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Physics Unit one

Physics Unit one. What is in Unit One?. Nuclear Physics and Radioactivity Electricity One of … Astrophysics Astronomy Energy from the Nucleus…. What is required?. Pass the SACs in the three areas of study Turn up to <90% of classes Complete all weekly homeworks

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Physics Unit one

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  1. Physics Unit one

  2. What is in Unit One? • Nuclear Physics and Radioactivity • Electricity • One of … • Astrophysics • Astronomy • Energy from the Nucleus…

  3. What is required? • Pass the SACs in the three areas of study • Turn up to <90% of classes • Complete all weekly homeworks • Optional … 40+ club homework

  4. What do I need? • Yr 11 Physics Textbook • Scientific Calculator (CAS not allowed in exam) • Workbook

  5. Nuclear physics and radiation

  6. What is an atom Three key historic figures in discovery of what an atom is: • Democritus – concept of atomos • J. J. Thompson – the plum pudding model • Lord Rutherford – the solar system model

  7. Rutherford’s Gold foil experiment A small amount of the radiation is deflected by very large angles Fired radiation at a thin sheet of gold (only 200 atoms thick) Most radiation particles pass through undeflected

  8. Rutherford’s gold foil experiment • He concluded two things: • 1) The atom is mainly empty space • 2) It has a small, but dense and positive nucleus.

  9. Activity – work in groups • Your teacher wants to make a scale model of an atom for the classroom. We want to use a tennis ball for the nucleus and marbles will be the orbiting electrons. If the size of a nucleus is r = 1x10-14m, and the electrons orbit at a distance of 1 x 10-10m; how far away do my electron marbles have to orbit the tennis ball?

  10. Activity: Extension • How small will the scale model nucleus have to be such that the entire scale model atom will fit in the classroom?

  11. Electrons orbit the nucleus The atom Nucleus: Made of neutrons and protons

  12. The atom Particles that live in the nucleus are called nucleons. Protons and neutrons are nucleons.

  13. Electromagnetic Force and Nuclear Force • Protons have the same charge, and just like having two magnets with similar poles, protons repel each other • How does the nucleus stick together? • There is a force called the strong nuclear force. It only works at very small distances (when nucleons are almost touching). • Adding neutrons to a nucleus increases the attractive strong nuclear force without increasing the repulsive electromagnetic force

  14. Electromagnetic Force and Nuclear Force • This is similar to adding a piece of metal between “like” poles of two magnets. It helps them stick!

  15. Isotopes • Isotopes are atoms of the same element that differ in the number of NUETRONS they have. • Isotopes have similar chemical properties but different physical properties. Example: • Carbon has 3 isotopes: Carbon -12, 13, and 14. All three isotopes have 6 protons. • C-12 has 6 neutrons. • C-13 has 7 neutrons. • C-14 has 8 neutrons.

  16. Describing an atom Z E Mass Number: Protons + Neutrons A Symbol of element Atomic Number: Protons This is the isotopic symbol for an atom

  17. Isotopic Symbol • Why are there mass numbers with decimal places on the periodic table? eg • How can you half a neutron? • On the periodic table, the mass numbers are called the relative atomic mass numbers. • They are an average of all isotopes of that element found on earth.

  18. Questions: Complete the table

  19. Radioisotopes • Sometimes the nucleus of a particular isotope is unstable, and it may undergo a radioactive decay or transmutation • This is known as a radioisotope • Radiation is emitted • Every element heavier than Bismuth (A = 83) is radioactive. • Every element heavier than Uranium (A = 92) has to be produced artificially.

  20. Nuclear Radiation • Three types of radiation can be emitted in nuclear decay • Alpha (α) • Beta (β) • Gamma (γ)

  21. Radiation Two protons, two neutrons (helium nucleus) electron energy +2 -1 0 None Light (0.0005 a.u.) Heavy (4 a.u.) 5MeV 1MeV 0.1MeV

  22. Radiation Many metres 1 or 2 metres Few cm Sheet of paper Many cm of Pb cm of Al Poor Reasonable High 0.1c 0.9c c c = the speed of light

  23. What is an electron volt? (eV) • We normally measure energy in the unit Joules (J) • Atoms and electrons often contain very small amounts of energy if measured in Joules (~10-19J) • 1eV = 1.6x10-19J • It is the energy 1 electron gains after being accelerated by 1V

  24. Questions • 1) Convert into Joules • 5eV b) 1keV c) 3MeV d)0.2keV • 2) Convert into eV a) 3.2x10-19J b) 6.4x10-14J c) 10-15J

  25. Prac: Detecting radiation • Cloud chamber and glow-in-the-dark and geiger

  26. How radiation is detected This collisions causes an electron to be ejected. It is attracted to the positive anode • 1) Geiger counter When it touches the anode, it becomes an electrical pulse. The counter counts these electrical pulses Incoming radiation strikes an atom in the Geiger counter

  27. How radiation is detected • 2) Cloud Chamber Incoming radiation Super saturated alcohol vapour The radiation ionizes some of the alcohol atoms Clouds form around ionized alcohol atoms

  28. How radiation is detected • 3) Fluorescent Materials – Some materials (like glow in the dark stickers, or phosphorus screens) will emit light when bombarded with nuclear radiation.

