Quantum Flashcards
What is the photoelectric effect
When light incident on a metal surface causes electrons to be emitted from the surface
When was the photoelectric effect discovered and by who
1887
by Heinrich Hertz, but it took 18 more years for Albert Einstein to explain it
Which visible light colour has the highest frequency and what does this mean
Blue/violet
Most likely to cause a photoelectric effect
2 main points about increasing the frequency in terms of the photoelectric effect
Below a certain frequency (threshold) no electrons are emit and above it they are
Increasing the frequency increases the kinetic energy of the emitted electrons
What is the threshold frequency
The minimum frequency of light incident on a metal surface needed to cause electrons to be emitted
Different for each metal and a property of the metal
Effect of increasing the brightness/intensity if below the threshold frequency
No effect
Although more waves are transmitted per second and more waves strike the metals surface per second
The photons of light still do not have a frequency above the threshold frequency
So no electrons are emit
Effect of increasing the frequency above threshold
Photons have an increased frequency so an increased energy
Electrons emit from the metals surface with greater maximum kinetic energy
Effect of increasing the brightness/intensity if above the threshold frequency
More waves emitted per second from the light source
More photons of light striking the metal surface per second
Increased number of electrons emitted from the metals surface per second
What is the effect of moving a lamp twice as far away from the source
4x less photons of light strike the surface of the metal
Due to the inverse square law
But the kinetic energies of the photons remains the same
Since the frequency is the same
So 4x fewer electrons emitted per second
With the same kinetic energy
Why is it common for metals with one electron in their outer shell to exhibit the photoelectric effect in visible light
Less energy has to be absorbed for the electron to be emitted
What are photoelectrons
The emitted/liberated electrons
That are liberated from their metals surface due to the photoelectric effect
Why is it more accurate to refer to kinetic energy as maximum kinetic energy
The emitted electrons have a range of energies
What are the axis for the energy frequency graph
x; Frequency of radiation (x10^14Hz)
y; Maximum kinetic energy (x10-19J)
y intercept for the energy frequency graph
The work function
What is the x intercept for the energy frequency graph
The threshold frequency
Gradient of the energy frequency graph
Planck’s constant
h
In formula book (6.63x10^-34Js)
Comment on the speed electrons are emit during the photoelectric effect
Instantaneously
Negligible delay between light hitting the metal surface and electrons being emitted
What does the number of electrons emitted per second depend on
The intensity of the light
Why does light behave the way it does in the photoelectric effect
Electrons are held by electrostatic forces onto the surface of the metal
The light has to provide enough energy to rip the electron free from the metals surface and break free from these forces
What did the wave theory anticipate that was not true
If a wave hasn’t got enough energy to liberate an electron then you need a higher amplitude wave (brighter light)
But in practice if the light is below the threshold frequency then no electrons are produced even if brightness is increased
So to explain it Einstein had to move away from the wave theory
What was Einstein’s proposition
Instead of travelling in waves light can travel in discrete packets known as photons
What happens when a photon strikes a metals surface
Either absorbed if it has the sufficient energy
Or not absorbed at all
Will knock out an electron if its energy is larger than the energy holding the electron in the metal
Energy of photon equation
E=hf=hc/λ
E; Energy of each photon in Joules
h; Planck’s constant in Js
f; Frequency of each photon
What is the work function
Φ
Minimum energy required to liberate an electron from the surface of a metal
A property of the metal and different for each metal
What happens if monochromatic light is incident on metals surface at the threshold frequency
All the photons would have a frequency equal to the threshold frequency
So each photon would have just enough energy to cause the photoelectric effect to occur and electrons to be emit
hf
Photon energy is less than the work function
No photoelectrons are emitted
Photon frequency is less than the threshold frequency
hf=Φ
Photon energy is equal to the work function
Photoelectrons emitted with zero kinetic energy
Photon frequency=threshold frequency
hf>Φ
Photon energy is greater than the work function
Photoelectrons are emitted with a range of kinetic energies up to the maximum kinetic energy
