and comments at starting points. Submit your contributions by the end of the day each Monday, if possible, to give me Tuesday to post them and to find resources that will help.
To think about
• Tyson's story of two atoms: in what sense is it true?
The last chapter of chemist and holocaust survivor Primo Levi's celebrated book The Periodic Table, is an essay, "Carbon", about the fascinating and tortuous life and times of a single carbon atom, down through millennia of its existence, in carbonate rocks, in the atmosphere, in many and varied life forms, down to the present day, when it is involved in the life of the author himself. Here's what he says about his story:
It is possible to demonstrate that this completely arbitrary story is nevertheless true. I could tell innumerable other stories, and they would all be true: all literally true, in the nature of the transitions, in their order and data. The number of atoms is so great that one could always be found whose story coincides with any capriciously invented story.
What do you think?
• Watch this demonstration:
Think about how this illustrates the concept of a chain reaction. At YouTube are many versions of this demo, some with slow motion replays.
Here's another good one:
Nuclear vs Chemical Energy
• Burning natural gas (mostly methane), produces 55.5 kj per gram of the gas (called the heat of combustion).
• Converting the same mass, 1 g, completely to energy (E = mc^2) produces 89,880,000,000,000 kj/g.
So converting one gram of any mass to energy produces about 1.6 trillion times as much as the energy yield from combustion of 1 g natural gas. In fact, in nuclear fission or fusion bombs, only a tiny fraction of the total mass of the fuel is converted to energy, but it is still a tremendous amount of energy compared with the yield from combustion. In a one-megaton nuclear bomb explosion, the mass converted to energy is about 47 g or 1.6 ounces. (All calculations by the WolframAlpha app on iPhone.)
• How do we express quantities of energy ?
Probably the most familiar unit for talking about energy is the calorie (cal). The chemical energy content of a Twinkie is about 130 nutritional calories (Cal, or kilocalories -- kcal, to a chemist, which means a thousand cal). This (and any) energy quantity can be expressed in many other units, among them joules (one joule equals 0.239 cal), watt-hours, foot-pounds, megatons of TNT, electron volts (eV), British Thermal Units (BTU) . . . . In chemistry, the most common energy unit is the kilojoule (kj).
A quantity expressed in any of these units can be converted into any other energy units. For example, the energy change caused by any process can be expressed in kcal. Scientists usually express energy in units that are convenient for the size or scale and the nature of the process under study. For example, the energy content of a Twinkie, 130 kcal, equals the energy of 0.0000000013 megatons of TNT. So kcal is not such a convenient unit if you are talking about the energy released by an atomic explosion, now is it?
Read about conversion of units at Wikipedia. In particular, look at the section "Energy" and see how many different units of energy are in use.
• What is energy?
Energy is defined in basic physics as the capacity to do work. For example, the energy required to push an object a distance of one foot, while exerting a force of one pound, is one foot-pound or pound-foot (lb-ft). One lb-ft equals 0.324 cal. It can also be expressed in any of the energy units mentioned above.
• Energy is a scientific term, but also a commonly-used term in other contexts. What other kinds of things do we use the word energy to talk about? Which of them are really energy (as defined above), and which of them are something else, perhaps something that can't be expressed in Cal, or even thought of as a quantity. If it cannot be expressed as a quantity, or cannot be converted into kcal, it's not energy, in the scientific sense.
• What is ionizing radiation, and how does it differ from other kinds of radiation, such as light from a lamp, or heat from a fireplace, or radio waves?
Questions for Discussion
Student contributions will appear here
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Student-recommended readings:
• Six Stars, Six Eclipses: ‘The Fact That It Exists Blows My Mind’ https://nyti.ms/365Cnz8
• Did an Alien Life-Form Do a Drive-By of Our Solar System in 2017? https://www.nytimes.com/2021/01/26/books/review/extraterrestrial-avi-loeb.html
• Two books by Richard Rhodes: The Making of the Atomic Bomb, and Dark Sun: The Making of the Hydrogen Bomb
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I got thinking about Tyson’s remark about the uranium atom’s fate of decaying. What is the probability that the uranium atom would have lasted - without decaying - from its formation until now? I looked up the half life of U-238, an astounding 4.5 billion years. If the uranium atom had been formed near the beginning of the universe - 13.5 billion years ago, the answer is easy to see. If you started with a lump of pure U-238 13.5 billion years ago, then half the lump would remain after 4.5 billion years: 9 billion year ago. A quarter of the lump would remain after another 4.5 billion years: 4.5 billion years ago. Finally, an eight of the lump would remain after another 4.5 billion years: today! So of every eight U-238 atoms forged near the beginning of the universe, 1 would have lived until today.
Of course if it was a U-235 atom - with half life of only 0.7 billion years - its probability of surviving til today would be only 1/2 raised to the 13.5/0.7 power which is 0.000001.56.
[[ From GR: All sound reasoning, seems to me, except for one little thing. How early in the universe could uranium have possibly been formed? How recently could it have been formed? ]]
See these articles:
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Other Resources
• Cloud chamber: a (relatively) simple way to see the effects of single radioactive atoms decaying.
A simple one you can make at home:
For inexpensive, harmless, legal radioactive sources, get these welding rods*, which contain a small percentage of radioactive thorium. In Portland, you can buy dry-ice pellets at Vessel Services on Portland Fish Pier. Pure isopropyl alcohol is a good liquid for the chamber.
* [[ When not using these rods, keep them in their plastic container. Avoid long-term contact with the rods, such as carrying a piece around in a pocket. The handling you need to do for this demo should not be harmful. Wash hands after handling. ]]
Here is a more sophisticated, better lighted cloud that shows much more detail:
• Tyson remarks that Curie family members won a total of 5 Nobel prizes. This does not count Marie's second daughter Ève, who was a director at UNICEF when the organization won the Nobel Peace Prize in 1965. Ève was born in 1904 and died in 2007 at age 102.
Do you think people might live a lot longer if they avoid radioactive materials? Marie was easily the longest lived of the family's science prize winners, succumbing to aplastic anemia (a consequence of radiation exposure) at age 66. It appears that all of their lives were shortened by their exposure to radioactivity.
• Marie Curie is the only woman to win two Nobels, and the only person to win science Nobels in different fields (1903, with Pierre in physics, shared with Henri Becquerel; and 1911, sole winner, in chemistry). Her life has been depicted -- not that badly -- in (at least) two movies: Madame Curie (1943), portrayed by Greer Garson; and Radioactive (2020), portrayed by Rosamund Pike.
Women Who Won the Nobel Prize (Note how few winners in chemistry and physics.)
For invention of the CRISPR system of gene editing, the 2020 Nobel Prize in Chemistry was shared by two women, Emmanuel Charpentier (currently at Max Planck Institute, Berlin) and Jennifer Doudna (U. of California, Berkeley). This was the first science Nobel ever awarded to two women.
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