3.2.2 ELECTROMAGNETIC RADIATION AND QUANTUM PHENOMENA Flashcards
What is the photoelectric effect?
When electrons are emitted from the surface of a metal after EM radiation of a certain frequency is directed at it
What is intensity?
The rate of energy transfer to an area (Watts/m^2)
What is the photon explanation of threshold frequency?
The minimum frequency of an incident photon required for photoemission
What is the work function?
The minimum energy of a conduction electron to escape the surface of a metal when the metal is at zero potential.
(The minimum energy required to cause photoemission)
What is the photoelectric equation?
hf = Φ + Ek (max)
Explain why photoelectrons leave a material with varying kinetic energies?
Depends on closeness of electron to surface
Electrons collide with other electrons + lattice ions, transferring energy
What is stopping potential?
The minimum negative voltage applied to the anode to stop the photocurrent
- Causes the Ek of electrons to fall
to zero, as Ek is converted to potential energy
What is the equation for stopping voltage?
eV = hf - Φ [eV = Ek(max)]
What is the relationship between intensity of radiation and number of electrons emitted?
- Intensity is proportional to number of electrons emitted, given that the radiation is above the threshold frequency.
- Photoelectric emission occurs without delay, and each electron can only absorb one photon at a time, so increasing the intensity of radiation under the threshold frequency will not cause photoemission
How is light “carried”?
In discrete packets of energy (photons)
energy of a photon = hf
Describe a method to determine the stopping potential
- Connect a battery at 0V to an ammeter and 2 plates (anode
and cathode) in an evacuated tube - Direct radiation above the threshold frequency of the plate
onto the cathode and record the current produced - Connect the positive terminal of the battery to the plate
emitting photoelectrons, and the negative terminal to the
plate receiving them - Adjust voltage until current recorded = 0
Why can’t current be used solely to find the stopping potential/work function of a material?
The current only tells us the rate of flow of charge, and not the Ek of the electrons
Why is an evacuated tube used to determine stopping potential?
To prevent collisions with air molecules, which would also do work against photoelectrons (as well as work done by battery), which could make the calculated stopping voltage lower than it should be
What happens in an evacuated tube where photoelectrons are being emitted when a stopping potential is applied?
photoelectrons accelerate towards the anode, but do not come into contact with it, and return to the cathode
How is the kinetic energy of a photoelectron converted into potential energy?
The electric fields produced by the charged plates opposes the motion of the electrons
What is the rule for electrons absorbing photons?
An electron can only absorb one photon of energy at a time, so a single photon can eject only one electron
How does the photoelectric effect prove that light acts as a particle?
- Increasing intensity didn’t affect the maximum Ek of the electrons, suggesting that one photon was absorbed by one electron - proved light existed as “quanta” - tiny bits of energy rather than one continuous wave.
- If light were simple a wave-like phenomenon then increasing the intensity and thereby increasing the total energy falling on the surface would be expected to eventually provide enough energy to release electrons no matter what the frequency.
Describe the effect of intensity on photoemission
No effect on photoemission, as intensity is the energy per second per m^2 (energy is related to number of photons)
Increasing intensity increases the rate of energy absorbed on an area of the material, but not the energy received by each electron/atom
What is the equation for the power of a beam (of photons)
Power = nhf
where n is the number of photons passing a fixed point each second. A beam contains photons of the same frequency, so the power of the beam is the rate at which energy is transferred from the photons
Why does intensity of light not affect the maximum kinetic energy of a photoelectron?
The energy gained by an electron is only due to the absorption of ONE photon, so increasing the intensity (number of photons) won’t affect Ek
Describe the relationship between intensity and current
Over the threshold frequency, intensity is proportional to the size of the photocurrent. Light intensity is a measure of energy per second per m^2. Energy is proportional to the number of photons per second. Above the threshold frequency, each electron must have absorbed one photon in order to be liberated and produce a current. By increasing the intensity, the number of electrons liberated (charge carriers) increases, so intensity is proportional to current, given that the incident light is above the cathode’s threshold frequency
How do you calculate the number of photoelectrons transferring per second from the cathode onto the anode for a photoelectric current 𝑰
n = 𝑰/e
Where e is the elementary charge. The photocurrent is the rate of flow of electrons/charge (Q/t) , and each electron has a charge e, therefore the current can be rewritten as ne/t , where n is the number of electrons passing per second, and e is the elementary charge. Dividing by e gives the number of electrons flowing per second
i am getting better at explaining things
What is the definition of ionisation energy?
