UCSD
 DEPARTMENT OF PHYSICS
Winter Quarter 2000 | |
PHYSICS 8: THE PHYSICS OF EVERYDAY LIFE
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1. Discuss the claim: "most energy which we use originates in nuclear reactions". Is this correct? Give examples. Give counter examples. The energy produced in USA 1994 was: 35% from coal, 31% from gas, 25% from oil, 4% from bio-fuels, 4% nuclear, and 1.4% hydroelectric. (from Energy Guide Book, Ramage, p.34, listed under references on our www page) Nuclear energy powers the Sun. Consider the following pathways for this energy: a. Sunlight is energy source for plants, which decompose unto coal, for gasoline, for example. Plants also feed animals and feed people, for one form of chemical energy. b. Sunlight evaporates water which forms clouds which rain more water,and so on. Running water is then used in hydrolectric power. c. Sunlight heats the Earth unevenly, eventually generating wind and waves which can be harnessed to make electricity. d. Nuclear reactors use a fuel which is a heavy element *Uranium) made in by nuclear reactions in stars. ************************************************************************ 2. Compare and contrast visible radiation (e.g. red light) and infrared radiation. What is similar, and what is different? Similarities: electromagnetic radiation/waves; all normal wave properties (refraction, diffraction), speed in vacuum Differences: wavelength, frequency (color), energy, origin (red light from hotter objects than infrared) (answers like "spelling" don't count) ************************************************************************ 3. If two materials have the same number of atoms per cubic centimeter, could their densities (mass/volume) differ? Discuss why or why not. Yes, their densities could differ. This because each atom individually can have a different weight. See the Periodic Table of the Elements. It's analogous to having 20 pennies or 20 nickels - both are 20, but each individual (coin, atom) differs in weight. ************************************************************************ 4. Please list two every day examples for each of the three methods of heat energy transport: radiation, conduction and convection. Possible answers include: RADIATION: sunlight, streetlight, infrared light from hot objects (buld, body) CONDUCTION: stove heating element TOUCHING pan/kettle, holding ice cube CONVECTION: storm cloud updrafts, chimney smoke rising,circulating water about to boil, miso soup, hot air rising from the pavement on a hot day, air rising form a convection heater... ************************************************************************ 5. Sketch a machine which could measure the weight of items up to 100 kg in weight, using a spring which can support weights up to 10 kg. (For weights above 10 kg, the spring distorts, and does not return to its original length.) There are a few ways to do this. The key is to use pulleys (or levers, etc) to reduce weight load upon spring by factor of 10. So 100 lbs weight produces a force of 10kg or less. The change is by the same factor for all weights, hence 50kg produces a force of 5kg, 20kg as 2kg, etc. The spring can be made to read the true weight by a scale. We did this as a demonstration in class: hanging weights on the spring. We knew the weight, and measured the length of the spring. Once the spring is thus calibrated, we can use its length to measure an unknown weight. The fact the spring gives way at 100 lbs need not be illustrated. Page 57 of the book shows pulleys which give the required 10:1 force reduction. Page 24-5 shows the use of levers in everyday devices. The first page of notes for Jan 25 shows the idea behind the lever. Homework 2, question 3 was similar. |
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