Quantum Flashcards
What is the Photoelectric Affect?
The process of removing an electron from the surface of a metal due to light incident on its surface
What is the Photoelectric Equation?
hf = Φ + Ek
What is Threshold Frequency?
The minimum frequency required to emit electrons from the surface of a metal
What is Work Function?
The minimum energy required to remove an electron from the surface of a metal
What is Ionization?
When an atom either gains or loses an electron, giving a charge
What is Excitation?
Where an atom absorbs energy, causing an electron to move to a higher energy level with ionisation
What happens in De-Excitation?
An electron moves from a higher energy state to a lower energy state and emits a photon in the process
What is the Emitted Photon Equation?
hf = E1 - E2
What is the De Broglie Equation?
λ = h / mv
What is the conversion for eV to J?
1eV = 1.6x10^-19J
-Charge of an electron
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
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?
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
What components are in a vacuum photo cell circuit?
Photoemissive electrode, wire, microammeter, variable DC supply
What is a Vacuum?
No particles
When the photons enter the vacuum photo cell what leaves the photoemissive electrode and what does this create?
Electrons, a current
If we increase frequency in a vacuum photocell circuit what happens to current, 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
What is 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
What happens to current as p.d increases
Decreases
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
What will the energies needed to promote an e- to a higher energy level be measured in?
eV
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
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 = hf
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
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
EM radiation can behave as what two things?
Wave or particle
What are the qualities of EM radiation that suggest it’s a wave? (5)
Reflection, refraction, diffraction, interference, polarisation
What are the qualities of EM radiation that suggest it’s a particle?
The photelectric affect
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
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 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
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
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-
