Hope Disneyland was lots of fun!
Here are my answers to the Review Problems. Just click! (An error in number 4 on this file was corrected on Monday, May 20) And here are the equations that will be provided next week. Be sure to know how to describe nuclear fission and nuclear fusion. They are very different processes. Also be able to state actual places at which each of these processes are occurring.
We also received a few problems to practice for our final. The solutions to these problems will be posted this weekend. Until later.... In fission, U-235 or Pu-239, two fissionable nuclei, are already on the verge of repelling themselves apart. All that is required is for a neutron to hit them, and then the split into two sizable pieces. Then these two pieces repel each other away, each acquiring a lot of kinetic energy. This is where the energy comes from in fission processes, electrical repulsion by the electric field. In fusion, two protons can get close enough together for their strong fields to kick in and pull them together despite their mutual electric repulsion. The strong force is about 100 times stronger than the electric repulsive force. It wins, a gamma photon is created, carrying away energy so that the two protons can no longer escape. The loss of strong field energy as the protons "fall" together ends up producing this gamma photon, which carries the energy to the surroundings. The surroundings thereby warm up. This happens in the hot cores of stars. And so stars are hot. The energy that goes to the surroundings due to nuclear fusion comes from the strong field ultimately.
We saw the endpoint of our classical physics today: There is no classical way to explain how an alpha particle exits a nucleus like Polonium-210. While the Po nucleus is only about 8 fm in radius, the alpha particle that leaves it somehow shows up about 45 fm out. How does it just "show up" like that? Classical physics has no answer, but quantum mechanics, which was first worked out by Werner Heisenberg in 1924 or so, describes that this kind of thing can and does happen all the time.
So we have seen two phenomena that cannot be explained by classical physics, the interference of electrons and the "tunneling" of alpha particles out of their parent nuclei. In reality there are many such quantum mechanical phenomena. For example, one can account for the structure of atoms only with quantum mechanics, and that's where all the electron configuration information came from in chemistry. After this interlude we returned to radioactivity by figuring out the isotopes involved in the uranium decay series. Assignment:
Here are objectives tailored to our Semester 2 Final Exam: Sem 2 Objectives for Final Exam
We rehearsed our account of how we can see rainbows, and we introduced radioactivity. Earnest Rutherford and others figured out what the radioactivity that Bequerel discovered was made of, namely, alpha particles, beta particles, and gamma rays: More radioactivity stories on Monday!
We finished our consideration of thin-film interference today and introduced rainbows. You should have a basic understanding of where you, the sun, and the raindrops that produce your rainbow are relative to each other. You should also know why we see red at the top (outside) of the bow and violet on the bottom (inside).
We presented our solutions to Ch 24 P: 1-5 today, and it went nicely. Following that we considered light reflected from thin films like soap bubbles. It turns out the it is similar to two-slit interference, because there are two places where the coherent light comes from on its way to our eyes, namely, the front surface and the back surface of the thin film. That means that light waves reflected from the front surface might be out of phase (i.e. out of step) with light reflected from the back surface. Or it might be IN phase, depending on the thickness of the film and the wavelength of the light.
In any case, one gets some nice interference patterns. We represent the interference of waves we observed yesterday by using the diagram above. We shined a laser through two slits. This diagram captures distances relevant to the resulting interference patterns we saw on the wall. The distance d is the distance between the two slits, S1 and S2, and we designate it as "d." AS2 is the difference in the lengths of the paths light takes as it goes from S1 to point P and from S2 to point P.
We infer the path length difference between PS1 and PS2 when we look at the interference pattern. If P is on the first antinode from the central antinode, we know that the path length difference between the paths from S2 and S1 to P will be one full wavelenth. [You know why, don't you?] We represent that path difference in the picture above with AS2. Assignment:
Two-slit interference patterns were the phenomenon of the day. We need to use a wave model for light to explain two-slit interference patterns. The key is to understand the difference in path length from the two slits to a point on a node or antinode (aka "fringe") in the interference pattern. If you can figure that out by inspecting the interference pattern, the math we will use tomorrow is sensible will make sense. |
Physics IIMr. Swackhamer Archives
May 2019
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