Photon frequency is greater than the threshold frequency
Kinetic energy of electron emitted =
Energy of incident photon - Energy needed to remove electron
Comment on the electron depth and energy
If an electron is at the very surface of the metal the energy required is the work function and so has the maximum kinetic energy
those deeper require more energy so emitted with slightly less kinetic energy
Hence a range of kinetic energies
Energy of incident photon equation
Energy needed to remove the electron (Φ) + Kinetic energy of emitted electron
hf = Φ + Ekmax hf = hf0 + Ekmax (in J)
Provided
Work function equation
hf0
Planck’s constant x the threshold frequency
Not provided
Explain the wave model prediction for the photoelectric effect
Electrons gradually absorb energy from multiple light waves to gain enough energy to break free from the metal
Brighter light means quicker electron accumulates required energy
Energy of a beam is determined by the number of waves emitted per second not the frequency
Changing the colour of light has no effect on the emission of electrons
Explain the particle model explanation for the photoelectric effect
Electrons either completely absorb photon energy if its energy is greater than that to break free from the metal or the photon passes through
1 to 1 interaction
Electron cannot absorb more than one photon of light
So brightness doesn’t have any affect if the photon doesn’t have enough energy
Energy of each photon is determined by the frequency/colour of light
Increasing the brightness increases the number of photons hitting the metal surface so increases the number of electrons emitted
Explain the first half of the gold leaf experiment
Rub plastic rod with cloth to transfer electrons from cloth to rod
Rod has overall negative charge
Touch rod to zinc plate to transfer electrons
Leaving plate with negative charge
Negative charge flows down the metal stem and into the gold leaf
The stem and gold leaf are both negatively charged so repel eachother
Gold leaf rises
Explain the set up of the electroscope in gold leaf experiment
Zinc plate on a metal cap Attached to a metal stem That goes into an early metal cage Insulated in a vacuum to avoid charge leaking away to the air Stem has a gold leaf attached
Explain the second stage of the gold leaf experiment
If light with a frequency above the metals threshold frequency is incident on the plate
Electrons are liberated from the metal surface
The plate, metal stem and gold leaf lose their excess charge
The gold leaf and metal stem no longer repel one another
Leaf falls
Why wont light below the threshold frequency cause the golf leaf to fall
Photons do not have enough energy to liberate the electrons from the surface
Energy is lower than the work function
Leaf and stem remain charged as the excess charge is not removed
Why wont the gold leaf fall when the brightness of the light is increased when below the threshold frequency
Frequency still below the threshold frequency
Even though there are more photons hitting the metals surface per second, since photons and electrons interact 1-1, each individual photon still doesn’t have enough energy to liberate an electron
Leaf and stem remain charged as the excess charge is not removed
Energy of a single photon
hf-Φ
Explain why the photoelectric effect is not observed below the threshold frequency
Frequency of light is not high enough
Energy of each photon is lower than the work function
So don’t have enough energy to liberate electrons from the metal surface
Photons aren’t absorbed and instead pass through
What is validated evidence
Experimental evidence
That can be replicated
eV
Electronvolt
The kinetic energy gained by one electron passing through a potential difference of 1 volt
Do you use energy in joules or eV
Joules
eV x 10^-19
What is the stopping potential
The voltage of the battery when the current is zero
In stopping potential, the energy added by light is equal to what
The energy the battery removes
How do you place the battery in the stopping potential and why
With the negative terminal pointed towards the wire electrode
To resist the flow of electrons and decrease current
Increasing the strength of the battery increases its voltage until the current reaches zero
Stopping potential equation
Maximum kinetic energy of emitted electrons=Energy removed by stopping potential
Ekmax=eVs
Maximum kinetic energy of the emitted electrons
e; charge of an electron
Vs; stopping potential
Why does increasing the frequency of radiation increase the stopping potential
Vs=Ekmax/e
Increasing the stopping potential increases the maximum kinetic energy of the photons
Since e is constant, Vs must increase
So each electron absorbs more energy
3 types of spectra
Continuous
Emission line
Absorption line
Explain continuous spectrum
Light is emitted across a range of wavelengths
E.g bulb or filament lamp
Explain emission line spectrum
