| Energy question... |
[Dec. 22nd, 2009|10:45 am] |
So I'm finishing up a course on Physics, Energy & the Environment, and I have a few problems giving me a really hard time. I've approached the one that follows in several ways and am not coming up with anything near the answer I am supposed to get.
"The oceans contain about 1.3x10^24cm^3 of water. Deuterium constitutes .028% (.00028) by MASS of natural Hydrogen. What is the total energy (in Joules) available from this deuterium via D-D fusion? Assume 4.0 MeV per fusion event. (ans 3.9x10^30J) For how many years could fusion reactors of 50% efficiency supply a power requirement of 2.0 million MW?" (ans. 3x10610 years)
Ok, so I know I have to figure out how much H is in the water to figure out how much D is in the water, so I can't just multiply 1.3x10^24 by .00028, but using 11.11% (2 H to 16 O in H2O by mass) got me an end result way too small. I also know to convert MeV to J (1.062x10^-13), and I guess we're assuming all of the D will be fused. None of my answers come close, and the book doesn't have a similar example. I can't ask my prof about this because he's just not helpful and I'm p.o.'d at him and that's another story. Anyone have thoughts on setting this up? Seems it should be as easy as getting the amount of H from the water, multiplying that by .00028, multiplying that by 4MeV and tada! but if I don't get the first steps right, we all know what happens after that. I don't think I need Avogadro's number as I don't have moles (and this book hasn't used it really). The book also mentions earlier that D constitutes one in 7000 H, so @3500 water molecules are HDO but I don't think this is useful. I'd like to believe I'm missing something obvious here. Ideas?
Thanks! |
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