  29. Decay equations Polonium-210 decays via alpha emission. What product is formed? • The mass numbers must balance on both sides of the equation • The atomic numbers must balance on both sides of the equation x= 206 z= Pb y= 82

  30. Decay equations • Carbon-14 decays via beta emission. Note about beta decay: An antineutrino ( a particle of very small mass, but no charge) is also emitted. Symbol: ῡ x= 14 z= N y= 7

  31. Decay equations • The product nucleus is called the daughter nucleus • Any further decay will lead to a grandaughter nucleus

  32. Questions: Write equations for the following: • 1) Uranium-238 decays via alpha emmision • 2) Francium-222 decays via beta emission • 3) Polonium-214 decays via alpha emission • 4) Carbon-12 emits gamma radiation • 5) A new isotope is formed by adding a neutron to U-238

  33. Decay equations prac

  34. Half-life Prac

  35. Half life • It is random when a radio-isotope might decay. • However, with a large number of atoms, they obey an exponential decay. • Half the atoms have decayed in one half life. • Half the remaining atoms have decayed in a second half life.

  36. Half Life Activity is a measure of atomic decays. • Units are Becquerels • 1 Bq = 1 decay per second

  37. Half Life Questions • From the following graph, estimate the half life of the sample. • Using this half-life, estimate at what time the sample will have an activity of 100 kBq

  38. Half Life Questions • A hospital keeps a sample of Iodine-131 (I-131) for use in radiation therapy. It is noted that the activity of the sample has reduced by 75% in 17 days. What is the half life of I-131?

  39. Half-Life Question • Carbon-14 is a radioisotope that is useful in obtaining the age of carbon containing materials (such as wood) up to about 50’000 years. Carbon-14 is a naturally occurring radioisotope that plants will absorb from the atmosphere, along with the non-radioactive isotopes carbon-13 and carbon-12. After the plant dies, it stops absorbing carbon, and the carbon-14 within the plant decays away. Carbon-14 decays through beta decay with a half-life of 5730 years.

  40. Half-Life Questions • A team of archaeologists have discovered the remains of what looks to be a fire-pit used by primitive homo sapiens. They measure the amount of carbon-14 in samples taken from the fire-pit and find the measured ratio of carbon-14 to be approximately 6.25% of the ratio of atmospheric carbon-14. • Approximately how many half-lives have elapsed since this sample was alive? • Approximately how old is the sample?

  41. Half-Life Questions

  42. Radiation Isn’t always a bad thing! Has many uses • Medical Imaging and treatment • Carbon Dating • Energy production (nuclear fission) • Smoke alarms • But in large quantities it can be harmful…

  43. Which radiation is the most harmful? • Alpha radiation has the highest ionizing ability. • Ionizing: The radiation removes electrons from atoms in our cells/DNA. This causes the cells/DNA to mutate/die. • However, alpha radiation is also the easiest to shield • Our skin will effectively shield it. • However, if we breath it in, or eat contaminated food = GAME OVER!

  44. Which radiation is the most harmful? What radiation are these Fukishima workers being protected from? How do you know?

  45. Absorbed Dose • The absorbed dose is the amount of radiation energy that has been absorbed per kilogram. • Units: Gray (Gy). 1 Jkg-1 = 1 Gy

  46. Dose Equivalent • Different forms of radiation have different ionising abilities, and so cause varying amounts of damage to humans. • As each type of radiation can affect tissue differently, the Dose Equivalent is used compare accurately radiation effects.

  47. Dose Equivalent • Units: Sieverts (Sv)

  48. Effective Dose • Different organs in the body have different sensitivities to radiation doses. eg. A person’s lungs would be more likely to develop cancer than the liver if they were both given the same amount of radiation. • W = Weighting • Σ = “the sum of”

  49. Effective Dose

  50. Effects – Short Term • Called Somatic effects • Radiation ionizes molecules, causing cells to mutate or die • Immediate Sickness/Death

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