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# l22p - Today Announcements HW#9 is due April 2 at 8:00 am...

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ISP209s8 Lecture 21 -1- Today • Announcements: – HW#9 is due April 2 at 8:00 am – Voting for the Spring Break Story contest is due April 2 at 8:00 am – HW#10 will be due April 9 at 8:00 am • Atoms and Nuclei ISP209s8 Lecture 21 -2- History • ca. Fourth century B.C.: Democritus proposes that matter is made of indivisible units called atoma . • 1896: Henri Becquerel discovers radioactivity by chance. • 1911: Ernest Rutherford discovers the nucleus. He found something hard and small at the center of atoms ISP209s8 Lecture 21 -3- From Large to Small Atoms Atomic Nucleus A Proton Made of nuclei and electrons. Size: 10 -9 m Made of neutrons and protons. Size: 10 -14 m Made of quarks. Size: 10 -15 m A neutron has ddu quarks. ISP209s8 Lecture 21 -4- The Strong force at work Strong force actually is mostly the result of exchange of pairs of quarks. Looks like The probability of an interaction decreases exponentially with the mass of the exchanges particle. cm h t c range h t mc t E π 4 4 2 = Δ = Δ = Δ Δ

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ISP209s8 Lecture 21 -5- The Nucleus • The nucleus contains more than 99.95% of an atom’s mass, but only 1E-15 of its volume! •T h e nucleons are held together by the strong force , even though the protons repel each other very strongly. 161 272 111 Rg Mass Number Atomic Number (or Proton Number) Neutron Number Element ISP209s8 Lecture 21 -6- Atomic Nuclei and Elements
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l22p - Today Announcements HW#9 is due April 2 at 8:00 am...

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