The energy required to liberate an electron from the ground state of an atom
(aka principal ionisation energy)
What is the ground state?
The lowest/most stable energy level of an atom
What is excitation?
The process of an atom (electron) absorbing energy WITHOUT being ionised.
Describe the process of excitation
Energy from an oncoming particle (photon/electron/neutron) is absorbed by an electron. The electron moves up to a discrete energy level (Ek), before dropping down to the original energy level, either directly or indirectly, releasing a photon/photons equal to the energy absorbed.
When an electron has moved to a higher energy level, the atom is said to be in an excited state
What are conditions for excitation with photons vs other particles (neutrons/electrons/ions)?
The energy of the photon must be EXACTLY equal to the difference in energy levels, and will be pass through the atom if it isn’t
With other particles, an electron can absorb all or some of the Ek of an oncoming particle. For excitation, the energy absorbed will be equal to the difference in energy lvls, and any remaining unabsorbed energy will remain in the initial oncoming particle
Describe how the ionisation energy of a gas can be measured
Set up a circuit w a glass tube containing gas at low pressure, with a negative filament on on end of the tube, and an anode on the other end. Measure current in circuit w milliammeter (due to some electrons reaching anode) , and then increase pd across filament and tube to increase the Ek of electrons emitted from filament. Eventually the emitted electrons have enough Ek to ionise gas atoms in the tube, increasing number of charge carriers reaching the anode. Each electron arrives at the anode with abt the same energy required to ionise it (from oncoming electron). By measuring the voltage just as current begins to increase, ionisation energy can be calculated as eV, as the work done on each electron to ionise the atom,
Gas is low pressure to reduce collisions and allow electrons to reach anode
How can the excitation energies of a gas in a glass tube be determined?
Increasing voltage across tube, and measuring voltage before current drops.
(As excitation will occur when energy of oncoming electron has been completely absorbed by atom, so no remaining Ek to provide a current)
What is monochromatic light?
Light of a fixed frequency
Why is the electron configuration of an excited atom unstable?
The excited electron has left a vacancy in the shell it moved from
What causes each concentric ring in an electron diffraction pattern?
(from a metal)
Electrons diffracted by the same amount from the same amount of grains of different orientations, at the same angle to the incident beam
Why is it important to test new theories by testing its predictions experimentally?
If the test does not provide supporting evidence,
the prediction is incorrect
so the theory is incorrect and must be changed
Explain why the Ek of photoelectrons ranges up to a maximum value
Incident photon has fixed energy -
photon loses all its energy in a single
interaction
Electron can lose various amounts of energy in reaching surface of metal, e.g. through collisions with other electrons
Why is the gas in a fluorescent tube at low pressure?
There must be sufficient distance between collisions for the electrons to gain enough energy for the required excitations to occur
The vapour must not completely absorb the
electrons
Explain how a fluorescent tube containing mercury vapour with a phosphorous coating would emit visible light
Mercury vapour emits ultraviolet radiation
UV radiation excites the atoms of
the coating
Coating then emits electromagnetic
radiation of longer wavelengths
Some of which is in the visible region
Describe how electrons can behave as both particles and waves
- They can be diffracted, or show
interference effects - Can be deflected in electric or
magnetic fields, or make collisions
with atoms
Explain why an electron and muon accelerated from rest by the same pd have different wavelengths
Both experience the same increase in energy. As W=QδV Wavelength is inversely proportional to momentum
gain in momentum is different for the muons and electrons. The smaller mass has the largest acceleration
+ Using λ = h/mv + Ek = p^2/2m
deduce λ proportional to 1/√m
So muon has a lower wavelength
Describe wave Vs particle properties
Wavelength, massless, frequency, diffraction and refraction
Ek, linear momentum
What does the average Ek of a conduction electron depend on
The temperature of the metal
Outline aspects of the photoelectric effect that can’t be explained by the wave model
Wave model predicts that wave energy would accumulate and therefore photoemission would occur eventually, however no photoemission under threshold frequency (zero time delay)
Wave model predicts that all frequencies of light would cause photoemission as energy depends on amplitude, isn’t the case
Stopping voltage independent of intensity. Wave model predicts that increasing intensity would increase energy given to each electron. However intensity is related to the size of the current, while V is related to Ek of each electron
Is current very related to speed
Current is related to the speed of electrons but it is not the speed of electrons! Current is the amount of charge that passes through a cross sectional area in one second
Current is given as N×A×V×E
where:
N is the number of free electrons per unit volume
A is the area of cross section
V is speed of free electrons and
E is electron charge