Physics 12: Homework #6: due May 14


In addition to the assigned problems from the book, part of this week's assignment is adapted from Chapters 5 and 6 of the book. Requests for "full notation" mean that you should provide mass number, proton number, and neutron number as superscript and subscripts in the appropriate positions.

  1. (Adapted from Q&P 5.11) Estimate the number of Btu that would be generated annually in the United States if all the municipal waste were incinerated. Assume 1000 pounds of waste per year per person at 4300 Btu per pound (about 2.4 Cal/g). How does this number compare with our total energy consumption in the U.S. (useful to find value in QBtu for the comparison)?

  2. If we moved to a biomass-dominated energy resource (burning firewood), we would be releasing about 1.5 pounds of CO2 into the atmosphere for every pound of firewood burned. Assuming that we are harvesting wood at the same rate as we are replenishing it with new growth, does the net amount of CO2 in the atmosphere increase? Given that CO2 is the primary culprit in global warming, will a sustained biomass reliance worsen our global warming problem? Think hard about this one—and, of course, explain your reasoning.

  3. List as many reasons as you can why ethanol from corn is not a viable way to go in trying to replace our dependence on oil.

  4. In the following reaction for the induced fission of 235U, apply an appropriate conservation law to determine the number of neutrons (n) emitted.
    23592U143 + n → 9737Rb60 + 13755Cs82 + (?)n

    Are all particles incoming and outgoing accounted for? As an aside, the products in the reaction above are short-lived: within a few seconds, the rubidium beta decays several times through strontium and yttrium, pausing at zirconium for about a day, then a minute as niobium, and finally as a stable isotope of molybdenum (9742Mo55). The cesium lasts for about 30 years (half life) and beta-decays to a stable barium isotope (13756Ba81).

  5. (Adapted from MC 6.5) The half-life of 137Cs (a friend from above) is 30 years. In how many years will a pile of 137Cs that emits 64×1012 electrons per second via beta decay calm down to "only" emitting 2×1012 electrons per second?

Hint: You may benefit from a re-phrasing of problem 6.1 (assigned) as follows:

  1. Calculate the number of joules of energy that can be obtained from the fissioning of 1 kg of 235U, assuming 200 MeV (200×106 eV) average energy release per fissioned nucleus. It is helpful to know that 1 eV is 1.6×10−19 J, and that 235U has a mass of 235 grams per mole, where one mole is 6×1023 atoms.
  2. Using the previous results, how much energy can be obtained from the small amount of 235U contained in one kilogram of natural uranium?



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