Particle Physics Flashcards
What is the photoelectric effect?
A metal can emit electrons if illuminated by electromagnetic radiation under specific conditions.
Who first observed the photoelectric effect?
Heinrich Hertz
What does the photoelectric effect depend on?
- Frequency
- Wavelength
What doesn’t the photoelectric effect depend on?
Intensity
What is the relationship between max kinetic energy of the emitted electrons and light intensity?
They are independent of each other.
What is the time delay between the light striking the metal and the first electrons being emitted?
There is no delay.
How is the photoelectric effect demonstrated?
- A metal plate is placed on top of a negatively charged golden leaf electroscope.
- If the EM radiation releases electrons, the metal will become positively charged.
- Electrons then move up the electroscope so the gold foil within will return to a vertical position.
Who developed the solution to the photoelectric effect problems?
Einstein and Planck
What were the three problems with the photoelectric effect?
- The ability of a metal to emit electrons didn’t depend on intensity.
- The kinetic energy of the electrons is unaffected by the intensity.
- No time delay was observed.
What was Planck’s theory regarding the photoelectric effect?
Energy was quantised (ie. discrete not continuous)
How did Einstein develop Planck’s work on the photoelectric effect?
He showed that EM radiation could be considered as quanta of energy called photons.
What is a photon?
A quanta of electromagnetic radiation.
What has modern physics discovered about the properties of EM radiation?
It can also have particle like properties.
How is the energy carried by a photon calculated?
E = hf
Or
E = hc / λ
What led to the theory of wave particle duality?
- Interference and diffraction can only occur with waves.
- The photoelectric effect can only occur with particles.
How are Joules converted into electron volts?
Energy in J = energy in eV x 1.60x10^-19
How are electron volts converted into Joules?
Energy in eV = energy in J / 1.60x10^-19
What is the energy carried by an EM wave dependent on?
Frequency or wavelength
In terms of the photoelectric effect, where do electrons within a metal sit?
In a potential energy well.
What is the work function of a metal?
The minimum energy required to remove an electron from the surface of a metal.
What happens when a photon collides with an electron in a metal?
The photon transfers all of its energy to the electron so the photon is absorbed.
How long does the absorption of a photon into a metal surface take?
The process is instantaneous.
How many photons can electron absorb?
An electron can only absorb one photon.
What is the threshold frequency?
The lowest of EM radiation that will emit electrons from the surface of the metal.
How is threshold frequency calculated?
fT = ϕ/ h
How is the maximum kinetic energy of a photoelectron calculated?
Ek = hf - ϕ
What does Ek represent?
Maximum kinetic energy
Why doesn’t intensity affect the Ek of emitted electrons?
- hf remains constant
- More photons are emitted meaning more electrons are emitted but with the same Ek.
Why isn’t there a time lag between the EM radiation striking the metal and the electron being emitted?
The energy from the photon is transferred instantly so the electron can escape instantly.
When drawing a graph of Ek against f, what will the gradient equal?
Planck’s constant
When drawing a graph of Ek against f, what will the y-intercept represent?
The work function of the metal
When drawing a graph of Ek against f, what does the x-intercept represent?
The threshold frequency of the metal.
How is time lag calculated from the photoelectric effect?
t = ϕ/IA
How is time lag calculated from the photoelectric effect?
t = ϕ/IA
What is an isotope?
Atoms of the same element that contain different numbers of neutrons.
What is the specific charge of a particle?
The charge on the object divided by the mass.
What is the unit of specific charge?
C/kg
How are diffraction gratings different to using a prism?
Gratings produce a repeating pattern
What are the repeats from a diffraction grating called?
Orders
How is an electron that orbits the nucleus arranged?
It can only sit at a specific energy level.
Give two properties of energy levels.
- The energy levels are quantised.
- The energy associated with each level is unique to the element.
How many energy levels can there be in an atom?
An infinite number
What is a ground state?
When all the electrons in the atom occupy their lowest allowed energy value.
What is an excited state?
Supplying energy to an atom can excite the electrons causing them to move up to higher energy levels.
When is an electron considered to be excited?
When it sits at any energy level other than n=1
What is ionisation energy?
The energy required to remove an electron from its ground state away from the atom.
What is released when an electron falls back to a lower energy level?
A photon with energy equal to the energy difference of the levels.
Why do emitted photons have a specific value?
The energy levels are fixed so the energy difference between the levels has to have a specific value.
Why do emitted photons have fixed frequency?
They have fixed energies.
When can an electron move up an energy level?
- When a photon of the exact energy difference is absorbed.
- When another particle with at least the energy difference collides.
What happens when the energy of a photon is greater than the ionisation energy?
The electron is emitted with the remaining kinetic energy.
When do hydrogen atoms release visible light?
When electrons return to n=2
If a material can emit photons at a specific wavelength, what must also be true?
The material can also absorb photons at the same wavelength.
What is an absorption spectra?
The wavelengths at which an element emits radiation and, therefore, absorbs it.
How does a fluorescent light bulb work?
- The tube contains mercury vapour at low pressure.
- The vapour is excited by the passage of an electric current.
- The excited electrons return to lower energy levels releasing UV EM radiation.
- The UV photons are absorbed by electrons in the phosphor causing the molecules to be excited.
- The excited electrons return to lower energy levels releasing photons of visible light.
- The solid phosphor means there is an overlap in energy levels so the tube emits a continuous spectrum.
Why must the mercury vapour in a fluorescent tube be at low pressure?
To ensure the electrons flowing through the tube don’t undergo too many collisions which would cause them to lose their energy.
What does ϕ stand for?
Work function
What does ϕ stand for?
Work function
What is the strong nuclear force?
