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Old May 27th 2008, 11:56 AM   #1
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Physics Explanations [mixed]

Hi, I have a series of problems that I believe I have answered correctly, and I 'd like your help making sure that they are in fact accurate- as well as clearly expressed.

Thanks for your help!
_______________________________________________
Lake Michigan is located in a relatively cold climate. It is 494 kilometers long and 190 kilometers at its widest, and reaches a depth of 281 meters.

A) During the winter months, the entire lake is not covered with ice, but near the shore, ice does form sufficiently thick for ice fishing. Explain.

B)Does the lake need to absorb or loose energy for the ice to melt? How much of an energy change must the lake under go for each gram of ice melted?

C)would the process of the ice melting change the temperature of the lake?Why or why not?
------------------------------------------------------
a) The water close in to the shore is generally shallower than in the middle of the lake. In order to cause ice to form all water in a given area must be cooled below 4C, since the area closer to the shore is shallower this means that the water can be cooled to that level faster than in the middle of the lake.

b) The lake must absorb 80 calories for every gram of ice melted.

c) The process of melting the ice will not change the temperature of the water until all the ice is melted.
_______________________________________________

A) What kind of pattern of light would you get of you shone a monochromatic light onto a diffraction grating? What pattern of light would you get if you shone white light?

B) Is it correct to say the differences in the patterns produced by monochromatic light and white light is caused by the fact that monochromatic light waves interfere with each other when they pass through a diffraction grating, while the rays of white light do not? Explain.
------------------------------------------------------
a) If a monochromatic light were shone onto a diffraction grating the pattern of interference would be a series of closely spaced light and dark fringes, while white light would produce no discernible diffraction pattern at all.

b) Yes, it would be correct because the interference pattern of one wavelength of light would be "filled in" by the interference pattern of another wavelength in the white light.
_______________________________________________
A) You and a friend see an oil slick in a puddle. Your friend wonders why oil comes in so many colors. What do you tell your friend in explanation?

B) After you've explained how the oil slick works, your friend says it's a shame oil isn't really colored because he would like to photograph the colors he sees, now he won't be able to, since the colors are only a trick of the mind. What do you tell your friend?
------------------------------------------------------
a) The swirling patterns he sees are due to the fact that there is both constructive and destructive interference affecting the light reflected off the water and the oil. The refraction of traveling through the oil, to the water and out again takes the light out of phase with the light reflecting off the oil, causing destructive interference that cancels the reflected light. Since different thicknesses of the oil correspond to different wavelengths they cancel out different colors of light, meaning that instead of being monochromatic the effect is more kaleidoscopic.

b) I'd tell him that it's not a trick of the mind. What causes the effect is that certain wavelengths of light are canceled, and since cameras work by recording those wavelengths as a picture that he would certainly be able to take a picture of it.
_______________________________________________

A magnet is placed directly between a Geiger counter and a material that is known to be radioactive. If the Geiger counter does not register any radioactivity, what kind of particles is the radioactive substance emitting? Why doesn't the Geiger counter register any radioactivity?
------------------------------------------------------
The material must be emitting either Alpha particles or electrons, since those types of particles can be effected by magnetic fields. The Geiger counter doesn't register any radioactivity because the beam of radiation is being influenced by the field of the magnet, so that the particles never reach the counter.
_______________________________________________

The scientists who worked with radioactive materials in the late nineteenth and early twentieth centuries did not realize how dangerous it was to their health. For example, 1903 Nobel Prize winner Henri Becquerel once placed a radioactive sample of Barium into a glass tube, wrapped it in paper, put in a cardboard box, and left it in the pocket of a coat he was wearing for six hours. When he removed the coat he discovered, to his surprise, that the radiation from the sample had burned his skin. What kind of radiation was responsible for Becquerel's burn? How do you know?
------------------------------------------------------
The radiation that caused the burn is likely Beta radiation, as Alpha radiation would not have had much of any effect and Gamma radiation has a far higher penetration strength and would have damaged more than his skin.
_______________________________________________

The decay of 266/88Ra results in the release of some energy. Your book tells you that energy released in a nuclear reaction when mass is changed to energy. Yet there are the same number of nucleons before the decay as there are after the decay. Is the energy released in this reaction without a conversion of mass? If not what mass is converted?
------------------------------------------------------
Yes, energy is released in the form of: the kinetic energy of both the alpha particle and the Rn particle, and some gamma radiation.
_______________________________________________
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Old May 27th 2008, 01:53 PM   #2
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Originally Posted by haflore View Post
Hi, I have a series of problems that I believe I have answered correctly, and I 'd like your help making sure that they are in fact accurate- as well as clearly expressed.

Thanks for your help!
_______________________________________________
Lake Michigan is located in a relatively cold climate. It is 494 kilometers long and 190 kilometers at its widest, and reaches a depth of 281 meters.

A) During the winter months, the entire lake is not covered with ice, but near the shore, ice does form sufficiently thick for ice fishing. Explain.

B)Does the lake need to absorb or loose energy for the ice to melt? How much of an energy change must the lake under go for each gram of ice melted?

C)would the process of the ice melting change the temperature of the lake?Why or why not?
------------------------------------------------------
a) The water close in to the shore is generally shallower than in the middle of the lake. In order to cause ice to form all water in a given area must be cooled below 4C, since the area closer to the shore is shallower this means that the water can be cooled to that level faster than in the middle of the lake.

b) The lake must absorb 80 calories for every gram of ice melted.

c) The process of melting the ice will not change the temperature of the water until all the ice is melted.
_______________________________________________

A) What kind of pattern of light would you get of you shone a monochromatic light onto a diffraction grating? What pattern of light would you get if you shone white light?

B) Is it correct to say the differences in the patterns produced by monochromatic light and white light is caused by the fact that monochromatic light waves interfere with each other when they pass through a diffraction grating, while the rays of white light do not? Explain.
------------------------------------------------------
a) If a monochromatic light were shone onto a diffraction grating the pattern of interference would be a series of closely spaced light and dark fringes, while white light would produce no discernible diffraction pattern at all.
White light diffraction makes a "prism" rainbow appear near every maximum. There are no dark lines in the rainbow, but there is a dark band between successive maxima.

b) Yes, it would be correct because the interference pattern of one wavelength of light would be "filled in" by the interference pattern of another wavelength in the white light.
_______________________________________________
A) You and a friend see an oil slick in a puddle. Your friend wonders why oil comes in so many colors. What do you tell your friend in explanation?

B) After you've explained how the oil slick works, your friend says it's a shame oil isn't really colored because he would like to photograph the colors he sees, now he won't be able to, since the colors are only a trick of the mind. What do you tell your friend?
------------------------------------------------------
a) The swirling patterns he sees are due to the fact that there is both constructive and destructive interference affecting the light reflected off the water and the oil. The refraction of traveling through the oil, to the water and out again takes the light out of phase with the light reflecting off the oil, causing destructive interference that cancels the reflected light. Since different thicknesses of the oil correspond to different wavelengths they cancel out different colors of light, meaning that instead of being monochromatic the effect is more kaleidoscopic.

b) I'd tell him that it's not a trick of the mind. What causes the effect is that certain wavelengths of light are canceled, and since cameras work by recording those wavelengths as a picture that he would certainly be able to take a picture of it.
_______________________________________________

A magnet is placed directly between a Geiger counter and a material that is known to be radioactive. If the Geiger counter does not register any radioactivity, what kind of particles is the radioactive substance emitting? Why doesn't the Geiger counter register any radioactivity?
------------------------------------------------------
The material must be emitting either Alpha particles or electrons, since those types of particles can be effected by magnetic fields. The Geiger counter doesn't register any radioactivity because the beam of radiation is being influenced by the field of the magnet, so that the particles never reach the counter.
There would be another explanation why you might not see any radioactivity. Depending on the substance the magnet is made of and how thick it is, the physical magnet itself might block x or gamma rays from reaching the counter.
_______________________________________________

The scientists who worked with radioactive materials in the late nineteenth and early twentieth centuries did not realize how dangerous it was to their health. For example, 1903 Nobel Prize winner Henri Becquerel once placed a radioactive sample of Barium into a glass tube, wrapped it in paper, put in a cardboard box, and left it in the pocket of a coat he was wearing for six hours. When he removed the coat he discovered, to his surprise, that the radiation from the sample had burned his skin. What kind of radiation was responsible for Becquerel's burn? How do you know?
------------------------------------------------------
The radiation that caused the burn is likely Beta radiation, as Alpha radiation would not have had much of any effect and Gamma radiation has a far higher penetration strength and would have damaged more than his skin.
I have no idea if you are right or wrong here. Though I will say that it is not likely that Becquerel would know if there were damage beneath the skin's surface.
_______________________________________________

The decay of 266/88Ra results in the release of some energy. Your book tells you that energy released in a nuclear reaction when mass is changed to energy. Yet there are the same number of nucleons before the decay as there are after the decay. Is the energy released in this reaction without a conversion of mass? If not what mass is converted?
------------------------------------------------------
Yes, energy is released in the form of: the kinetic energy of both the alpha particle and the Rn particle, and some gamma radiation.
But where did the energy come from is the question that is being asked. Compare the mass of the Radium 266 and the Francium 266 nucleus and you will find that the mass of the Radium nucleus is larger. That is where the energy comes from. Specifically the energy comes from the nuclear binding energy: the Radium nucleus has more protons and thus sits at a higher energy level.
_______________________________________________
.........
-Dan
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Old May 27th 2008, 02:17 PM   #3
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Wow topsquark, thanks very much for your help! My textbook isn't that advanced and I'm very glad to learn more about these subjects!
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