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Modern Physics

Modern Physics. A Presentation by Neil Bronks. Mass Number - Number of protons + Neutrons. Atomic Number - Number of protons. Atoms. C. 12. 6. In a neutral atom the number of electrons and protons are the same. In Carbon it is………. 6. Hydrogen.

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Modern Physics

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  1. Modern Physics A Presentation by Neil Bronks

  2. Mass Number - Number of protons + Neutrons. Atomic Number - Number of protons Atoms C 12 6 In a neutral atom the number of electrons and protons are the same. In Carbon it is……… 6

  3. Hydrogen The simplest atom has one negative electron orbiting one positive proton. The electron is very light compared to the proton. Electron Proton

  4. Helium In this atom we see two neutrons and two protons forming the nucleus. The Neutron has no charge but is the same mass as the proton. Electron Neutron Proton

  5. 4 Forces of Nature (Order of strength) Electromagnetic – Positive and negative • Strong Nuclear – Holds nucleus together - Very Short Range Weak Nuclear- Associated with beta decay • Gravitational - Only Positive - Very long range

  6. Dodgy Beard J’ai fait ça ! Radiation • Decay of nucleus by the emission of a particle or a ray. • Discovered by Henri Becquerel • Units 1 Bq is one decay per second • Natural happens without outside bombardment • Artificial happens due to bombardment

  7. Safety • Wear Gloves or Apron of lead • Don’t point at anyone • Don’t eat!!!

  8. Nuclear Equations • Top and bottom must add up • Top is mass number • Bottom is atomic number • Proton H Neutron electron • And Alpha

  9. Alpha Particles  • Helium Nuclei • Positive Charge • Heavy so not very penetrating • Very Ionizing 147N +42He178O + 11H

  10. Beta Particle ß • Fast electron from the nucleus • Negative charge • Moderately Penetrating • Moderately ionizing 147N 148O + 0-1 ß

  11. Gamma Ray • High energy e-m wave (A Photon) • No charge - not deflected by field • Very penetrating – Need lead to stop most of them • Not very ionizing • Release energy after reaction

  12.   Penetrating Power Al foil Paper Concrete

  13. H/W • LC H 2007 • Q12(d)

  14. Charged particles move in a circular path as the force is always at right angles to the direction of motion- Fleming's Left Hand Rule Particles in Fields    Radioactive Source Cloud Chamber

  15. Click here for internet demo

  16. Ionisation • We can prove that radiation creates ions as we bring a source close to a charged electroscope 

  17. Ionising Power • Alpha is heaviest and so does most damage – poison with Polonium • Beta is only moderately ionising • Gamma is only slightly ionising but difficult to stop

  18. Solid State Detector This a P-N junction in reverse bias. This creates a huge depletion layer. - + P N A piece of radiation passes through the depletion layer and creates enough electron -hole pairs to allow one pulse of current.

  19. Geiger Muller Tube-incident particle ionises gas

  20. H/W • LC Ord 2004 • Q10

  21. Experiments • All experiments the same stick a DETECTOR in front of a source and count the decays. • Move it away for distance and plot • Time for half life and plot • Put things in front for penetration

  22. Penetration A Gieger Muller Tube and Counter. Plot the activity against the thickness or the type of barrier

  23. Distance r A Gieger Muller Tube and Counter. Plot the activity against the distance r.

  24. Half Life Time it takes for half the atoms to decay A Gieger Muller Tube and Counter. Plot the activity against the time

  25. Half-Life –time it takes for half the radioactive particles to decay Atoms Not Decayed Time 1 2 3 4

  26. Half life demo from internet click here

  27. Half Life Calculations 4000 particles time=0 2000 particles time=3s 1 half-life 1000 particles time=6s 2 half-life 500 particles time=9s 3 half-life 250 particles time=12s 4 half-life 125 particles time=15s 5 half-life

  28. Calculations – we use the decay constant λ in our calculations. =0.693/T½ =0.693/3s =0.231s-1

  29. Activity Calculations Rate of Decay =  x number you started with dN/dt = - x N Start with 4000 particles and =0.231 Activity = 4000 x 0.231=924 Bq

  30. Calculations • 1) You start with 100 grams of sulfur-35, which has a half life of 87.51 days. How much time will it take until only 12.5 grams remain? • How many half lives? • 100>50>25>12.5 so 3 half lives • Time = 3 x 87.51 = 262.53 days

  31. Calculations • 2) You measure the radioactivity of a substance, then when measuring it 120 days later, you find that it only has 25 % of the radioactivity it had when you first measured it. What is the half life of that substance? • How many half lives • 100%>50%>25% • 2 half lives =120 days • 1 half life = 60 days

  32. Calculations • 3) Your professor gives you 64g of phosphorus-32 (half life = 14.263 days). • (a) What is its decay constant ? • (b) What is its activity (Rate of Decay)? • (a) Using the formula =0.693/T½ • =0.693/(14.263x24x60x60) • = 6.62 x 10-7 s-1 Must be in seconds

  33. Calculations • 3) Your professor gives you 64g of phosphorus-32 (half life = 14.263 days). • (a) What is its decay constant ? • (b) What is its activity (Rate of Decay)? • = 6.62 x 10-7 s-1 • (b) Using Activity =dN/dt = -N • N= Moles x 6x1023 = 2 x 6x1023 • Activity = 6.62 x 10-7 x 12x1023 = • = 7.3 x1017Bq

  34. Isotopes • Same atomic number different mass number

  35. Isotopes • Same Atomic number different Mass number

  36. %C-14 time Carbon-14 Dating • At death all animals contain the same ratio of C-14 to C-12 • The rate of decay of C-14 is fixed • The C-14 left tells us how long ago it died

  37. Isotope pp

  38. Uses of Radioactive Isotopes • Medicine – treatment and imaging • Smoke detectors • Food Irradiation • Carbon-14 Dating

  39. H/W • LC Ord 2005 Q12(d) • LC Higher 2003 Q11 • LC H 2007 12(d)

  40. Do I look like Freddie? Rutherford Scattering

  41. Rutherford on internet

  42. Rutherford Scattering – alpha particles fired at gold foil. Most pass unaffected - So the nucleus is very small

  43. Rutherford Scattering – alpha particles fired at gold foil. Nucleus A small number of high energy alphas are Deflected Some reflected completely back - Nucleus totally positive.

  44. Rutherford Scattering – alpha particles fired at gold foil. Nucleus A small number of high energy alphas are Deflected More pass unaffected - So the nucleus is very small Some reflected completely back - Nucleus totally positive.

  45. Proton Accelerated by An huge electric Field (700000v) Alpha strikes the ZnS screen Producing a flash that Is seen with the microscope Alpha  Alpha  Lithium Target Cockcroft and Walton Hydrogen discharge tube Al p h a Al p h a a

  46. Internet explanation

  47. Nobel Prize for Physics • Proton + Lithium  2xAlpha + Energy • Proves Einstein’s Law E=mc2 • First Transmutation by artificial Bombardment of an element Ernest Walton

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