LECTURE 11 COMPOSITION OF MATTTER CLASSICAL EXPERIMENTS PHYS 420-SPRING 2006 Dennis Papadopoulos

EXTRA - PROBABILITY IN PHOTOGRAPHY 1000 PHOTONS PHOTONS

IF MATTER COMPOSED OF ATOMS WHAT ATOMS ARE COMPOSED OF? FARADAY –ELECTROLYSIS THOMSON – ELECTRONS q/m MILLIKAN – ELECTRONIC q RUTHERFORD – NUCLEAR MODEL

DEFINITIONS AVOGADRO’S NUMBER ( N A = 6.022X10 23 ) :NUMBER OF INDIVIDUAL PARTICLES (ATOMS) CONTAINED IN 12 gr OF 12 C. MOLE OF A SUBSTANCE: QUANTITTY OF SUBSTANCE (in gr) THAT CONTAINS N A INDIVIDUAL PARTICLES VALENCE: NUMBER OF ELECTRONS THAT AN ATOM THAT CAN FORM IONIC MOLECULES CAN GAIN OR LOOSE IN FORMING IT. Cl - O 2- N 3- Na + NaClNa 2 ONa 3 N Mg 2+ MgCl 2 MgOMg 3 N 2 Al 3+ AlCl 3 Al 2 O 3 AlN Dalton

Fig. 4-2, p. 109

number of coulombs in a mole mass of liberated substance Cl - -> Cl+e - BaCl 2 Ba has 2N A charges in one mole(137) to pair with N A charges of Cl we need 137/2 Ba to 35.5 of Cl Matter composed of atoms

FOUND THAT NEGATIVE PARTICLES EMITTED FROM METALS HAVE THE SAME q/m. -> ELECTRONS ARE CONSTITUENT OF ALL MATTER

Fig. 4-5, p. 111

Fig. 4-6, p. 111

V, L, and d are all measurable characteristics of the apparatus If you can measure v x, then you can determine e/m. A magnetic field can be used to just cancel the deflection and determine v x.

Determining the electron charge e separately. Spray small droplets of oil which quickly reach terminal velocity due to air resistance. Small number of droplets fall between two plates into to a region of constant electric field. Velocity of fall can be estimated by measuring the time to fall a distance d. Ionizing radiation then charges the droplet, introducing an electric force. Charge is quantized. By measuring the velocity of a number of particles with the field on and off and assuming that the electric charges must be multiples of each other, e can be determined.

Fig. 4-7, p. 114 Measure v t to find r

Fig. 4-7b, p. 114

Fig. 4-8, p. 115

Fig. 4-9, p. 116

Fig. 4-9a, p. 116

Fig. 4-9b, p. 116

a: radius of drop  : viscosity of oil v: terminal velocity Find a by observing drop in free-fall  : density of oil  : viscosity of air Turn on field: Reverse field direction:

80.708s22.335s22.390s22.368s s s34.748s34.762s29.286s29.236s Rise times:

The discovery of the atomic nucleus: Rutherford Back Scattering what Rutherford expected from the “plum pudding” model the clever experiment the (surprise!) result “It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Fig. 4-10, p. 120

Fig. 4-11, p. 121

Fig. 4-12, p. 123

When aiming a beam at a thin sample two processes occur: Particles loose energy due to the material’s stopping power There are head on collisions at high angles due to “Rutherford backscattering” scattering depends on mass/size and charge of nucleus Particles loose energy in glancing collisions and interactions with electrons-particles scattered from deep in material are scattered with less energy.

positive charge is concentrated at the center of the atom in an area ~1/1000 th the size of the atom the mass of the electron is very small compared to the mass of the atom (one thousand times less than the hydrogen atom