a-brief-history-of-physics

A Brief History of Physics (in 23 PowerPoints) Two pillars of physics Matter Energy Matter = Stuff Properties of matter • Takes up space (volume) • Has weight (today physicists would say “mass”) Density = mass / volume A digression on units Scientists use SI units (Système International) [derived from the metric system] Length: metre (m) [1/10,000,000th of distance from pole to equator] Volume: derived from length units [e.g., cubic decimetre (dm3), or litre (l)] (1 dm = 0.1 m = 10 cm) Mass: kilogram (kg) [originally the mass of litre of water] Therefore density has units like kg/m3 or kg/dm3 (= kg/l) Time: second (s) Matter is composed of atoms ? Democritus Kinds of atoms [Elements] Properties of atoms • Almost indestructible (but in the late 19th century, some were found to naturally self-destruct [radioactivity], and in the 20th century, we humans learned how to destroy them) • Radius: ~50-250 pm (picometer = 10-12 m) • Mass: ~10-27-10-25 kg (about 1-250 times the mass of a hydrogen atom, or ~1-250 u [u = unified atomic mass unit]) But what are atoms made of? Electron • From the Greek word for amber [ήλεκτρον] • Electrons can be easily knocked out of atoms (e.g., by rubbing with fur) • Mass is slightly more than 1/2000th of the mass of the lightest atom (hydrogen) • Carries one basic unit of negative electric charge • Given the symbol e– But what are atoms made of? • Atoms are electrically neutral, so they must contain a positive charge equal to their number of electrons • At first, this was thought to be uniformly distributed throughout the atom • Then in 1912, Ernest Rutherford discovered that the positive charge was located in a tiny region in the centre of the atom, its nucleus But what is the nucleus made of? Proton • Mass about the same as a hydrogen atom (~6×1023 [Avogadro’s number] protons make one gram) • Carries one basic unit of positive electric charge • Given the symbol p+ But what is the nucleus made of? • A hydrogen atom contains one proton (99.9+% of its mass) and one electron • Opposite charges attract, keeping the two together to form the hydrogen atom • Heavier atoms were assumed to have more protons and electrons • But like charges repel, so what keeps the protons together in the tiny nucleus? But what is the nucleus made of? Neutron • Finally in 1932, James Chadwick discovered the nuclear glue—the neutron • Mass is ~1% greater than the proton’s • Zero electric charge (neutral) • Given the symbol n0 • Outside a nucleus, the neutron breaks down in ~15 minutes to a p+ and an e– We’re ready to build an atom • Decide what kind of atom you want, and find its atomic number (Z) in the periodic table • Count out Z protons • You will also need at least Z neutrons for glue—more for heavier atoms (about 50% more for lead) • Too few (or too many) neutrons will make your atom unstable (radioactive) • Combine the protons and neutrons to make your nucleus • Add Z electrons for electrical neutrality—they will automatically take up positions around the nucleus • Any atom with Z>83 will be radioactive (as will Z=43 or 61), but some will survive for a reasonable amount of time (say a few billion years) • Enjoy your new atom! Example: Building a helium atom Nucleus much too large! Building blocks of matter • Protons (p+) found in nucleus; mass ≈ u (1.007 276 u) • Electrons (e-) surround nucleus; mass ≈ 1/2000 u (0.000 549 u) • Neutrons (n0) found in nucleus; mass ≈ u (1.008 664 u) decay to p+ + e- • Neutrinos (ν) [Greek nu] emitted in nuclear reactions; mass