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Saturday Morning Physics Texas A M University Descent into the Proton A Journey Inside an Elementary Particle Dr Rainer J Fries March 22 2008 Zooming in on the World around us Descent into the Proton 2 Atoms 19th century chemistry confirms there are only 92 different elements from hydrogen H to uranium U Everything around us is built from combinations of these elements Democritus Greek philosopher 400 B C All matter is made up of very small indivisible elements He called them atomos 3 Descent into the Proton Atoms Today we can make atoms visible U of Oregon Chemistry Size of the smallest atom hydrogen Sandia National Lab 0 000 000 000 1 m meter 10 10 m 1 Angstrom How is it possible to see such tiny structures Descent into the Proton 4 Scattering Experiments Our vision the eye collects light reflected from objects and our brain processes the information Use this principle Shoot a ray of light or particles at an object Measure the scattered rays with a detector Resolution of the probe light particle is important The wavelength must be smaller than the size of the structure to probe Light wavelength 4000 7000 Angstrom too large to see an atom Better X rays electrons 5 Descent into the Proton Electrons What is electric current In wires there seems to be a flow of very small quantities of negative electric charge carried by tiny particles They are called electrons e In fact these quanta can be extracted from metals by heating them up cathode rays Basic properties of electrons measured around 1900 Electric charge is e e 1 6 10 19 C is called the fundamental charge Mass 1 2000 u 511 keV 1 u is the mass of the hydrogen atom Descent into the Proton J J Thomson 1897 Electrons are small parts of atoms The first subatomic particle was discovered 6 Taking a Look inside an Atom Atoms are neutral If they contain electrons there must be an equal amount of positive charge How does an atom look on the inside Compare the following two scattering experiments Professional scientist

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