2. Particles and Radiation Flashcards
What are the main constituents of an atom?
● Proton
● Neutron
● Electron
What is meant by specific charge?
The charge to mass ratio:
Specific charge = charge / mass
Units C/kg.
What is the specific charge of a proton?
Protons have charge +1.6 x10^-19 and mass 1.67 x 10^-27 kg
Specific charge = 1.6 x 10^-19/1.67 x 10^-27
= 9.58 x 10^7 C/kg
What is the letter associated with a proton number?
Z
What is a nucleon?
A constituent of the nucleus: a proton or a neutron.
What letter represents nucleon number?
A
Which is the correct notation?
A
Z X
or
Z
A X
A
Z X
What is an isotope?
A version of an element with the same number of protons but a different number of neutrons.
State a use of radioactive isotopes
Carbon dating - the proportion of carbon-14 in a material can be used to estimate its age
What is the strong nuclear force?
The fundamental force that keeps the nucleus stable by counteracting the electrostatic force of repulsion between protons.
Describe the range of the strong force?
● Repulsive up to 0.5fm
● Attractive from 0.5-3fm
● Negligible past 3fm
What makes a nucleus unstable?
Nuclei which have too many of either protons or neutrons or both.
How do nuclei with too many nucleons decay?
Alpha decay (emission of a helium nucleus formed of 2 protons and 2 neutrons)
How do nuclei with too many neutrons decay?
Beta minus decay in which a neutron decays to a proton by the weak interaction (quark character has changed from udd to uud).
How was the existence of the neutrino hypothesised?
The energy of particles after beta decay was lower than before, a particle with 0 charge (to conserve charge) and negligible mass must carry away this excess energy, this particle is the neutrino.
What is meant by beta minus decay?
When a neutron turns into a proton, the atom releases an electron and an anti-electron neutrino.
What is an alpha particle?
A particle contains two protons and two neutrons, the same as a helium nucleus.
What is an antiparticle?
For each particle there is an antiparticle with the same rest energy and mass but all other properties are the opposite of its respective particle.
True or false:
‘Every particle has a antiparticle’
True
What is the name of the antiparticle of an electron?
Positron
What is the antiparticle of π0 (pion with 0 charge) ?
π0, its antiparticle is itself
What occurs when a particle and antiparticle meet?
Annihilation:
The mass of the particle and antiparticle is converted back to energy in the form of 2 gamma ray photons which go in opposite directions to conserve momentum.
What is pair production?
A gamma ray photon is converted into a particle-antiparticle pair.
What is the minimum energy of a photon required to make a proton-antiproton pair?
2 x proton rest energy
2 x 938.257 = 1876.514 MeV
Name the 4 fundamental forces?
● Gravity
● Electromagnetic
● Weak nuclear
● Strong nuclear
The virtual photon is the exchange particle of which force?
The electromagnetic force.
What type of particles are affected by the strong nuclear force?
Hadrons
What is the exchange particle of the weak nuclear force?
The W boson (W+ or W-)
What does the electromagnetic force act on?
It acts on charged objects, for example when a positively charged ball repels another positively charged ball.
When does weak nuclear interaction occur?
When quark character changes (a quark changes into another quark), it affects all types of particles.
Which properties must be conserved in particle interactions?
● Energy ● Charge ● Baryon number ● Lepton number ● Momentum ● Strangeness (only for strong interactions)
What is a hadron?
Both baryons and mesons are hadrons, hadrons are made of 2 or more quarks held together by the strong nuclear force.
What are the classes of hadrons?
● Baryons (three quarks)
● Mesons (1 quark, 1 antiquark)
The pion and kaon are both examples of which class of particle?
Mesons
The pion can be an exchange particle for which force?
The strong nuclear force
What particle does a kaon decay into?
A kaon decays into a pion
Give some examples of baryons
● Proton - uud
● Neutron - ddu
What is significant about a proton?
● It is the only stable baryon
● All baryons will eventually decay into protons
What are some example of leptons?
● Electron
● Muon
● Neutrino
● (the antiparticles of the above)
What does a muon decay into?
An electron
What is the strangeness value of a strange quark?
-1
True or false:
‘Strangeness is always conserved in a weak interaction’
False
Strangeness is only conserved in the strong interaction, in weak interactions it can change by 0, -1 and +1.
Complete the sentence:
Strange particles are produced through the _____________ and decay through the ____________.
Strange particles are produced through the strong interaction and decay through the weak interaction.
Are electromagnetic waves transverse or longitudinal?
Transverse
What phenomenon can be used to show that light behaves as a particle?
The photoelectric effect
Describe the photoelectric effect
When light above a particular frequency is shone on metal, electrons are released - these released electrons are “photoelectrons”.
What is the threshold frequency?
The minimum frequency of light required for an electron to be emitted.
What equation is used to determine the energy of a photon?
E = hf = hc/𝜆 Energy = Planck’s constant x frequency Energy = (PC x 3 x 10^8) / wavelength
Why does a photon need to have a minimum frequency in order to liberate an electron?
The energy of the photon is determined by its frequency, the photon’s energy must be greater than the work function (energy needed to break bonds holding the electron) in order for an electron to be emitted.
If a photon has a frequency higher than the threshold frequency, what would occur?
The electron will be liberated and the remaining energy is the kinetic energy of the electron.
If light is incident on a metal and photoelectric emission does NOT occur, what is the effect of increasing light intensity?
● If it is more intense then there would be more photons incident on the metal each second
● However each photon still carries the same amount of energy as before
● Therefore it still does not contain enough energy to liberate an electron
● No effect
What is the photoelectric equation?
h 𝑓 = ø + Ek(max)
Planck’s constant x frequency = work function +
maximum kinetic energy of the photoelectrons
Define the work function
The energy required by an electron to overcome the metallic bond holding it in the metal.
What is an electron volt?
The kinetic energy of an electron that has been accelerated from rest through a potential difference of 1V.
How do you convert electron volts to joules (and vice versa)?
Electron volt(eV) x 1.6x10^19 = Joules(J) Joules(J) / 1.6x10^19 = Electron volt(eV)
How does a fluorescent tube work?
● High voltage applied across mercury vapour accelerates fast moving free electrons which collide with the mercury atoms.
● Mercury electrons are excited and then return to the ground state, releasing a UV photon.
● The tube’s phosphorus coating absorbs the UV photons and its electrons are excited, they cascade down the energy levels and emit visible light photons.
What can be used as evidence for the discrete energy levels in atoms?
Line emission and absorption spectra as the lines appear at discrete points which show where a light photon of specific frequency and wavelength has been absorbed or emitted, this shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level.
What is wave particle duality?
All particles have both particle and wave properties, waves can have particle properties
e.g. light acts as a particle in the photoelectric effect and as a wave when it is diffracted.
What is the equation for de Broglie wavelength?
λ = h / mv
Where mv is momentum.
