Friday, April 11, 2008
Announcements:
- Peter Bertone is teaching class while I am away.
- Our next exam is scheduled for Monday, April 21. If this date
does not suit you, please let me know.
Assignments:
- Read Chapter 17 and answer the recommended homework questions.
- Continue working on your Web Project, and send me the URL for
your group's website once it is posted.
- For 2 days worth of class participation credit, you are
encouraged to create your own personal website in your UNC server space
(www.unc.edu/~onyen). This website does not have to be anything fancy,
just something other than the current default message that appears for
a blank webpage. Once you have completed this assignmment, send
me your URL.
Chapter 16 - Fusion and Fission
Nuclear Fusion occurs
when two small nuclei (like H and He) are brought close together
(within ~fermi) so that the strong nuclear
force can fuse the nuclei together to form a single larger
nucleus. Energy is
needed to overcome the Coulomb repulsion of the two positive nuclei,
but the final product results
in a net release of energy (an exothermic reaction). This is the
process that fuels our Sun.
Nuclear Fission is
the process where a large nucleus (like that of uranium or plutonium)
captures a neutron and then divides
into two smaller "daughter" nuclei. When this happens, two or
three neutrons are typically released, and this can result in other
fission reactions (chain reaction). This process is used in
nuclear power plants and nuclear bombs.
Sample nuclear reaction
equations - mass and energy must balance
Nuclear binding energy - see Fig. 16.4
Does a nucleus have more mass than its constituent parts?
Why is more thermal energy needed to fuse He atoms than H atoms?
What is special about the radioactivity of U-235?
What kind of radiation is associated with the emission of neutrons?
How are the control rods in a nuclear reactor used?
Is it possible for a nuclear reactor to explode?
Is plutonium radioactive?
What kind of nuclear bomb would terrorists be likely to build?
What is the most likely kind of nuclear terrorism?
What is meant by "weapons grade nuclear material"?
The critical mass of enriched U-235 is 25 kg (about the size of a
cantelope), while only 8 kg of Pu-239 is needed (about the size of a
large orange).
What is meant by the term "critical mass"? How is this term used
in other contexts relating to people?
Video: Fat Man and Little Boy
- Scene
15 (Not Responsible), ~4:00 min
At the beginning of this scene, the post-doc
from the University of Chicago, Michael Murrayman, (John Cusack)
questions Robert Oppenheimer about the ethics of their project, to
which Oppenheimer responds that "we are responsible for solving a
technical problem, to build this device. We are not responsible
for its use." What do you think about this? Who is
responsible for the application of scientific knowledge? As
graduation approaches, you might consider joining the Graduation Pledge Alliance
which was initiated at my alma mater, Manchester College.
In the second part of this scene, there is a
radiation lab accident where a radioactive source goes
"critical". Michael tells everyone in the lab to mark their
locations. Why? Michael's radiation exposure was more than
1000 rad. This is about three times higher than the LD50 limit of
300 rad, which means that 50% of the people exposed at this level will
die within 60 days. The LD100 limit (Lethal Dose for 100% of the
exposed population even with the best available treatment) is 800 rem,
so this is why Michael states that everyone in the room will live
except for him.
Science provides us with knowledge, but each individual person has a
responsibility to evaluate the merits of this information and decide
how this information will be used. This challenge is inherent in
all real-world problems (not just those related to physics), and this
is why evaluation is the highest level of cognitive reasoning (per Bloom's
taxonomy). Hopefully this
course has improved your critical reasoning skills to make you a
well-functioning member of our society.