3.2 Particles and radiation Flashcards
What is the charge of the proton and the electron?
(-)1.60*10^-19C
What is the charge of a +2 ion in coulombs?
3.2*10^-19 C
What is nucleon number (A)?
Mass number- number of protons and neutrons in the nucleus.
What is nuclide notation?
A Z [X]
What is an Isotope?
Atoms with the same number of protons and different number of neutrons.
What is specific charge?
Charge divided by mass.
Which subatomic particle has the highest specific charge?
The electron
What is the strong nuclear force?
It keeps the nucleus together. It has a range of 3-4 fm. It has the same effect between a proton or a neutron. At distances less than 0.5 fm, it is a repulsive force, to prevent neutrons and protons from touching. (It overcomes the electrostatic force of repulsion.)
What is radiation caused by?
Unstable nuclei
What is alpha radiation?
Consists of 2 protons and 2 neutrons.
What is beta radiation?
Fast-moving electrons. (A neutron in the nucleus turns into a proton, also emitting an antineutrino. Atomic number increases by 1)
What is gamma radiation?
Electromagnetic radiation that can pass through thick metal plates. It has no charge or mass.
Write down the general equation for alpha decay.
Write down the general equation for beta- decay.
How was the neutrino hypothesized?
It accounts for the conservation of energy in beta decay.
Does every particle have an antiparticle?
Yes
What is the relationship between the charge, mass and rest energy of a particle and its antiparticle?
They are the same.
What is the correspondence between the rest energy and rest mass of a particle?
The rest mass of a particle corresponds to the rest energy locked up as mass.
What is the antiparticle of an electron?
A positron
What is the antiparticle of a proton?
An antiproton
What is the antiparticle of a neutron?
An antineutron
What is the antiparticle of a neutrino?
An antineutrino
What is the Planck constant?
6.63*10^-34 Js
What is rest energy?
The energy a particle has in the form of mass. (Calculated using E=mc^2)
What are photons?
A name given to the bursts of electromagnetic radiation.
When are electromagnetic waves produced?
When a charged particle loses energy like
-When a fast moving electron stops, slows down or changes direction.
-An electron in the shell of an atom moves to a different shell of lower energy.
How is photon energy calculated?
E=hf
How do you calculate the power of a beam?
Power=nhf
What is 1MeV in joules?
1.60*10^-13 J
When does annihilation occur?
When a particle and its antiparticle meet and their mass is converted into radiation energy. 2 photons are produced in the process.(a single photon cannot ensure a total 0 momentum after collision)
How do you calculate the minimum energy of each photon?
Energy of the 2 photons= Rest energy of the particle and the antiparticle
What does a laser beam consist of?
Photons of the same frequency
What occurs in pair production?
A photon creates a particle and a corresponding antiparticle, and vanishes in the process. The minimum energy of the photon is equal to 2E0.
Minimum energy of photon = rest energy of particle + rest energy of antiparticle.
What happens to the extra energy in pair production?
It becomes Kinetic energy in the particles.
Which 2 groups are subatomic particle split up into?
Leptons and hadrons
What are the properties of hadrons?
They contain quarks
They feel the strong force
What are the 2 subgroups of hadrons?
Baryons and mesons
What does a baryon consist of?
It either contains 3 quarks or 3 antiquarks.
What are some examples of baryons?
Protons
Neutrons
What do mesons consist of?
1 quark and 1 antiquark.
What are some examples of mesons?
Pions
Kaons
What are some examples of leptons?
Electrons
Muons
Neutrinos
What is the quark structure of a proton?
uud
What is the quark structure of a neutron?
udd
What is the relative charge of an up quark?
+2/3
What is the relative charge of a down quark?
-1/3
What are the 2 quantum numbers?
Baryon number
Lepton number
What is the baryon number of a quark?
+1/3
What is the most stable baryon?
The proton. Other baryons eventually decay into a proton.
What happens during a proton proton interaction? Draw the Feynman diagram for this.
They both experience electromagnetic force caused by the exchange of virtual photons.
What is the weak nuclear force?
It causes beta- and beta+ decay. (It can be electromagnetic force as a neutron is uncharged.)
What are the 3 different types of quarks?
Up
Down
Strange
How are neutrinos created?
Radioactive decay
Nuclear reactions in the sun
What are the properties of neutrinos?
Elementary particles
Travel close to the speed of light
Extremely small mass
Uncharged
What is a radioisotope?
An isotope that is radioactive.
What are the products of beta- emission?
An electron
An anti electron neutrino
What happens in beta+ decay?
The weak nuclear force causes a proton to turn into a neutron. A W+ boson is produced and this decays after a very short time into a beta+ and neutrino which are emitted. An up quark changes into a down quark.
