Download PHY 231 Lecture 29 (Fall 2006)

Physics 213
General Physics
Lecture 26
Last Meeting: Nuclear Physics I
Today: Nuclear Physics II,
Applications
Practice Problems
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Fusion in the Sun
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All stars generate energy through fusion
The Sun, along with about 90% of other
stars, fuses hydrogen
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Some stars fuse heavier elements
Two conditions must be met before
fusion can occur in a star
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The temperature must be high enough
The density of the nuclei must be high
enough to ensure a high rate of collisions
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Proton-Proton Cycle
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The proton-proton cycle
is a series of three
nuclear reactions
believed to operate in
the Sun
Energy liberated is
primarily in the form of
gamma rays, positrons
and neutrinos
2 H is deuterium, and
1
may be written as 21D
H  11H  12H  e 
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1
H  12H  23He  
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Then
H  23He  24He  e 
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or
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He  23He  24He  2 11H
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Fusion Reactors
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Energy releasing fusion reactions are
called thermonuclear fusion reactions
A great deal of effort is being directed
at developing a sustained and
controllable thermonuclear reaction
A thermonuclear reactor that can
deliver a net power output over a
reasonable time interval is not yet a
reality
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Energy Consumption on Earth
Oil will last ~ 20-- 50 yrs, given the current
rate of consumption of 7 x 1012 J/s.
 Coal will last hundreds of years.
 Uranium (fission) will last thousands of
years.
 Deuterium (fusion) will last hundreds of
million of years.
 Solar energy (?) will last billions of years.
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(Roentgen equivalent man – same biological effectiveness as
1 Rad of x-ray)
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/ yr
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What energy must be added or given off in a reaction where two
hydrogen atoms and two neutrons are combined to form a helium
atom? (Atomic masses for each: hydrogen, 1.007 825 u; neutron, 1.008
665 u; helium, 4.002 602 u; also, 1 u = 931.5 MeV/c2)
1. 10.7 MeV added
2. 10.7 MeV given off
3. 28.3 MeV given off
4. 0.931 MeV added
5. None of the above, the
reaction is not possible due to
conservation of energy.
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If a fossil bone is found to contain 1/8th as much Carbon-14 as the
bone of a living animal, what is the approximate age of the fossil?
(half-life of 14C = 5 730 years)
1. 2,640 years
2. 17,200 years
3. 32,900 years
4. 45,800 years
5. 60,300 years
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