lecture 9 nucleus - forces astr 340 fall 2006 dennis papadopoulos
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FIRST IN CLASS EXAM ON THURSDAY OCT. 5 3.30-5.00 PM.
BRING CALCULATOR. YOU CAN HAVE ONE PAGE OF NOTES
EXAMS COVERS ALL LECTURES – GO OVER THE POWERPOINTS IN WEB, CHAPTERS 1- 4 AND CHAPTER 6 PAGES 165-173.
Mass-Energy Equivalence
PREVIEW E=mc2 EQUIVALENCE OF MASS AND ENERGY
MASS+ENERGY CONSERVED
MASS TRANSFORME INTO ENERGY AND ENERGY INTO MASS
E=9x1016 (m/kg) Joules
From Lecture 5 we found energy of a 1kg (e.g. steak) 4.5 MJ (1000 cal)
In chemical reactions we get an efficiency of transforming mass into energy approximately 4.5x106/9x1016 =5x10-11
Chemical bond – Electromagnetic Force – e.g. NaCl
In chemical reactions only the energy stored in outer electrons is released
Nucleus does not play any role
Fig. 13-3, p.468
Nucleus involves only protons and neutrons (nucleons)
Electrostatic repulsion of protons balanced by the nuclear force. Strong but short range – nearest neighbor.
The Nuclear Force
Binding Energy:
A nucleus is dismantled by removing a nucleon at a time andthe amount of work done in the process is measured.
Next if we next reassemble the nucleons in the form of the original nucleus, an amount of energy equal to the work done would be released.
This is the called the binding energy of the nucleus. It indicates howtightly bound is.
Key quantity is the binding energy per nucleon. It is the bindingenergy divided by the number of nucleons.
Curve of the binding energy
DOUBLING TIMES
Doubling time Growth factor 1 21=2 2 22=4 4 24=16 10 210=1024 25 3.3x107
50 1.1x1015
80 1.2x1024
Critical Mass Energy per U235 fission 235 MeV
Energy per nucleon required to put together the nucleus of an element as a function of the mass number
Radioactivity alpha decayRa(226,88)->Rn(222,86)+He(4,2)U(238,92)->Th(234,90)+He(4,2)
Beta decay
C(14,6)->N(14,7)+e-+
Fig. 13-15, p.482
Radioactive Dating
U 238 ->Pb 206 determines when rocks were solidified
3.9 byears, meteorites 4.6 byears