What is the magnitude of the repulsive electromagnetic force between the protons in the nucleus?
60N
What do pions contain?
Up or down quarks
What do kaons consist of?
A strange quark
An up or down quark
What does a neutron decay into?
A proton
An electron
An antineutrino
What are the 4 fundamental forces?
The strong force
The weak force
The EM force
Gravitational
Which particles feel the strong force?
Quarks
Which particles experience the weak force?
Quarks
Leptons
Which particles experience the electromagnetic force?
Any particle with charge.
Which particles experience the gravitational force?
Any particle with mass.
What does the weak force do?
It causes particles to decay
It’s responsible for beta decay
What is the exchange particle for the electromagnetic force?
A virtual photon
What is the exchange particle for the strong force?
The pion
What is the exchange particle for the weak force?
The W boson
What are the properties of W bosons?
Have a non-zero rest mass
Have a range of 0.001fm
Can have a positive or negative charge
What happens in beta- decay?
The weak nuclear force causes the neutron to change into a proton, W- boson is produced. Decays after a short time into beta- and an antineutrino, which are emitted. A down quark turns into an up quark.
What happens in an electron -proton collision?
An electron and a proton collide at a very high speed, they interacts through the weak force. The proton turns into a neutron and the electron turns into a neutrino by the exchange of a W- boson.
What happens during electron capture?
A proton can interact through the weak force with an inner shell electron. The proton turns into a neutron and the electron into a neutrino by the exchange of a W+ boson.
What is strangeness?
Properties of particles, with strange particles, being assigned a strangeness number.
What is the charge, strangeness and baryon number of a strange quark?
charge: -1/3
strangeness: -1
baryon number: +1/3
What happens to strangeness in strong interactions?
Strangeness is conserved
What happens to strangeness in weak interactions?
It can change by a factor of one.
What are the properties of strange particles?
They are created in pairs
They decay by weak interactions.
Why are particle studies carried out by a large international research team?
Research is expensive, requires funding from many countries.
Both scientists and engineers are required
Research in multidisciplinary
Research is round-the -clock
They are needed to process the large amounts of data produced
What are the 2 types of lepton number?
Electron
Muon
What does a kaon eventually decay into?
A pion
What does the muon decay into?
An electron
How are strange particles produced?
Through the strong interaction and decay through the weak interaction.
How were exchange particles hypothesised?
You can’t have instantaneous action at a distance, there must be some form of interaction between the 2 particles involved.
What are the properties of pions?
They are the lightest mesons
There are 3, each with a different charge
What are the properties of kaons?
They’re heavier than pions
They’re more unstable than pions
Have a very short lifetime and decay into pions
How were mesons discovered?
In cosmic rays. They can be observed in a cloud chamber.
Describe the conservation in particle interactions.
Charge is always conserved.
Baryon Number is always conserved.
Strangeness is conserved in Strong Interactions.
Strangeness can change by 1 for Weak interactions.
Lepton number must be conserved.
When does alpha emission typically happen?
In big nuclei like uranium and radium.
Where does beta decay typically occur?
In isotopes that are unstable from being neutron rich.
What is threshold frequency?
the minimum frequency of light needed to cause photoelectric emission (release electrons.)
What is work function?
The minimum energy required to release an electron.
What is stopping potential?
The potential required to stop all the electrons from reaching the opposite plate.
What is the photoelectric equation?
hf = φ + Ek (max)
What is Ek max in the photoelectric equation?
The maximum kinetic energy of the photoelectrons.
What is excitation?
When an electron is promoted into higher energy levels.
What causes excitation?
Collision with an external electron.
By absorption of a photon
What happens when electrons de-excite?
A photon is emitted
If de-excitation happens in steps, you emit a photon in each step.
How does a fluorescent lamp use excitation and ionisation?
A high voltage is applied and this accelerates free electron which ionise the mercury atoms, producing more free electrons.
When these electrons collide with electrons in other mercury atoms, the electrons are excited to higher energy levels.
When the electrons de-excite, they release photon in the uv range.
A phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to much higher orbits. These electrons then de-excite in steps, emitting many lower energy photons in the form of visible light.
The photons have specific frequencies as energy levels have discrete values.
What is a line spectrum?
Light emitted from an atom produces a line spectrum. A series of lines against a dark background.
What is an absorption line spectrum?
In an absorption spectrum a series of dark lines (appears against a bright background/within a continuous spectrum)
What is 1eV?
1eV = 1.60*10^-19 J
What did line spectra provided evidence towards?
Each line corresponds to a particular wavelength of light emitted by the source.
Since only certain photon energies are allowed, you only see the wavelengths corresponding to these energies.