Emit specific wavelengths of light
By exciting a gas with heat or an electrical current
E.g a hot gas
Absorption spectrum
If a continuous spectrum is shone though a cold gas the gas will absorb specific wavelengths of light
So when dispersed through a prism a continuous spectrum is seen but with some wavelengths missing
Why does hydrogen/gases form an emission line spectrum
Can only emit light at certain discrete frequencies across the spectrum
Due to having electrons in discrete energy levels
Which only emit specific wavelengths of light/photons of specific energies
Neils Bohr propositions
Electrons only travel in certain allowed orbits known as energy levels
When electron is in an allowed orbit it does not radiate but stays at a constant energy
An electron in an atom can only emit or absorb energy as it moves from one orbit to another
This explains why only certain wavelengths of light are emitted from certain gases
Hydrogen has a ground state of 13.6eV
What does this mean
13.6eV energy is required to hold an electron in hydrogens ground state
Lowest energy level
2 key points for energy level diagrams
Negative
Given in eV
How is kinetic energy defined at ground state and infinity
Zero for highest state/infinity
So the energies for lower levels must be increasingly more negative
Since larger orbits have larger kinetic energies
How do electrons move between energy levels in terms of speed
Instantly
Cannot exist between energy levels
What is the energy absorbed equivalent to in energy level diagrams
The difference between the two energy levels
What happens in de-excitation
Loses the exact right amount of energy to fall to a lower energy level
Lost energy of electron is emitted as a photon
Equation for energy and frequency for photons
E=hf
E=hc/λ
E, the energy of a photon, must be in joules
How do you calculate the frequency or wavelength of an emitted photon
Calculate the difference in energy between the two energy levels
Convert to joules
E=hf or E=hc/λ
Why do different gases form different emission spectra
Each element has its own unique set of energy levels
Meaning de-excitations between the same two energy levels for two different elements will release a photon with different energies, frequencies and wavelengths
3 parts of the hydrogen emission spectrum
Lyman
Balmer
Paschen
Energy level transitions for Lyman
n=1 to n=6
Wavelengths for Lyman in nm
122 103 97 95 94
Smaller means a bigger transition
Energy level transitions for Balmer
n=2 to n=6
Wavelengths for Balmer in nm
656
486
434
410
Smaller means a bigger transition
Energy level transitions for Paschen
n=3 to n=6
Wavelengths for Paschen in nm
1875
1282
1094
Smaller means a bigger transition
3 ways in which an electron can be excited to a higher state
Incoming photon collides with an electron in an energy level
Incoming electron from current collides with an electron in an energy level
Heat transfer to an electron in an energy level
What happens when an electron absorbs an amount of energy
If its the correct amount
Excited or ionised
What is ionisation
When an electron gains enough energy to leave the atom
The energy required is always equal to the energy of the ground state but positive
How does ionisation relate to work function
It doesn’t
Never mention it in emission spectra
Only applies to the photoelectric effect
Explain excitation by a photon
Photon incident with an energy that is the exact difference between two energy levels
Photon is completely absorbed
Electron increases its kinetic energy
Instantaneously moves to a higher energy level
Explain excitation by collision of an electron
Incident electron carries energy greater than or equal to the difference between two energy levels
Loses its energy to excite the orbital electron
Orbital electron increass uts kinetic energy
Instantaneously moves to the higher energy level
Orbital maintains any left over energy to move away (its Ek)
Explain ionisation by an electron
Incident electron carries greater kinetic energy than the ground state energy
Orbital electron uses this amount to free the electron from the atom
Any left over kinetic energy used by the two electrons
Due to conservation of energy
Compare excitation by a photon and by an electron
Photon is absorbed but electron is not absorbed
Photon must have an energy equal to the difference between two energy levels but incident electron must have an energy greater than or equal the difference between two energy levels
Photon involves photon electron involves electrons
Explain multiple de-excitation
Can de excite more than one time and emit a photon each time it de excite
The path the electron takes is usually random
How does the existence in the spectrum of lines of a definite wavelength support the view that atoms have discrete energy levels
Fixed energy levels means only certain transitions are allowed
Producing photons of a few different wavelenths corresponding to the differences in energy levels
