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LECTURE 7

PHYS 420-SPRING 2006 Dennis Papadopoulos. LECTURE 7. ENERGY TRANSFORMATIONS. Energy : The measure of a system’s capacity to do work. Units of Energy: Joule = Nt x m, eV= 1.6 x 10 -19 J, Cal = 4.2 x 10 3 J. Examples : It takes 100 Joules to lift 10 kg by 1 meter against the

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LECTURE 7

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  1. PHYS 420-SPRING 2006 Dennis Papadopoulos LECTURE 7 ENERGY TRANSFORMATIONS

  2. Energy : The measure of a system’s capacity to do work • Units of Energy: Joule = Nt x m, eV= 1.6 x 10-19 J, Cal = 4.2 x 103 J • Examples : It takes 100 Joules to lift 10 kg by 1 meter against the • Earth’s gravity (g= 10 m/sec2); It takes .4 MJ to accelerate a 1000 kg • car to 30 m/sec (105 km/hr); It takes about 1010J to accelerate a • missile to 5 km/sec. [E=1/2 M(kg) v2 (m/sec) J] • Chemical Energy Storage : Chemical energy is stored in the • chemical bonds of molecules. As an order of magnititude a few eV • per bond. A 1 kg steak store approximately 1000 Cal or approximately 4 MJ. This is the amount (4-10 MJ/kg)stored in one kg of chemical explosives (TNT). Also a typical battery has few MJ of stored energy. • Energy Transformations: Energy has many forms, e.g. potential, • kinetic, chemical,acoustic, radiation, light etc. Each can be transformed • to the other, but overall energy is conserved.

  3. Explosion : In ordinary, e.g. TNT, explosion chemical energy is • transformed in kinetic energy of the fragments, acoustic energy of • the snow-plowed air (shock or blast wave) and some radiation from • the heated air. In an explosion we deliver the energy fast. • How fast and why? Faster than it takes for the air to push back. Power: P= Energy/time. Units are Watt = J/sec. Hair dryer 1 kW, Light bulb 100 W. In ordinary explosives the trigger time is of the order of the size of the explosive divided by the speed of sound (about .3-1 km/sec). • Overpressure: Atmospheric pressure is 1 atm, corresponding • to a force per unit area of one 105 Pascals ( N/m2) (14psi). Overpressure • is the difference between blast wave pressure and atmospheric • pressure. Overpressure of 1/3 of atm starts to create damage. For • a 1 kT explosive this occurs inside a radius of 1-2 km. For a 1 MT • it will occur over 15-20 km.-

  4. All matter is an Assembly of Atoms

  5. CHEMICAL BINDING ENERGY TWO OXYGEN ATOMS ATTRACT EACH OTHER TO FORM O2 WHILE RELEASING 5 eV OF ENERGY. THEREFORE 2 OXYGEN ATOMS ARE HEAVIER THAN AN OXYGEN MOLECULE BY Dm= 5 eV/c2 =9x10-36kg MASS OF OXYGEN MOLECULE IS 5x10-26kg. Dm/m=2x10-10 FORM 1 GRAM OF O2 AND GET 2x104 JOULES

  6. Atomic Mass Unit = 1.66x10-27kg = =930 MeV/c2 ISOTOPES -> same Z different A ISOTONES -> same Z different N

  7. Rare Earths Actinide

  8. Binding Energy: A nucleus is dismantled by removing a nucleon at a time and the mount 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 how tightly bound is. Key quantity is the binding energy per nucleon. It is the binding energy divided by the number of nucleons. Curve of the binding energy

  9. CURVE OF BINDING ENERGY SPLIT 160 TO 100 AND 60 HOW MUCH ENERGY I GET BACK? (60x8.8 + 100x8.6)- 160x8.2=76 MeV=DMc2

  10. FISSION One case of the fission of 236U. The net mass of the initial neutron plus the 235U nucleus is 219,883 MeV/c2. The net mass of the fission products (two neutrons, a 95Mo nucleus and a 139La nucleus) is 219,675 MeV/c2 - smaller because of the stronger binding of the Mo and La nuclei. The "missing mass'' of 208 MeV/c2 goes into the kinetic energy of the fragments (mainly the neutrons), which of course adds up to 208 MeV.

  11. 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 Energy per U235 fission 235 MeV Critical Mass

  12. Radioactivity alpha decay Ra(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-+n

  13. PAIR PRODUCTION Electron-positron PAIR PRODUCTION by gamma rays (above) and by electrons (below). The positron (e+) is the ANTIPARTICLE of the electron (e-). The gamma ray (g ) must have an energy of at least 1.022 MeV [twice the rest mass energy of an electron] and the pair production must take place near a heavy nucleus (Z) which absorbs the momentum of the g .

  14. BOHR -> RADIOACTIVE PROPERTIES DUE TO NUCLEUS BUT CHEMICAL DUE TO NUMBER AND DISTRIBUTION OF ELECTRONS CHEMISTRY BECAME PHYSICS

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