A force that overcomes the electrostatic repulsion of the protons to hold the nucleons together.
What is the range of the strong nuclear force?
3-4 femtometres
What does an alpha particle consist of?
Two protons and two neutrons
What does an alpha particle consist of?
Two protons and two neutrons
What is released in Beta decay?
An electron and an antineutrino
How is the power of a laser beam calculated?
nhf
What is released from annihilation?
Two gamma photons
When does positron emission take place?
When a proton changes into a neutron.
When does positron emission take place?
When a proton changes into a neutron.
What does annihilation take place between?
A particle and its antiparticle
How is the minimum energy of each photon from annihilation calculated?
hfmin = 2E0
E0 = rest energy fmin = minimum freq.
What is pair production?
When a photon creates a particle and antiparticle.
How is the minimum energy of each photon required for pair creation calculated?
hfmin = 2E0
E0 = rest energy fmin = minimum freq.
What is the minimum energy of a photon required for the creation of an electron and positron?
2 x 0.511MeV = 1.022 MeV
What is the minimum energy of a photon required for the creation of an electron and positron?
2 x 0.511MeV = 1.022 MeV
Why are some photons described as virtual?
They can’t be detected directly
What is a W boson?
An exchange particle of the weak nuclear force.
What is the weak nuclear force?
The force responsible for beta decay.
Describe a neutron and neutrino interaction.
A W^- boson is transferred from the neutron releasing a proton and an electron.
Describe a proton and antineutrino interaction.
A W^+ is transferred from the proton releasing a neutron and positron.
Describe the process of beta decay.
A neutron becomes a proton and a W^- decays into an electron and neutrino
Describe the process of beta decay.
A neutron becomes a proton and a W^- decays into an electron and antineutrino.
Describe the process of positron decay.
A proton becomes a proton and a W^+ decays into a positron and neutrino.
Describe the process of electron capture.
A proton becomes a neutron and a W^+ is transferred causing an electron to become a neutrino.
What is a muon?
A negatively charged particle with a rest mass over 200 times an electron’s
What is a muon?
A negatively charged particle with a rest mass over 200 times an electron’s.
What is a pion?
A meson that can be positively or negatively charged or neutral.
What is a pion?
A meson that can be positively or negatively charged or neutral.
What five particles can kaons decay into?
- Pions
- Muons and antineutrinos
- Antimuons and neutrinos
What five particles can kaons decay into?
- Pions
- Muons and antineutrinos
- Antimuons and neutrinos
What can a charged pion decay into?
- Muon and antineutrino
- Antimuon neutrino
What does a neutral pion decay into?
High energy photons
What does a neutral pion decay into?
High energy photons
What three interactions can protons undertake?
- Strong
- Weak decay
- Electromagnetic
What two interactions can neutrons undertake?
- Strong
- Weak decay
What two interactions can electrons undertake?
- Weak
- Electromagnetic
What interaction can neutrinos undertake?
-Weak
What two interactions can muons undertake?
- Weak
- Electromagntic
What two interactions can pions undertake?
- Strong
- Electromagnetic
What two interactions can kaons undertake?
- Strong
- Electromagnetic
What two interactions can kaons undertake?
- Strong
- Electromagnetic
What are hadrons?
Particles that can interact through the strong interaction.
What are leptons?
Particles that do not interact through the strong interaction.
What are leptons?
Particles that do not interact through the strong interaction.
What is a baryon?
A hadron that decays into a proton.
What are mesons?
Hadrons that don’t decay into protons.
What are hadrons made of?
Quarks
What can happen in an interaction between a lepton and a hadron?
A neutrino or antineutrino can change into or from a corresponding charged lepton.
Give an example of an interaction between a lepton and a hadron.
v + n —> p + e
When is strangeness always conserved?
In a strong interaction
What is the charge of a strange quark?
-1/3
What is the charge of a strange quark?
-1/3
What are the two conservation rules only used for particle interactions?
- Lepton number
- Strangeness
- Baryon number
What are the two conservation rules only used for particle interactions?
- Lepton number
- Strangeness
- Baryon number
What are the two conservation rules only used for particle interactions?
- Lepton number
- Strangeness
- Baryon number
On an Ek vs freq graph, what does the gradient equal?
Planck’s constant
On an Ek vs freq graph, what does the y-intercept equal?
-ϕ
On an Ek vs freq graph, what does the x-intercept equal?
Threshold frequency
On an Ek vs freq graph, what does the x-intercept equal?
Threshold frequency
What is an ion?
A charged atom
What is an ion?
A charged atom
What is an ion?
A charged atom
What is an electron volt?
A unit of energy equal to the work done when an electron is moved through a pd of 1 volt.
What is excitation?
When atoms absorb energy without becoming ionised.
Why does excitation occur?
An electron moves from an inner shell to an outer.
What is excitation energy?
The energy required to make an electron jump to an outer shell..
What is excitation energy?
The energy required to make an electron jump to an outer shell.
What is a ground state?
The lowest energy state of an atom.
What happens when an electron returns to an inner shell?
A photon is emitted
How is the energy of an emitted photon calculated?
hf = E1 - E2
Give an example of the wave-like nature of electromagnetic waves.
Diffraction of light
Give an example of the particle-like nature of electromagnetic waves.
Photoelectric effect
Explain de Broglie’s diffraction of electrons experiment.
- A narrow beam of electrons is aimed at a thin film.
- The electrons are diffracted in certain directions.
- A pattern of rings is formed due to diffraction of electrons at different orientations.
What is a scanning tunneling microscope used for?
Map atoms on solid surfaces
What is a transmission electron microscope used for?
Obtaining detailed images of objects too small to see with optical microscopes.
What is a SQUID used for?
Detecting weak magnetic fields.
