3.2.2 Electromagnetic Radiation and Quantum Phenomena Flashcards
What do electrons do in a metal and what are these referred to as?
They move around, delocalised/free electrons.
Why don’t free electrons escape the metal?
Still electrostatically attracted to the positive ions so stay in the metal. Don’t have the energy to overcome the attraction.
Define work function.
The minimum amount of energy required to remove an electron from the surface of a metal.
What is work function measured in and should you convert?
MeV or J, convert to J.
What symbol do we use for work function?
ϕ.
Define threshold frequency?
The minimum frequency of the EM radiation required to remove an electron from the surface of a metal.
What do we measure threshold frequency in?
Hz.
What is the symbol for threshold frequency?
f₀.
What is Einstein’s photoelectric effect equation?
hf = Φ + Ek(max)
In Einstein’s photoelectric equation what is hf?
Photon energy.
In Einstein’s photoelectric equation what is Φ?
Work function.
In Einstein’s photoelectric equation what is Ek(max)?
Maximum kinetic energy.
What do electrons escape the metal with and what is the range?
Ek, ranging from 0 to Ek(max).
Why do electrons escape with a range of Ek?
Because some are deeper in the surface of the metal and have to do more work to escape.
In a graph showing Ek max against frequency, what is the gradient?
h, planck constant.
In a graph showing Ek max against frequency, what is the x intercept?
f₀.
In a graph showing Ek max against frequency, what is the y intercept?
-Φ.
In a graph showing Ek max against frequency, if the work function is bigger how does the graph change?
Shifts to the right.
What components are in a vacuum photo cell circuit?
Photoemissive electrode, wire, microammeter, variable DC supply.
What is a micro amp in amps?
x10-6.
What is a vacuum?
No particles.
In a vacuum photocell what goes in?
Photons.
What are the two electrodes in a vacuum photocell?
Wire, photoemissive.
When the photons enter the vacuum photo cell what leaves the photoemissive electrode and what does this create?
Electrons, a current.
If we increase f in a vacuum photocell circuit what happens to A, why?
No change, same number of electrons crossing gap per second (they just have more Ek(max)).
If we increase intensity in a vacuum photocell circuit what happens to current, why?
Current increases, more photons arrive per second, more e-s per second cross the gap, same Ek(max) as before.
Define stopping potential.
The minimum p.d. required to stop all of the e-s crossing the gap (even ones at Ek(max)), the current becomes 0.
As the p.d increases the current…?
Decreases.
Work done by p.d = Ek …?
Lost by e-s.
QV =
1/2 m v^2.
eV=
1/2 m v^2
eVs = (complex)
1/2 m vmax^2
eVs = (simple)
Ek(max)
What is the e in eVs?
Charge of electron.
What can happen when an orbiting electron gains energy?
It can be promoted to a higher energy level.
When an electron is promoted to a higher energy level what do we call it?
Excited.
How does an electron gain energy to get excited?
Collision with a passing electron or absorbing a photon of EM radiation.
What do we call an electron when it’s in it’s normal energy level?
Ground state.
How do we represent in a diagram the different energy levels an e- can be promoted to?
Using straight lines stacked on top of eachother, get closer together as we go up.
What will the energies needed to promote an e- to a higher energy level be measured in?
eV.
How do we convert eV into J?
x by charge of an electron. 1.60 x 10^-19.
What happens after an e- is excited?
It relaxes and returns to the ground state.
Does an e- have to return directly to the ground state?
No.
What does the excited e- emit when it returns to the ground state?
A photon of energy.
Change in E = ?
E1 - E2.
A photon that is emitting by an e- going directly to a ground state has what?
Highest energy and highest frequency.
A photon that is emitted by an e- relaxing from a higher energy level to a lower energy level but not returning directly to the ground state has what?
Lower energy, longer wavelength.
How do you calculate the length of the wavelength of a photon emitted by an excited e- relaxing?
Convert eV into J, use E = hc/λ, rearrange to find λ.
What is the equation that connects E, h and f?
E = h f.
What is the equation that connects E, h, c and λ?
E = hc/λ.
What is the equation that connects h, f, E1 and E2?
hf = E1 - E2.
What do we call it when an excited e- returns to ground state?
De-exciting or relaxing.
Each element has a specific set of?
Discrete energy levels.
Define discrete.
Separate, not continuous data.
When an electron de-excites and releases a photon the photon can only have what? What is this equal to?
Certain energy values, equal to the difference in energy levels.
The frequency and wavelength are dependant on the ______?
Photon energy.
What is the visible light range?
400nm to 700nm.
How do you set up an absorption spectra?
Shoot white light through a cloud of gas and observe with a telescope.
As white light passes through a gas why do certain photons get absorbed?
Because their energies match possible gaps in the atom’s energy levels.
On absorption spectra what do the missing photons show up as?
Dark lines.
What is an ion?
A charged atom.
What is ionisation?
