WJEC Physics for AS: Student Bk

80 Terms & definitions Terms & definitions Terms & definitions Until the late 19th century, the atom was regarded as an elementary particle. The periodic table of the elements, first published by the Russian chemist, Dmitri Ivanovich Mendeleev in the 1860s, strongly suggested an underlying structure to atoms and by the end of the century, the negatively charged electron had been identified as a universal component of atoms. The positively charged atomic nucleus, which contained virtually all the mass of the atom, was discovered from the work of Rutherford, Geiger and Marsden between 1908 and 1913 and both its main constituents – protons and neutrons – identified by the early 1930s and initially thought to be elementary. The development of understanding of atomic structure since that time is shown schematically for a deuterium (heavy-hydrogen) atom in Fig. 1.7.1. With the development of quantum theory in the 1920s it was realised that electrons occupied a region a fraction of a nanometre across which is typically 100 000 × the diameter of the nucleus. This was shown in the 1930s to consist of protons ( p ) and neutrons ( n ), which are collectively known as nucleons. The results of collision experiments known as deep inelastic scattering in the 1960s and 1970s showed that nucleons are composed of 3 particles called quarks bound together by the so-called strong interaction . These quarks are thought to be elementary particles. The middle decades of the 20th century produced a veritable smorgasbord of particles, called variously hadrons, fermions, bosons, mesons, nucleons, baryons, neutrinos…. The standard model of particle physics goes a long way towards simplifying this picture: this will be described briefly here. 1.7.1 The standard model – three generations of leptons and quarks Almost all the normal matter in the universe (that is, ignoring the mysterious dark matter which is dealt with later) is composed of heavy protons and neutrons and the much lighter electrons. We infer the existence of other almost massless particles called neutrinos (see Section 1.7.4) and notice that, if we bash protons and neutrons together, we produce showers of other intermediate mass particles, which we call mesons . When cosmic ray particles collide with atoms in the upper atmosphere they produce showers of particles, called muons, which form part of the background radiation which we can detect using a Geiger-Müller tube. We now know that the electrons, muons and neutrinos (and some others) are elementary particles, which we call leptons . The other particles, the heavy ones, are called hadrons , and are not elementary – they are composed of combinations of quarks. Table 1.7.1 contains the elementary particles in the standard model . 1.7.2 Units of mass and energy Particle physicists usually express energy in electron volts ( eV ) or its multiples, keV , MeV , GeV and TeV . This makes calculations of kinetic energy gain very straightforward: an electron accelerated through a pd of 100 V  gains a kinetic energy of 100 eV ; a particle with a charge of 2 e (e.g. an alpha particle) would gain a kinetic energy of 2 × 100 = 200 eV . 1.7 Particles and nuclear structure The word atom comes from the Greek word atomos (atomos) which means indivisible. A particle is elementary (or fundamental) if it is not a combination of other particles. Lepton – low mass, elementary particles, e.g. electron, neutrino. Quark – elementary particle, not found in isolation, which combines to form hadrons and baryons, e.g. up, down. Hadron – high mass particle consisting of quarks and/or anti- quarks . Baryon – hadron composed of 3 quarks, e.g. proton, neutron. Antibaryon – hadron composed of 3 antiquarks, e.g. antiproton. Meson – hadron composed of a quark and an antiquark, e.g. pion. Fig. 1.7.1 The structure of a deuterium atom electron ~10 –18 m nucleons ~10 –15 m quarks ~10 –18 m p n electron cloud ~10 –10 m