PHYS 202. Lecture 24 Professor Stephen Thornton April 27, 2005

Transcription

1 PHYS 202 Lecture 24 Professor Stephen Thornton April 27, 2005

2 Reading Quiz 44 20Ca has how many neutrons? 1) 44 2) 24 3) 20 4) Can t be determined

3 Answer: 2 Mass number is 44 Atomic number is 20 Neutron number is 24

4 Final Exam Friday, May 13, pm, in this room 50 multiple choice questions. There will be additional questions from Chs You may bring 2 sheets of paper with equations, constants, etc. Must have calculator, but you may not use constants, equations, etc. in calculators.

5 Teaching Help: Doing homework problems is essential. I prefer the following method: 1) Honor system. Assign problems, but do not collect or grade them. 2) Turn in problems at discussion section. Have TA grade one random problem per week. 3) Use WebAssign where we have feedback.

6 Teaching Help: The thing I dislike most about doing WebAssign problems is 1) that they count towards my grade. 2) they are due every week. I would rather them be due every other week. 3) there is not enough office hour help from TAs or professors. 4) really nothing.

7 Teaching Help: The thing I like best about WebAssign is that 1) I receive immediate feedback as to whether I got the problem correct. 2) I receive multiple submissions. 3) random numbers makes students more honest. 4) all three of the above. 5) 1) and 2) above.

8 Teaching Help: I would prefer WebAssign problems be due on 1) Monday 2) Tuesday 3) Wednesday 4) Thursday 5) Friday

9 Teaching Help: Consider the following statement: I think using WebAssign problems is overall a good practice and helps learning. 1) Yes. 2) No.

10 Teaching help Which statement best describes your feelings about the Reading Quiz? 1) They are helpful, because they make me prepare for class. 2) They are okay, but I think they should count more than 2% of our grade. 3) They are okay, but I don t think they should count as much as 2% of our grade. 4) I dislike them, because they make me come to class, and I don t need to.

11 Teaching Help Which combination of credit towards final grade should the Reading and Conceptual Quizzes count? 1) RQ = 2%, CQ = 1% 2) RQ = 1%, CQ = 2% 3) RQ = 3%, CQ = 1% 4) RQ = 1%, CQ = 1% 5) RQ = 2%, CQ = 2% (this semester)

12 Teaching evaluations should now be available for you to fill out on the web. Please do so.

13 Last Time Review hydrogen spectra Rydberg equation Bohr model of the atom Quantum mechanics and quantum numbers Pauli exclusion principle Periodic table Atomic radiation Lasers Fluorescence and phosphorescence

14 Today Nuclear constituents and properties Radioactive decay Radioactive decay law 14 C radioactive dating Nuclear binding energy Nuclear fission and reactors Nuclear fusion and reactors Energy content of fuels Work problems

15 A = Z + N Nuclear constituents Mass number = atomic # + neutron # Mass of 44 Ca A X 20 Z u = x kg 1 u = MeV/c 2 C is 12.0 atomic mass units Nuclear force is very strong; n-n, n-p, p-p

16 Nuclear Density r ( m) = A 15 1/ 3

17 N and Z for Stable and Unstable Nuclei

18 Alpha Decay of Uranium-238

19 Beta Decay of Carbon C N + β + ν β decay (electron) O N + β + ν β decay (positron) N + e C + ν Electron capture

20 Energy of Electrons Emitted in β Decay. This curve led to discovery of neutrino. n p+ e + C N + e + ν e * 6 7 ν e

21 Radioactive Decay Series of 23592U

22 Gamma decay Gammas are electromagnetic radiation that originate from the nucleus. Another photon. 14 N* N + γ

23 Show Geiger counter and radioactive pieces: Fiestaware - uranium oxide lantern mantle thorium radium clock faces smoke detectors 241 Am (alpha)

24 Tossing Coins as an Analogy for Nuclear Decay Let's do experiment. Stand up and flip a coin.

25 Activity decays per second 1 curie = 1 Ci = 3.7 x decays/s 1 becquerel = 1 Bq = 1 decay/s λ = decay constant T T 1/2 1/2 = time for half of nuclei to decay = ln = λ λ λt 0 0 N = N e = N e tt / 1/2

26 Dependence on the Decay Constant Large decay constant λ, small T, and more rapid decay. 1/2

27 n C-14 radioactive dating CO 2

28 Activity of Carbon-14

29 Bones Iceman Died in Italian Alps during Stone Age over 5000 years ago.

30 Conceptual Quiz Which of the following particles is not likely to be emitted by the nucleus as a decay product? 1) alpha particle 2) electron (beta) 3) gamma 4) x-ray 5) positron

31 Answer: 4 When electromagnetic radiation is emitted from the nucleus, we call it a gamma ray, not an x ray.

32 Conceptual Quiz: Which of the following would not be used for radioactive C-14 age dating? 1) feather 2) mammoth bone 3) tooth 4) arrowhead 5) deer hide moccasin

33 Answer: 4 In order to use radioactive C-14 dating, the object must have been originally a living object that utilized carbon in CO 2.

34 The Concept of Binding Energy Ka-zaam! Lower mass E = ( mc ) 2 binding energy

35 Binding Energy of Nuclei Fission of heavy masses leads to energy release Fusion of light masses leads to energy release

36 235 U 236 U Nuclear Fission Ba 236 U * Kr 36

37 A Chain Reaction Do demo mousetrap atomic bomb

38 Do nuclear reactor simulation mos/applets/reactor.html Mike Timmins simulation

39 Boiling water reactor

40 Pressurized steam reactor

41 Natural nuclear reactor Oklo uranium mine in Gabon. 2 million years ago. How did it work?

42 Binding Energy of Nuclei Fission of heavy masses leads to energy. Fusion of light masses leads to energy

43 Proton-proton cycle

44 Fusion reaction 2 H 3 H

45 Possible fusion processes

46 Plasma

47 Toroid magnetic confinement

48 Possible fusion reactor

49 Z machine, Sandia Labs

50 Energy content of fuels

51 Work Problems 32-26, may not have time