The process of removing/adding an electron from an atom, turning it into an ion.
Define ionisation energy.
The minimum energy required to remove an electron in the ground state from an atom.
What are the three things inside a fluorescent tube?
Low pressure mercury gas atoms, free electrons, powder coating.
Why do we apply a high p.d to a fluorescent tube?
To remove electrons from some of the gas atoms, turning them into positive ions.
What do the positive ions do in the fluorescent tube?
Accelerate towards the negative electrode and dislodge electrons from the electrode.
What do the free electrons in a fluorescent tube do?
Accelerate towards the positive electrode and collide with mercury atoms along the way, exciting them.
When a mercury atom gets excited it releases a what, of which type?
A high energy UV photon.
What absorbs the UV from the mercury atom and what happens?
The powder coating around the tube, it excites.
What happens when the powder coating excites?
The atom de-excites through smaller intermediate energy levels, releasing lower frequency photons (visible light).
Why is there high pressure and high p.d in a fluorescent tube?
Ensures the e-s ca reach the required speeds (by accelerating between collisions) to excite the mercury atoms.
Why do we need a powder coating in a fluorescent tube, why can’t we just have mercury?
Because mercury emits photons with too high a frequency to see and it wouldn’t make light.
Why are compact fluorescent tubes funny shapes?
Increased surface area.
EM radiation can behave as what two things?
Wave or particle.
What are the qualities of EM radiation that suggest it’s a wave?
Reflection, refraction, diffraction, interference, polarisation.
What are the qualities of EM radiation that suggest it’s a particle?
The photoelectric effect.
What is the de Broglie hypothesis?
A particle, mass m, moving with speed v, has an associated wavelength, λ, the ‘de Broglie wavelength.
What is the equation for the de Broglie wavelength?
λ = h/p = h/mv
What does p stand for, and what is it measured in?
Momentum, kgms-1
How has the de Broglie wavelength been proved?
By electron diffraction experiments.
What is diffraction?
Where the edges of a wave spread out when it passes an obstacle or through a gap.
If λ is equal to gap size what is the diffraction?
Strong.
If λ > gap size what is the diffraction?
It reflects and doesn’t pass through.
If λ < gap size what is the diffraction?
Weak.
When getting long numbers in a calculation answer, like calculating v, what do we do?
Put them in standard form.
When asked to define Ek in Einstein’s photoelectric equation, what do we always write?
Ek max, maximum kinetic energy an e- can have after escaping the surface of a metal.
When asked the minimum p.d for an e- to be accelerated through for ionisation how do we answer?
For ionisation to occur it must supply difference from ionisation level and ground state, put this value into eV, e- is accelerated through eV value in just V.
How do we answer questions asking about doubling light intensity? What assumptions are made?
x2 photons, x2 e-s, current = rate of flow of charge, x2 current. Ek max stays constant and unchanged. Assume 1 photon removes 1 e-, assume all photoelectrons are collected.
How do we answer new metal and new Φ questions?
No e-s have enough energy to escape, Φ>hf and Φ>original Φ.
What happens in an atom when line spectra are produced?
e-s excited from one energy level to another, e-s are emitting or absorbing definite λ/ f/ colour/ photon energy hf.
When in eV, how to convert into J?
x 1.60 x10-19
What shows e-s are particles?
Deflection in EM fields.
Why does the gold leaf fall when radiation is absorbed into electroscope?
Energy of rad > Φ so photoelectrons are emitted and the electroscope discharges (becauses e-s and their - charge is leaving), leaf and metal stem no longer repel and the leaf falls.
Why won’t only visible light being absorbed (for zinc) let the gold leaf fall?
VL f < UV f, or VL E < Φ, so leaf doesn’t fall becuase no e-s are escaping so everything is still charged and repelling.
When plate is given + charge why doesn’t gold leaf fall?
Higher voltage, harder for e-s to leave, gold leaf doesn’t fall.
What does an e- have to pass through to diffract?
The gap between two nuclei.
What is observed on the fluorescent screen when the e-s diffract?
A diffraction pattern.
Roughly how big is the atomic diameter?
10^-10m
Why does there being no time delay for the photoelectric effect prove light is a particle?
Light travels as photons, transfers E in discrete packets in 1 to 1 interactions.
Why does Ek have a max value?
hf is energy available/ always the same energy from the photons, energy required varies (some e-s deeper) so Ek varies.
Why only certain values of f cause excitation
e-s occupy DISCRETE energy levels, need to absorb exact energy levels to move to a higher level, photons need certain E to provide f (E= hf), energy needed is same for a certain atom, all energy of photon is absorbed, 1 to 1 interaction between photon and e-.
What does the atom receive in both ionisation and excitation?
Exact amount of E.
How do we prove a cathode is negative?
Use a magnetic field.
What must happen to modify/ replace a theory?
Theory must meet predictions tested, repeatable results and peer reviewed.
