Particles test 11/23 Flashcards
Specific charge definition and how it’s calculated
The specific charge of a particle is the charge to mass ratio and is calculated by dividing a particles charge by its mass -> charge/mass
What is the Photoelectric affect?
Where photoelctrons are emitted from the surface of a metal after a light above a certain frequency is shone on it.
What is the frequency required for different types of metals called?
Threshold frequency
What is suggested in the photon model of light?
- EM waves travel in discrete packets called photons, that have an energy directly proportional to frequency
- each electron can absorb a single photon, thus a photo electron is only emitted if above the threshold frequency
- if the intensity of the light is increased and frequency above threshold more photoelectrons are emitted per second.
What is the work function?
Minimum energy required for electrons to be emitted from the surface of a metal
What is the stopping potential?
Potential difference you need to apply across the metal to stop the photo electrons with the max KE
How photons become Photoelectrons?
- delocalised electrons near the surface of the metal absorb the energy (=hf) of a single incident photon
- the KE (Ek) of the delocalised electron gains the same amount of energy (=hf)
- if the KE is now bigger than the work function (phi) then the electron will escape from the surface of the metal
- while escaping the surface of the metal and becoming a photo electron, the electron loses/uses an amount of energy equal to the work function
- thus a photoelectron have a max KE calculated by:
- KE max = energy of photon - work function
- KE max = (planchs constant x frequency of photons) - work function
Photoelectric graphs
-KE max on y axis, frequency on x axis
- gradient = Planck constant
- minimum frequency found where KE max = 0 and from equation f(min) = work function/ plan is constant
- value of -(phi) found on y-intercept
- fmin = threshold frequency
How to find the stopping potential?
- create a circuit including
- sensitive ammeter
- photoemissive plate in an evacuated tube with an electrode opposite
- variable voltage supply - when monochromatic light is shone on the plate a current will flow
- increase the voltage until the current reads as 0, this is rigs stopping potential/voltage
Converting eV to Joules
(eV) x 1.6x10^-19
Converting joules to eV
(Joules) / 1.6x10^-19
What does the stopping potential give us and why?
The maximum KE because the electrons with KE max have been stopped.
- KE max = V * 1.6*10^-19 (joules)
What are discrete energy levels?
Where electrons in atoms can exist
Define excitation
Electrons in atoms gain energy from collisions with free electrons
- causes them to move up in energy levels
Define ionisation
When electrons in atoms gain enough energy from collisions with free electrons to be removed from the atom entirely
When does ionisation occur?
If the energy of the free electron is greater than the ionisation energy
What happens if an electron becomes exited?
It will return it it’s original energy level, thus release the energy gained in the form of a photon
How to get a line spectrum with a fluorescent tube
Pass the light from a fluorescent tube through a diffraction grating or prism
What does each line in a line spectrum represent
Different wavelength of light emitted by the tube
- contains discrete values of wavelength
What is the relationship between photon energies and wavelengths?
The correspond to one another
When does annihilation take place and what happens when it does ?
-When matter and anti-matter meet
- they destroy each other causing the particles to disappear from existence and a pair of photons to be produced
Three examples of corresponding matter and antimatter
- electron and positron
- proton and antiproton
- neutron and antineutron
What must be conserved in annihilation?
Momentum - as always
Energy - will change from particle rest energy into another form.
What is produced in the process of annihilation?
An identical pair of gamma protons with the same frequency as each other
Why is momentum conserved in annihilation?
Because the gamma photons will head in completely opposite directions
- thus having equal and opposite momentum
What is the total momentum before the collision and why?
- the momentum of particles before the collision is equal and opposite thus total momentum before the collision is zero
What is conserved in annihilation and pair productions?
Charge
Lepton number
Baryon number
Strangeness - unless weak interaction then can be 1 away
Momentum
Energy