So, this provided evidence for discrete energy levels.
What factors affect intensity?
The number of photons released per second.
The energy of each photon.
How do we know that electrons have wave behaviour?
Electron diffraction:
Light produces interference and diffraction patterns — alternating bands of dark and light.
These can only be explained using waves interfering constructively or interfering destructively.
How do we know that electromagnetic waves have particle nature?
The photoelectric effect:
The beam of light acts like a series of particle-like photons.
If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one-to-one way.
All the energy in the photon is given to one electron.
What is the effect the increasing momentum on the diffraction of electrons?
If you increase the voltage, the momentum and kinetic energy increases. The wavelength of the particle decreases and frequency increases. And the size of the diffraction rings decreases.
How do you reduce the diffraction of electrons?
Reduce the wavelength
Increase aperture width
What is the ionisation energy (physics)?
The energy that an electron in the ground state must absorb to be emitted.
Describe how to carry out an experiment to demonstrate the photoelectric effect. [5 marks]
Zinc plate mounted on top of a gold leaf electroscope
Plate initially negatively charged
So, the gold leaf and metal rod repel each other
There’s insulation, so, the charge can’t leak away
UV lamp shined on plate
The leaf falls as charged particles leave plate and it neutralises
Experiment indicates electrons must be released from plate
The closer the source of UV is, the faster the leaf falls
If the electroscope is positively charged, no radiation will have any effect
Visible light has no effect, therefore energy must be linked to frequency
Describe how the photoelectric effect can be explained in terms of the physics of quantum behaviour. [5 marks]
Individual photons are absorbed by individual electrons on metal surface
Electron’s must have absorbed sufficient energy to overcome work function energy of the metal
Concept of instantaneous emission
(photo)electron released
Electrons emitted frequency > = threshold frequency
Energy is conserved
hf = φ + KE(max)
How does the electron diffraction experiment demonstrate the wave-like nature of the electrons? What happens to the appearance of the rings when the speed of the electrons is increased? [5 marks]
Wavelength = h / mv
Definition of symbols used in equations above
Electrons travel as a wave
Electrons are diffracted
By the spacing between the atoms
The electrons are diffracted when their wavelength is less than or comparable to the size of atoms / gap between the atoms
Superposing/interfering at screen.
When the speed of the electrons is increased the rings get smaller
At greater speed (of electrons) the wavelength is shorter
At greater speed (of electrons) there is less diffraction
Describe the process of peer review.
A scientist makes an observation
The theory isn’t accepted straight away
Other scientists evaluate the theory by peer review
The scientist then publishes their theory
The theory gets tested through experiments
When there’s enough evidence, the theory is validated by the scientific community.
Discuss how the discharge tube is made to emit electromagnetic radiation of specific frequencies.
A high potential difference accelerates electrons
electrons have to have sufficient energy to excite the atoms and raise them into higher levels
Visible spectrum results from excited electrons moving into a lower energy level
Each transition results in a photon of light
Energy of photon is the difference in the energies of the two levels
An electron beam is incident on a thin graphite target that behaves like the slits in a diffraction grating experiment. After passing through the graphite target the electrons strike a fluorescent screen.
Explain how the pattern produced on the screen supports the idea that the electron beam is behaving as a wave rather than as a stream of particles.
Particle behaviour would only produce a patch of light or
Particles would scatter randomly
Wave property shown by diffraction/ interference
Graphite causes (electron) waves/beam to spread out
Bright rings/maximum intensity occurs where waves interfere constructively
for a diffraction grating maxima when sin(theta) = n (lambda)/d
Discuss how the rate of loss of charge from the plate depends on the frequency and intensity of the incident radiation.
• The photoelectric effect is the emission of electrons (from a surface) when electromagnetic radiation is incident (on surface).
• Minimum energy required (work function) to release an electron from the metal surface.
• A photon must supply this energy in one interaction.
• The energy of a photon is directly proportional to its frequency.
• Increased intensity (at same frequency) results in more photons per second incident on metal.
• Must increase the number of photons (per second) even if frequency increases.
• More electrons released from surface every second so loses charge more rapidly.
Explain how bombarding electrons causes the atoms of gas to emit light and how the line spectra provides evidence towards discrete energy levels.
Electrons bombard atoms of vapour and give energy to electrons in the atom
Electrons move to a higher energy level
Atoms are excited
Atomic electrons move down to lower energy levels, losing energy by emitting photons
Photons have energy hf = energy difference between the levels that electron falls between
Only certain photons (of characteristic frequencies) emitted from atoms of a particular element
Only certain transitions available to electron this is because atoms have discrete energy levels which are associated with particular energy values