If the electron energy was continous a continous spectrum would be formed
What is fluorescence
When UV light is absorbed by certain substances or materials
Which them emit visible light
Uses of fluorescence
Lighting homes
Streets
Marker pens
Fluorescent inks used in bank notes for security
Why are fluorescent tubes more efficient than traditional tungsten filament lamp
Fluorescent tubes convertost of the energy of the supplied into light with only a few watts lost as heat
Why is Mercury vapour at low pressure used in fluorescent tubes
Lower pressure means lower collisions per second
Allowing electrons to pass through easily
So a suffienct current can flow
Purpose of the phosphor coating atom
Orbitals are close together so the enegy difference between each is less than the mercury
Orbital electrons of phosphor absorb the visible light emitted by mercury
This excites its orbital electrons
And when they de-excite they emit visible light
Because the energy levels are close together
Explain how the fluorescent tube works
- Mercury atoms collide with eachother and with electrons in the tube
- Electrons in Mercury absorb energy
- Mercury atoms become excited/ionised
- Electrons return to ground state in Mercury atoms
- Emitting ultraviolet photons as they de excite
- UV photons absorbed by electrons in the atoms of the tube coating
- Coating atoms became excited
- Coating atoms de excite emitting visible light photons
Evidence for light as a wave
Diffraction
Refraction
Polarisation
Evidence for light as a particle
Photoelectric effect
Line spectra
What is wave particle duality
Light behaves like a wave under some circumstances and a particle under others and sometimes a mixture of the two
De Broglie wavelength formula
4
De Broglie wavelength formula
λdb=h/mv
Or
λdb=h/p
What is the de broglie wavelength
The effective wavelength of a particle
As determined by its momentum
When will a particle diffract
If the de broglie wavelength is similar to the gap size
Will experience diffraction effects just like a wave
When does maximum diffraction occur
De broglie wavelength is close to the size of the gap
What can be used to examine atomic structure
Electron diffraction
Where the spacing between atoms (aperture) is the order of 2x10-¹¹m
Why is electronic diffraction seen as rings and not linear
In 2 dimensions
Still a single slit interference but in 2D there are lots of different patterns at angles to form rings of constructive and destructive (light and dark) interference
What happens in the electron diffraction
Electrons fores from a hot metal filament (electron gun)
And accelerated to some kind of crystalline structure (in this case graphite)
Electrons have a λdb similar to the spacing of carbon atoms in graphite so behave like waves passing through a diffraction grating
Diffraction electrons hit fluorescent screen/phosphor coating and form an interference pattern
By measuring the separation of the rings the spacing of the carbon atoms in graphite can be calculated
CRYSTALLOGRAPGY
What is crystallography
Study of atomic arrangements in materials
State what is meant by the duality of electrons
Electrons behaving as both a wave and a particle
Why is the energy of photoelectrons normally less than the maximum kinetic energy
Some energy is lost in collisions when leaving the metal
How is ultraviolet light generated in a fluorescent tube
High voltage used to accelerate s small number of free electrons through the tube
These free electrons collide with Mercury gas atoms and ionise and excite them
When the excited atoms return to ground state they emit a ultraviolet photon of energy equal to the difference in energy levels
How does the production if bright rings in an electron gun suggest electeons behave like waves
Constructive interference/superposition where waves arrive in phase and produce maximum intensity
Why do electrons emitted have a range of kinetic energies up to a maximum
Photon energy depends on frequency and is constant
One to one interaction between photon and electron
Maximum kinetic energy = photon energy - work function
More energy required to remove deeper electrons
What must happen in order for an existing scientific theory to be modified or replaced with a new theory
Theory makes predictions tested by other scientists and reviewed
New evidence that is repeatable is checked by other scientists
How could you demonstrate cathode rays are negative charge particles
Pass them between charged particles
Use a magnetic field
What evidence does a cathode Ray tube show about nature of moving electrons
Cathode rays are negatively charged
Diffraction
Electron is behaving as a wave
Describe the process that occurs when positron collides with a free lepton in water
Missing energy is carried off by third particle
Law of conservation of energy appears to be violated when beta particle has less than 1.2MeV
