11/10/2014

1. 11/10/2014 AQA 2011 Physics Unit 2 This PowerPoint supports sections P2.5 and P2.6 of the 2011 AQA Physics Unit 2 module W Richards

2. P2.5.1 – Atomic Structure 11/10/2014

3. The structure of the atom 11/10/2014 ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”) The nucleus is around 10,000 times smaller then the atom! Atoms always have the same number of protons and electrons so they are neutral overall. They can gain or lose electrons to form ions.

4. Structure of the atom 11/10/2014 A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it… Ernest Rutherford, British scientist: I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment”

5. The Rutherford Scattering Experiment 11/10/2014 Alpha particles (positive charge, part of helium atom) Thin gold foil Most particles passed through, 1/8000 were deflected by more than 900 Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

6. The structure of the atom 11/10/2014 MASS NUMBER = number of protons + number of neutrons 4 He SYMBOL 2 PROTON NUMBER = number of protons (obviously)

7. Mass and atomic number revision 11/10/2014 How many protons, neutrons and electrons? 1 11 16 H B O 1 5 8 23 35 238 Na Cl U 11 17 92

8. Isotopes 11/10/2014 Notice that the mass number is different. How many neutrons does each isotope have? 16 O 17 18 O O 8 8 8 Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. An isotope is an atom with a different number of neutrons: A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

9. P2.5.2 – Atoms and Radiation 11/10/2014

10. Introduction to Radioactivity Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation at random intervals: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactive decay”. This process is NOT affected by temperature or other physical conditions.

11. Ionisation Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons: Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer.

12. Background Radiation 11/10/2014 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power

13. Background Radiation by Location In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event: Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc

14. Types of radiation 11/10/2014 New nucleus 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. Beta particle Unstable nucleus 3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Words – frequency, proton, energy, neutrons, helium, beta Unstable nucleus New nucleus Gamma radiation

15. Changes in Mass and Proton Number 241 α Am Np + 95 90 β Sr Y + 38 Alpha decay: 237 4 93 2 Beta decay: 90 0 -1 39

16. Blocking Radiation    Each type of radiation can be blocked by different materials: Sheet of paper (or 6cm of air will do) Few mm of aluminium Few cm of lead

17. Summary

18. Deflection by Electric Fields + + + + + - 2 protons, 2 neutrons, therefore charge = +2 Alpha and beta particles have a charge: 1 electron, therefore charge = -1 - Because of this charge, they will be deflected by electric fields: - 1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?

19. Deflection by Magnetic Fields + + + + Region of magnetic field Recall: 2 protons, 2 neutrons, therefore charge = +2 1 electron, therefore charge = -1 - Because of this charge, they will also be deflected by magnetic fields: - 1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?

20. Uses of radioactivity 1 Sterilising medical instruments Gamma rays can be used to kill and sterilise germs without the need for heating. The same technique can be used to kill microbes in food so that it lasts longer.

21. Uses of radioactivity 2 - Tracers Gamma source A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected. Tracers can also be used in medicine to detect tumours: For medicinal tracers, you would probably use a beta source with a short half life – why?

22. Uses of radioactivity 3 – Smoke Detectors Alpha emitter +ve electrode -ve electrode If smoke enters here a current no longer flows Ionised air particles Smoke detectors Alarm

23. Uses of Radioactivity 4 - Treating Cancer High energy gamma radiation can be used to kill cancerous cells. However, care must be taken in order to enure that the gamma radiation does not affect normal tissue as well. Radioactive iodine can be used to treat thyroid cancer. Iodine is needed by the thyroid so it naturally collects there. Radioactive iodine will then give out beta radiation and kill cancerous cells.

24. Dangers of radioactivity Alpha Beta Gamma Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia. OUTSIDE the body  and  are more dangerous as  radiation is blocked by the skin. INSIDE the body an  source causes the most damage because it is the most ionising.

25. Half life = radioisotope = new atom formed The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… After 2 half lives another half have decayed (12 altogether) After 3 half lives another 2 have decayed (14 altogether) After 1 half life half have decayed (that’s 8) At start there are 16 radioisotopes

26. A radioactive decay graph 1 half life 1 half life 1 half life Count Time

27. Dating materials using half-lives 1 half life later… 1 half life later… 1 half life later… 2 1 8 4 8 8 8 8 Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer: The sample was originally completely uranium… …of the sample was uranium Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.

28. An exam question… Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years

29. P2.6.1 – Nuclear Fission 11/10/2014

30. Nuclear fission 11/10/2014 More neutrons Neutron Unstable nucleus Uranium or plutonium nucleus New nuclei (e.g. barium and krypton)

31. Chain reactions 11/10/2014 Each fission reaction releases neutrons that are used in further reactions.

32. Nuclear power stations Nuclear power stations use the energy from each reaction to heat water and use the steam to drive turbines:

33. P2.6.2 – Nuclear Fusion 11/10/2014

34. Nuclear Fusion in stars 11/10/2014 Proton Neutron Nuclear fusion happens in stars but it’s not possible to use it in power stations yet as it needs temperatures of around 10,000,000OC

35. The Life Cycle of a Star

36. Stage 1: Nebulae A nebulae is a collection of dust, gas and rock. Some examples of nebulae…

37. Dark nebula

38. Emission nebula

39. Reflection nebula

40. Planetary nebula (This nebula is smaller and will only form a planet)

41. Stage 2: Protostar Gravity will slowly pull these particles together… As they move inwards their gravitational potential energy is converted into heat and a PROTOSTAR is formed

42. Stage 3: Main Sequence In a main sequence star the forces of attraction pulling the particles inwards are _________ by forces acting outwards due to the huge __________ inside the star. Stars are basically ________ reactors that use _______ as a fuel. During its main sequence a star will release energy by converting hydrogen and helium (light elements) into _________ elements and this is why the universe now contains a number of heavier elements. Our sun is an example of a main sequence star – it’s in the middle of a 10 billion year life span Words – heavier, balanced, hydrogen, nuclear, temperatures

43. Stage 4: Red Giant If the star is relatively small (like our sun) the star will become a RED GIANT If the star is big (at least 4 times the size of our sun) it will become a RED SUPERGIANT Eventually the hydrogen and helium will run out. When this happens the star will become colder and redder and start to swell…

44. Stage 5: The Death Red giant White dwarf Black dwarf What happens at this point depends on the size of the star… 1) For SMALL stars the red giant will collapse under its own gravity and form a very dense white dwarf:

45. 2) If the star was a RED SUPERGIANT it will shrink and then EXPLODE, releasing massive amounts of energy, dust and gas. Before After This explosion is called a SUPERNOVA

46. The dust and gas on the outside of the supernova are thrown away by the explosion and the remaining core turns into a NEUTRON STAR. If the star is big enough it could become a BLACK HOLE instead.

47. Stage 6: Second generation stars Our sun is believed to be a “______ ______ star” – this is because it contains some __________ elements along with hydrogen and ________. These heavier elements would have been the products of a previous star that have been thrown out by a ________. These heavier elements are also found on planets, indicating that they might have been made from remains of previous _______ as well. Words – helium, heavier, second generation, stars, supernova The dust and gas thrown out by a supernova can be used to form a new star…

48. The Life Cycle of a Star summary SMALL stars BIG stars Protostar Main sequence Red giant Red super giant White dwarf Supernova Black dwarf Neutron star Black hole Basically, it all depends on the size of the star!

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