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KEY TERM WHAT YOU NEED TO KNOW Absorption line spectrum A spectrum of discrete dark lines against the bright background of a continuous spectrum showing that only photons of certain exact energies have been absorbed. Lepton A class of fundamental particle of relatively small mass. Electrons, muons and neutrinos are leptons. Leptons are subject to the weak interaction. Alpha decay The process during which an unstable nucleus emits an alpha particle. Lepton number Alpha particle A particle made of two protons and two neutrons emitted from the nucleus of an unstable atom. It is identical to the nucleus of an atom of helium-4. A number which is conserved all particle interactions. Leptons have a lepton number of +1. Antileptons have a lepton number of -1. All non-leptons have a lepton number of 0. Annihilation When a particle and an antiparticle meet, they annihilate i. e. their total mass is converted directly into energy and momentum (one gamma ray photon with energy and momentum that is equal to the total of the two incoming particles). Maximum kinetic energy, Ek(max) Photoelectrons are emitted with a range of kinetic energies. A few of the photoelectrons will have the maximum kinetic energy, Ek(max). The maximum kinetic energy depends only on the frequency of light (i. e. its colour) and not on its intensity. Antimatter Matter made of antiparticles. Mega-electron-volt, Me. V 1 Me. V = 1 x 106 e. V = 1. 60 x 10 -13 J Antiparticle Every particle has a corresponding antiparticle e. g. proton(+)/antiproton(-) or electron(-)/positron(+). Mesons Composite particle made of two quarks which is subject to the strong interaction. Atom The smallest particle of an element. Atoms have neutral charge unless they are ionised. Neutrino, ν A type of lepton emitted during beta decay (represented by the Greek letter ν - nu ) A non-SI unit of mass used in nuclear and particle physics equal to 1/12 th of the mass of one atom of carbon-12. u = 1. 66 x 10 -27 kg Neutron A neutral (zero charge) particle found in the nucleus of an atom. Atomic mass unit, u Nucleon number, A The sum of the number of protons and number of neutrons in the nucleus. Baryon Composite particle made of three quarks which is subject to the strong interaction e. g. proton, neutron. Nucleus The +vely charged centre of an atom. It is extremely small compared to the radius of an atom and extremely dense. Baryon number A number which is conserved all particle interactions. Baryons have a baron number of +1. Baryons have a baryon number of -1. All non-baryons have a baryon number of 0. Pair production The opposite of annihilation. An interaction in which a particle with enough energy can create a particle and antiparticle at the same moment, whilst conserving lepton no. , baryon no. , energy, mass, momentum, strangeness and charge. Beta minus particle, β- A high energy electron emitted from the nucleus of an unstable atom. Photoelectric effect The emission of electron(s) from a metal surface when EM radiation is incident on the surface. The emitted electrons are sometimes called photoelectrons. Beta minus β- decay The process during which an unstable nucleus emits a negatively charged beta particle. This relates the photon energy, work function and maximum kinetic energy of the emitted photoelectrons. hf = Φ + Ek(max) Beta plus β+ decay The process during which an unstable nucleus emits a positively charged beta particle. Photoelectric equation Continuous spectrum Electromagnetic radiation arranged in order of frequency (eg. ROYGBIV) after it has passed through a prism or diffraction grating. A continuous spectrum has no gaps. Photoelectron An electron that has escaped from a metal surface due to the photoelectric eﬀect. Photon The particle that is exchanged when two particles interact via the EM force. It has a mass of zero. Coulomb, C The unit of electric charge. One coulomb is the charge transferred when a current of one amp flows for one second ie. Q = It Photon absorption When a photon is absorbed, an electron from a lower energy level is promoted to a higher energy level or escapes the system. de Broglie wavelength The equation that proposes that waves have particle-like properties and particles have wave-like properties. Photon emission When a photon is emitted because an electron drops down to a lower energy level. Diffraction The spreading out of a wave as a result of passing through a narrow gap or across an edge. Pion exchange The exchange particle for the residual strong interaction between composite particles (e. g. between two protons). A property which causes a particle to experience a force when placed in an electromagnetic ﬁeld. There are two types: positive (+ve) and negative (-ve). Unlike charges attract, like charges repel. Planck’s Constant, h A fundamental constant of nature h = 6. 63 x 10 -34 Js Electric charge Proton A +vely charged particle found in the nucleus. It is the only stable baryon into which other baryons eventually decay. Electron A negatively charged particle found “orbiting” the nucleus of an atom. Proton number, Z Electron capture A form of beta decay where the nucleus captures an inner electron (from the electron cloud) via the weak interaction. The number of protons in the nucleus of an atom. It is also the number of electrons present in the neutral atom. The proton number defines the element e. g. carbon atoms will always have Z = 6. Electron cloud An alternative model which pictures the electrons existing in a cloud around the nucleus. Quark An elementary particle with fractional electric charge that cannot exist in isolation. Electron diffraction A beam of electrons is diffracted by the gaps between atoms in the crystal. This shows that particles (electrons) can have wave-like properties associated with them. Radioactive decay The process whereby an unstable nucleus becomes more stable by emitting alpha or beta particles or a gamma ray photon. The process is spontaneous and random. Electron excitation An alternative model which pictures the electrons existing in a cloud around the nucleus. Rest energy From Einstein’s Theory of Special Relativity (for an object at rest). Mass can be expressed in units of energy. This is known as the rest energy. The rest energy is often given in the form of Me. V. Electronic relaxation The opposite of excitation i. e. a transition from a high to a lower energy level. Specific charge The charge per unit mass. qm = Q / m Electron-volt, e. V A non-SI unit of energy used in nuclear physics. It is the energy transferred when an electron moves through a potential diﬀerence of one volt. 1 e. V = 1. 