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\noindent {\bf PHYSICS 1020}
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{\Large \bf Homework set 2\\}
{\large 31 March 1997\\}
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\item[{\bf [17.5]}] Use Fig. 17.2 to estimate the percentage of U.S. energy
resource consumption that came from coal in each of the following years: 1900,
1920, 1940, 1960, 1980. Do the same for oil.
  \\
Answer:\\
From the figure, we deduce the following numbers:
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\begin{tabular}{|c|c|c|c|c|c|} \hline
year        &1900&1920&1940&1960&1980\\ \hline
\% from coal& 80 & 70 & 50 & 20 & 20 \\ \hline
\% from oil &  0 & 10 & 30 & 40 & 45 \\ \hline
\end{tabular}
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\item[{\bf [17.6]}] Total U.S. energy use was roughly exponential from 1880 to
1920. Use Fig. 17.2 to estimate its doubling time.  Find its annual percentage
increase.
  \\
Answer:\\     
Energy use roughly doubled from 1880 to 1900, and then doubled again from 1900
to 1920, so the doubling time was roughly 20 years. In section 7.7 it was stated
that the percent increase P in terms of the doubling time T is approximately 
given by 
P = 70/T. This leads to P = 70/20 = 3.5\%.


\item[{\bf [17.9]}] Every heat engine has a thermal energy input, a work output,
and a thermal energy output. At what places in a nuclear reactor 
(see fig. 17.6)  does each of these occur?
  \\
Answer:\\
Thermal energy input is in the core of the reactor; work output is at the
turbine which drives the electric generator;
thermal energy output is at the condenser.

\item[{\bf [17.12]}] Neutrons, created when particles from outer space hit the
atmosphere, often hit Earth's surface. Could these neutrons start a fission
chain reaction in natural uranium in the ground? Why?
  \\
Answer:\\
Since natural uranium contains only 0.7\% of the fissile U-235, the neutrons
from the cosmic radiation could not start a fission chain reaction in the
uranium in the ground. Most of the neutrons generated in a fission process would
be absorbed by a U-238 nucleus, and therefore no chain reaction could be
sustained.

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