PHYS1211—part 3; nuclear • Chapters 13, 14, and 15 of the textbook: “Energy—Its Use and the Environment”. • Prof. Michael Ashley, Room 137, Old Main Building,
[email protected], pph 9385-5465. • http://mcba11.phys.unsw.edu.au/˜mcba/PHYS1211/a.pdf The atomic hypothesis • Question: is matter indefinitely divisible? • Democritus (ca. 420 BC) proposed that all matter was composed of an indivisible component: at atom. • Aristotle: (384 BC–332 BC) matter composed of four “elements”: air, fire, water, earth. • Dalton (1766–1844) proposed that each element (lead, aluminium, hydrogen, etc) consisted of only one kind of “atom”, unique to that element. • Dalton conducted chemical experiments, John Dalton e.g., combining hydrogen & oxygen in precise ratios to produce water, which strongly supported his atom hypothesis. The electron charge-to-mass ratio Thomson’s experiment • J. J. Thomson measured the charge-to-mass ratio of the electron in 1897 using an evacuated tube. http://www.youtube.com/watch?v=IdTxGJjA4Jw • Electrons (blue) are produced by heating a filament, and are then accelerated by a voltage to the right. • Electric & magnetic fields deflect the electron beam. • From the strength of the two fields, the q/m ratio can be found. • We can do this experiment in the UNSW 2nd year laboratory. Protons and neutrons • All nuclei are composed of two particles: I protons—positively charged, with exactly the opposite charge of the electron, I neutrons—no charge • The masses of these particles are: I proton—1.0072766 amu I neutron—1.0086654 amu I electron—0.0005486 amu where an atomic mass unit (amu) is 1.66×10−27 kg.