60 x 10 -19 J Stable nucleus A nucleus that has no tendency undergo radioactive decay, due to its very long half life, e. g. carbon-12. Stopping potential, Vs The voltage or potential diﬀerence needed to “stop” all photoelectrons. This gives a measurement of the maximum kinetic energy of the photoelectrons. Strangeness, s A physical property that is conserved during the strong and electromagnetic interactions. Strangeness can change by 0, +1 or -1 in weak interactions. Strong nuclear force The force that holds the nucleus together against the electrical repulsion of protons. It is attractive at separations of up to 3 fm and repulsive at separations below 0. 5 fm. Threshold frequency, f 0 The minimum frequency that will release photoelectrons from a metal. Ultraviolet light A high frequency light with large photon energy that will produce photoelectrons from zinc. Unstable nucleus A nucleus that has a tendency to undergo radioactive decay e. g. carbon-14. Velocity of EM radiation, c A fundamental constant of nature c = 3. 00 x 108 ms-1 in a vacuum. λ= h mv Emission line spectrum A spectrum of discrete bright lines against a black background showing that only photons of certain exact energies have been emitted, shown by the equation hf = E 1 − E 2 Energy levels Electrons bound to atoms exist at discrete energy levels e. g. -13. 6 e. V is indicates that 13. 6 e. V are needed for the electron to be released from the atom. (Note that -5. 7 e. V is a higher energy level. ) Exchange particles Particles interact by exchanging particles e. g. the electromagnetic force is mediated by photons. Femtometre, fm A very small distance roughly equal to the radius of a hydrogen nucleus 1 femtometre = 1 fm = 1 x 10 -15 m Feynman diagram A diagram which represents an interaction in terms of the incoming and outgoing particles and the exchange particle e. g. electromagnetic repulsion of two electrons. Fundamental interactions Particles can aﬀect each other only by these interactions: gravitational, electromagnetic, strong and weak. Gluon The exchange particle for the strong interaction between quarks. Ground state The lowest energy level (most tightly bound to the nucleus). Virtual photon The exchange particle for the electromagnetic interaction. They are virtual because detection would prevent the exchange. Hadrons Composite particles made of quarks which are subject to the strong interaction. W boson/Z boson The exchange particles of the weak interaction. Ionisation The process of removing (or adding) electrons from (or to) atoms to create electrically charged ions. Wave particle duality Isotope A nucleus of a particular element that may have a diﬀerent number of neutrons e. g. carbon-12 (6 p, 6 n) and carbon-14 (6 p, 8 n). The Periodic Table lists only the most common isotopes as found in nature. The idea that as particles can have ‘wave-like’ properties (e. g. electron diﬀraction), so waves can have ‘particle-like’ properties (e. g. photons and the photoelectric eﬀect). Work function, φ The minimum energy “cost” for an photoelectron to escape from a metal.
PARTICLES QUARKS AND ANTIQUARKS Nuclide notation: Simple diagram of the atom Specific charge (C/kg) = charge (C) mass (kg) These quantities are always conserved in particle interactions: E = hf = hc λ Particle decay: (where h=Planck’s Constant) Baryon → proton Muon → electron Kaon → pion • • Mass Charge Baryon number Lepton number UNIT 2 - PARTICLES AND RADIATION FUNDAMENTAL FORCES ELECTRONS AND ENERGY LEVELS In fluorescent tubes a gas has an electric current pass though it. This causes electrons to become excited and ionise. When electrons return towards the ground state they lose energy which is emitted as UV radiation. This radiation is absorbed by the coating in the tube which emits visible light. STABLE AND UNSTABLE NUCLEI Spectrums provide evidence for electrons inhabiting discrete energy levels. The definitions for these spectra can be found in the key terms section. WAVE PARTICLE DUALITY The photoelectric effect suggests that EM waves have a particulate nature. It is the emission of electron(s) from a metal surface when EM radiation is incident on the surface. The emitted electrons are sometimes called photoelectrons. They are normal electrons, but their name describe their origin – emitted through the photoelectric effect. A photon with less than the threshold frequency (f 0) cannot cause electron emission so therefore: Φ = hf 0 • Strangeness • Momentum • Energy Wave particle duality is theory proposed by Louis de Broglie. It suggests that ‘wave-like’ like shows particle properties and that ‘particles’ like electrons should be expected to show wave-like properties. de Broglie Equation λ= h mv Electron diffraction suggests that particles possess wave properties. Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal. Changing the speed or mass of the particles changes the diffraction pattern – which fits in with de Broglie’s hypothesis. FEYNMAN DIAGRAMS