Forces in the nucleus

1. Forces in the nucleus • 3 forces • Electromagnetic (+ protons and – electrons) • Strong nuclear forces (hold together nucleus) • Weak nuclear forces (beta decay)

2. What is radioactivity? • Unstable nucleus • Emits particles or energy • To become stable • The nucleus of uranium-238 is radioactive. • Emits radiation (particles) • Transforms into a stable atom • Uranium-238 lead-206 • Takes about 5 billion years

3. Unstable isotopes emit 3 kinds of radioactive decay • Alpha decay • 2 protons & 2 neutrons released • Called alpha particle • Beta decay • Neutron splits into a proton and an electron • The proton stays in the nucleus but the electron is emitted – beta particle • Gamma decay • Release of high-energy, electromagnetic radiation from the nucleus of an atom

4. http://www.gcsescience.com/prad17-measuring-half-life.htm Half-life (nuclear reactions lab) • Length of time for half a sample of radioactive material to decay

5. Station A – Half life • What type of decay has Cesium undergone? • β • How do you know? • Mass unchanged, atomic number increased by 1 (neutron turned into a proton + electron) • If you started with 60 grams of Cesium-137, how many grams would be remaining after 30 years? 30 grams 60 years? 15 grams 90 years? 7.5 grams • How long would it take to decay 90% of the Cesium-137? • Between 90-120 years

6. B Questions • A radioactive substance has a half-life of 30 minutes. What fraction of the atoms will not have decayed after 1 hour? • 1/4 • Another substance has a half-life of 3 hours. What fraction of the atoms will have decayed after 6 hours? • 3/4 • A researcher measures 200 counts per minute coming from a radioactive source at midday.At 3 o'clock, she finds that this has dropped to 25 counts per minute.What is the half-life of the radioactive source? • 1 hour

7. C Questions • What is the isotope? • Sodium-24 • Using this graph, determine the ½ life of the substance shown. • 14-15 hours • How much radioactive substance is remaining after 30 hours? • 100 counts per second • How much radioactive sample has decayed after 60 hours? • 400-24=376 counts per second • How many neutrons are in this isotope of sodium? • 13

8. m

9. Nuclear reactions • Fission • Split one atom to make 2 • Fusion • Fuse 2 atoms to make one +

10. Nuclear fusion in the sun

11. light Wave or Particle? • Thomas Young’s double slit experiment • Interference pattern • Diffraction • Interference • http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/ • Photoelectric effect • High energy photons bounce off • Photon – particle with no mass • http://www.lpscience.fatcow.com/mgagnon/Photoelectric_Effect/photoelectriceffect1.htm

12. Photoelectric effect proved light travels as a particle • Blue light • Red light • Light particles are called photons • No mass therefore no momentum • Travel at near speed of light • A photon does not obey the laws of conservation of mass & energy

13. Thomas Young’s experimentproved light travels as a wave http://www.cavendishscience.org/phys/tyoung/tyoung.htm http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/ http://vsg.quasihome.com/interfer.htm http://video.google.com/videoplay?docid=-4237751840526284618

14. Quantum model of the atom • Predicts only probability of location of the electrons in orbitals • http://science.howstuffworks.com/atom8.htm Bohr Model of Atom

15. Heisenberg Uncertainty Principle http://www.aip.org/history/heisenberg/p08.htm • Both momentum and position cannot be known when dealing with subatomic particles • You can know one, but not both!

16. What is space-time?And how does it get warped? Space-time is the combination of 3 dimensions of space and 1 dimension of time and can be warped or distorted by large masses

17. Light bending due to warping of space-time

18. Twin paradox • http://www.pbs.org/wgbh/nova/einstein/hotsciencetwin/ • Time dilatation • http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm