Wednesday, April 9, 2008

Announcements:

Assignments:

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 society.