Physics 213 General Physics

1. 0 Physics 213General Physics Lecture 26

2. Last Meeting: Nuclear Physics I Today: Nuclear Physics II, Applications Practice Problems 2

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10. Fusion in the Sun • All stars generate energy through fusion • The Sun, along with about 90% of other stars, fuses hydrogen • Some stars fuse heavier elements • Two conditions must be met before fusion can occur in a star • The temperature must be high enough • The density of the nuclei must be high enough to ensure a high rate of collisions 10

11. Proton-Proton Cycle • 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 • 21H is deuterium, and may be written as 21D 11

12. Fusion Reactors • 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 12

13. 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. 13

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15. (Roentgen equivalent man – same biological effectiveness as 1 Rad of x-ray) 15

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20. 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) • 10.7 MeV added • 10.7 MeV given off • 28.3 MeV given off • 0.931 MeV added • None of the above, the reaction is not possible due to conservation of energy.

21. 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) • 2,640 years • 17,200 years • 32,900 years • 45,800 years • 60,300 years