KEY TERM
KEY TERM
WHAT YOU NEED TO KNOW
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.
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.
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)
Matter made of antiparticles.
Mega-electron-volt,
MeV
1 MeV = 1 x 106 eV = 1.60 x 10-13 J
Antimatter
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 1/12th of the mass of one atom of carbon-12.
u = 1.66x10-27 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.
Photoelectric
equation
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.
Photoelectron
An electron that has escaped from a metal surface due to the photoelectric effect.
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.
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 MeV.
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, eV
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 eV = 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,
f0
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 = E1 − E2
Energy levels
Electrons bound to atoms exist at discrete energy levels e.g. -13.6 eV is indicates that 13.6 eV are needed for the electron to
be released from the atom. (Note that -5.7 eV 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
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).
Isotope
A nucleus of a particular element that may have a different number of neutrons e.g. carbon-12 (6 p, 6n) and carbon-14 (6p,
8n). The Periodic Table lists only the most common isotopes as found in nature.
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
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
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 (f0) cannot
cause electron emission so therefore:
Φ = hf0
Strangeness
Momentum
Energy
UNIT 2 - PARTICLES AND RADIATION
FUNDAMENTAL FORCES
The photoelectric effect suggests that EM waves have a
particulate nature.
•
•
•
Wave particle duality is
the 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